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+4 0 -1 0 0 0 12 0.0000000 0 135 750 1395 2295 Data area\001 +4 0 -1 0 0 0 12 0.0000000 0 135 750 1395 2970 Data area\001 +4 0 -1 0 0 0 12 0.0000000 0 135 1485 1395 1980 Unused boot sector\001 +4 0 -1 0 0 0 12 0.0000000 0 135 1485 1395 2655 Unused boot sector\001 diff --git a/LDP/guide/docbook/sag/hd-layout.gif b/LDP/guide/docbook/sag/hd-layout.gif new file mode 100644 index 00000000..ab400b13 Binary files /dev/null and b/LDP/guide/docbook/sag/hd-layout.gif differ diff --git a/LDP/guide/docbook/sag/hd-mount-mounted.fig b/LDP/guide/docbook/sag/hd-mount-mounted.fig new file mode 100644 index 00000000..297ab507 --- /dev/null +++ b/LDP/guide/docbook/sag/hd-mount-mounted.fig @@ -0,0 +1,48 @@ +#FIG 3.1 +Portrait +Center +Inches +1200 2 +6 975 1350 2250 2175 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 1 + 1575 1950 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1575 1950 1575 1350 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1575 1350 1125 1950 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1575 1350 2025 1950 +4 0 -1 0 0 5 10 0.0000000 0 90 270 1425 2175 liw\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 975 2175 abc\001 +4 0 -1 0 0 5 10 0.0000000 0 120 270 1950 2175 ftp\001 +-6 +6 2625 1350 4125 2175 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 3375 1350 2775 1950 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 3375 1350 3375 1950 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 3375 1350 3975 1950 +4 0 -1 0 0 5 10 0.0000000 0 90 270 2625 2175 bin\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 3825 2175 lib\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 3225 2175 etc\001 +-6 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 375 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 975 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 1575 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 2325 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 2775 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 3375 1050 +4 0 -1 0 0 5 10 0.0000000 0 90 270 225 1275 bin\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 825 1275 dev\001 +4 0 -1 0 0 5 10 0.0000000 0 90 360 1425 1275 home\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 2250 1275 etc\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 2625 1275 lib\001 +4 0 -1 0 0 5 10 0.0000000 0 60 270 3300 1275 usr\001 +4 0 -1 0 0 5 10 0.0000000 0 120 90 1800 375 /\001 diff --git a/LDP/guide/docbook/sag/hd-mount-mounted.gif b/LDP/guide/docbook/sag/hd-mount-mounted.gif new file mode 100644 index 00000000..275a3d1d Binary files /dev/null and b/LDP/guide/docbook/sag/hd-mount-mounted.gif differ diff --git a/LDP/guide/docbook/sag/hd-mount-separate.fig b/LDP/guide/docbook/sag/hd-mount-separate.fig new file mode 100644 index 00000000..b0c63e9b --- /dev/null +++ b/LDP/guide/docbook/sag/hd-mount-separate.fig @@ -0,0 +1,48 @@ +#FIG 3.1 +Portrait +Center +Inches +1200 2 +6 4050 224 7350 1349 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 1 + 4651 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 4651 1050 4651 450 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 4651 450 4201 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 4651 450 5101 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6601 450 6001 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6601 450 6601 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6601 450 7201 1050 +4 0 -1 0 0 5 10 0.0000000 0 90 270 4501 1275 liw\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 5851 1275 bin\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 7051 1275 lib\001 +4 0 -1 0 0 5 10 0.0000000 0 120 90 6526 375 /\001 +4 0 -1 0 0 5 10 0.0000000 0 120 90 4576 375 /\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 4051 1275 abc\001 +4 0 -1 0 0 5 10 0.0000000 0 120 270 5026 1275 ftp\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 6451 1275 etc\001 +-6 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 375 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 975 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 1575 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 2325 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 2775 1050 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 1875 450 3375 1050 +4 0 -1 0 0 5 10 0.0000000 0 90 270 225 1275 bin\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 825 1275 dev\001 +4 0 -1 0 0 5 10 0.0000000 0 90 360 1425 1275 home\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 2250 1275 etc\001 +4 0 -1 0 0 5 10 0.0000000 0 90 270 2625 1275 lib\001 +4 0 -1 0 0 5 10 0.0000000 0 60 270 3300 1275 usr\001 +4 0 -1 0 0 5 10 0.0000000 0 120 90 1800 375 /\001 diff --git a/LDP/guide/docbook/sag/hd-mount-separate.gif b/LDP/guide/docbook/sag/hd-mount-separate.gif new file mode 100644 index 00000000..51b45661 Binary files /dev/null and b/LDP/guide/docbook/sag/hd-mount-separate.gif differ diff --git a/LDP/guide/docbook/sag/hd-schematic.fig b/LDP/guide/docbook/sag/hd-schematic.fig new file mode 100644 index 00000000..97517e25 --- /dev/null +++ b/LDP/guide/docbook/sag/hd-schematic.fig @@ -0,0 +1,155 @@ +#FIG 3.1 +Portrait +Center +Inches +1200 2 +5 1 0 1 -1 -1 0 0 -1 0.000 0 0 1 0 1525.830 7519.170 990 7110 1410 6855 1995 7035 + 0 0 1.00 75.00 135.00 +5 1 0 1 -1 -1 0 0 -1 0.000 0 1 1 0 1525.830 6880.830 990 7290 1410 7545 1995 7365 + 0 0 1.00 75.00 135.00 +5 1 0 1 -1 -1 0 0 -1 0.000 0 1 0 0 3872.565 3152.565 5715 3015 3870 1305 2025 3150 +5 1 0 1 -1 -1 0 0 -1 0.000 0 1 0 0 3872.565 3147.435 2025 3150 3870 4995 5715 3285 +5 1 0 1 -1 -1 0 0 -1 0.000 0 1 0 0 3872.280 3152.280 5940 3015 3870 1080 1800 3150 +5 1 0 1 -1 -1 0 0 -1 0.000 0 1 0 0 3872.280 3147.720 1800 3150 3870 5220 5940 3285 +5 1 0 1 -1 -1 0 0 -1 0.000 0 1 0 0 3870.900 3150.900 6165 3060 3870 855 1575 3150 +5 1 0 1 -1 -1 0 0 -1 0.000 0 1 0 0 3870.900 3149.100 1575 3150 3870 5445 6165 3240 +5 1 0 1 -1 -1 0 0 -1 0.000 0 0 1 0 3870.000 3127.500 3060 765 3870 630 4680 765 + 0 0 1.00 75.00 135.00 +6 2340 1620 5400 4680 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 3255 1650 3330 1860 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 4380 4440 4470 4650 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 2355 3750 2565 3675 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 5160 2625 5370 2535 +-6 +6 2340 1620 5400 4680 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 4470 1650 4380 1860 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 3330 4440 3255 4650 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 2355 2535 2565 2625 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 5160 3675 5370 3765 +-6 +1 3 0 1 -1 -1 0 0 -1 0.0000000 1 0.000 3870 3150 225 225 3870 3150 3915 2925 +1 3 0 1 -1 -1 0 0 -1 0.0000000 1 0.000 3870 3150 1620 1620 3870 3150 3960 1530 +1 3 0 1 -1 -1 0 0 -1 0.0000000 1 0.000 3870 3150 1395 1395 3870 3150 3960 1755 +1 3 0 1 -1 -1 0 0 -1 0.0000000 1 0.000 3870 3150 1170 1170 3870 3150 3915 1980 +1 3 0 1 -1 -1 0 0 -1 0.0000000 1 0.000 3870 3150 945 945 3870 3150 3870 2205 +2 1 1 1 -1 -1 0 0 -1 4.000 0 0 -1 0 0 2 + 2250 3150 2250 8190 +2 1 1 1 -1 -1 0 0 -1 4.000 0 0 -1 0 0 2 + 2475 3150 2475 8190 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 6615 4170 5940 5805 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 1035 6300 1575 6300 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 2355 8640 2355 8235 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 4095 6390 4095 7065 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 4095 7290 4095 7965 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 3645 7290 3645 7965 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 3645 6390 3645 7065 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 7110 8550 7110 5760 6840 5760 6840 8550 7110 8550 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 1 + 3825 3150 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 5850 1440 5175 2250 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 5265 990 4860 1665 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 6615 3690 5805 3150 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 3870 1530 3870 1755 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 3870 4545 3870 4770 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 2250 3150 2475 3150 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 5280 3150 5505 3150 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 2730 1995 2880 2160 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 4845 4140 5010 4290 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 2730 4290 2880 4140 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 4860 2145 5025 1995 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 6030 3285 6030 3015 5535 3015 5535 3285 6030 3285 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 7110 3420 7110 2880 6840 2880 6840 3420 7110 3420 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 3240 6840 3240 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 3060 6840 3060 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 6030 6120 6030 5940 5535 5940 5535 6120 6030 6120 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 5985 6840 5985 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 6075 6840 6075 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 6120 6165 6120 6390 1620 6390 1620 6165 6120 6165 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 6030 6615 6030 6435 5535 6435 5535 6615 6030 6615 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 6480 6840 6480 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 6570 6840 6570 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 6030 7020 6030 6840 5535 6840 5535 7020 6030 7020 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 6885 6840 6885 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 6975 6840 6975 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 6030 7515 6030 7335 5535 7335 5535 7515 6030 7515 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 7380 6840 7380 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 7470 6840 7470 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 6030 7920 6030 7740 5535 7740 5535 7920 6030 7920 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 7785 6840 7785 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 7875 6840 7875 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 6030 8415 6030 8235 5535 8235 5535 8415 6030 8415 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 8280 6840 8280 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 2 + 6030 8370 6840 8370 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 6120 7965 6120 8190 1620 8190 1620 7965 6120 7965 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 6120 7065 6120 7290 1620 7290 1620 7065 6120 7065 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 4 + 3645 8190 3645 8415 4095 8415 4095 8190 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 4 + 3645 6165 3645 5940 4095 5940 4095 6165 +4 0 -1 0 0 0 24 0.0000000 0 255 1755 405 810 From above\001 +4 0 -1 0 0 0 2 0.0000000 0 15 150 405 5535 From the side\001 +4 0 -1 0 0 0 16 0.0000000 0 165 855 3645 540 Rotation\001 +4 0 -1 0 0 0 16 0.0000000 0 165 570 5310 1035 Track\001 +4 0 -1 0 0 0 16 0.0000000 0 165 630 5895 1485 Sector\001 +4 0 -1 0 0 0 16 0.0000000 0 195 1605 6255 3960 Read/write head\001 +4 0 -1 0 0 0 16 0.0000000 0 225 870 2115 8865 Cylinder\001 +4 0 -1 0 0 0 16 0.0000000 0 165 660 375 6300 Platter\001 +4 0 -1 0 0 0 16 0.0000000 0 165 840 150 7200 Surfaces\001 diff --git a/LDP/guide/docbook/sag/hd-schematic.gif b/LDP/guide/docbook/sag/hd-schematic.gif new file mode 100644 index 00000000..03ee11aa Binary files /dev/null and b/LDP/guide/docbook/sag/hd-schematic.gif differ diff --git a/LDP/guide/docbook/sag/logins-via-terminals.fig b/LDP/guide/docbook/sag/logins-via-terminals.fig new file mode 100644 index 00000000..36a94788 --- /dev/null +++ b/LDP/guide/docbook/sag/logins-via-terminals.fig @@ -0,0 +1,100 @@ +#FIG 3.1 +Portrait +Center +Inches +1200 2 +6 2550 750 4350 1425 +6 2700 750 4200 1350 +4 0 -1 0 0 0 12 0.0000000 0 135 780 2700 1035 init: fork +\001 +4 0 -1 0 0 0 12 0.0000000 0 180 1455 2700 1305 exec("/sbin/getty")\001 +-6 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 4350 1425 4350 825 2550 825 2550 1425 4350 1425 +-6 +6 2550 1725 4350 2325 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 4350 2325 4350 1725 2550 1725 2550 2325 4350 2325 +4 0 -1 0 0 0 12 0.0000000 0 180 1440 2700 2100 getty: wait for user\001 +-6 +6 2550 4425 4350 5025 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 0 0 5 + 3450 4425 2550 4725 3450 5025 4350 4725 3450 4425 +4 0 -1 0 0 0 12 0.0000000 0 180 1140 2925 4800 do they match?\001 +-6 +6 2550 5325 4350 5925 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 4350 5925 4350 5325 2550 5325 2550 5925 4350 5925 +4 0 -1 0 0 0 12 0.0000000 0 180 1605 2700 5700 login: exec("/bin/sh")\001 +-6 +6 2550 6225 4350 6825 +6 2700 6225 4275 6750 +4 0 -1 0 0 0 12 0.0000000 0 135 1560 2700 6450 sh: read and execute\001 +4 0 -1 0 0 0 12 0.0000000 0 135 810 2700 6720 commands\001 +-6 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 4350 6825 4350 6225 2550 6225 2550 6825 4350 6825 +-6 +6 2550 7125 4350 7725 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 4350 7725 4350 7125 2550 7125 2550 7725 4350 7725 +4 0 -1 0 0 0 12 0.0000000 0 135 555 3075 7500 sh: exit\001 +-6 +6 150 4425 1950 5025 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 1950 5025 1950 4425 150 4425 150 5025 1950 5025 +4 0 -1 0 0 0 12 0.0000000 0 180 735 600 4800 login: exit\001 +-6 +6 2549 3524 4424 4199 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 4350 4125 4350 3525 2550 3525 2550 4125 4350 4125 +4 0 -1 0 0 0 12 0.0000000 0 180 1560 2700 3900 login: read password\001 +-6 +6 2549 2624 4424 3299 +6 2625 2699 4275 3224 +4 0 -1 0 0 0 12 0.0000000 0 180 1335 2626 3120 exec("/bin/login")\001 +4 0 -1 0 0 0 12 0.0000000 0 180 1635 2626 2850 getty: read username,\001 +-6 +2 2 0 1 -1 -1 0 0 -1 0.000 0 0 0 0 0 5 + 4350 3225 4350 2625 2550 2625 2550 3225 4350 3225 +-6 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 2550 4725 1950 4725 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 3450 1425 3450 1725 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 3450 2325 3450 2625 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 3450 2325 3450 2625 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 3450 3225 3450 3525 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 3450 4125 3450 4425 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 3450 5025 3450 5325 +2 1 0 1 -1 -1 0 0 -1 0.000 0 0 -1 1 0 2 + 0 0 1.00 75.00 135.00 + 3450 5925 3450 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00000000..8b649a8e --- /dev/null +++ b/LDP/guide/docbook/sag/overview-kernel.fig @@ -0,0 +1,181 @@ +#FIG 3.1 +Portrait +Center +Inches +1200 2 +6 300 1350 6000 1800 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 6000 1800 6000 1350 300 1350 300 1800 6000 1800 +4 0 -1 0 0 0 10 0.0000000 0 135 1290 2220 1620 System call interface\001 +-6 +6 300 2100 1800 2700 +6 375 2100 1725 2625 +4 0 -1 0 0 0 12 0.0000000 0 180 1365 375 2355 Virtual filesystem\001 +4 0 -1 0 0 0 12 0.0000000 0 150 960 375 2625 management\001 +-6 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 1800 2700 1800 2100 300 2100 300 2700 1800 2700 +-6 +6 300 4425 1800 5025 +6 525 4500 1575 5025 +4 0 -1 0 0 0 12 0.0000000 0 135 1005 525 4725 IDE harddisk\001 +4 0 -1 0 0 0 12 0.0000000 0 135 435 525 4995 driver\001 +-6 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 1800 5025 1800 4425 300 4425 300 5025 1800 5025 +-6 +6 2250 5625 3525 6000 +2 4 0 1 -1 -1 0 0 -1 0.000 0 0 7 0 0 5 + 3525 6000 3525 5625 2250 5625 2250 6000 3525 6000 +4 0 -1 0 0 0 12 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0 12 0.0000000 0 180 1530 2250 525 User level programs\001 diff --git a/LDP/guide/docbook/sag/overview-kernel.gif b/LDP/guide/docbook/sag/overview-kernel.gif new file mode 100644 index 00000000..00f1ef12 Binary files /dev/null and b/LDP/guide/docbook/sag/overview-kernel.gif differ diff --git a/LDP/guide/docbook/sag/sag.sgml b/LDP/guide/docbook/sag/sag.sgml new file mode 100644 index 00000000..20f53779 --- /dev/null +++ b/LDP/guide/docbook/sag/sag.sgml @@ -0,0 +1,7826 @@ + + + + + $Date$ + The Linux System Administrator's Guide + Version 0.7 + + Lars + Wirzenius + +
+ liw@iki.fi +
+ + + + Joanna + Oja + +
+ viu@iki.fi +
+ + + + Stephen + Stafford + +
+ stephen@clothcat.demon.co.uk +
+ + + + + + An introduction to system administration of a +Linux system for novices. + + + + Copyright 1993--1998 Lars Wirzenius. + Copyright 1998--2001 Joanna Oja. + Copyright 2001 Stephen Stafford. + + Trademarks are owned by their owners. + + Permission is granted to copy, distribute and/or modify this + document under the terms of the GNU Free Documentation License, + Version 1.1; with no Invariant Sections, with no Front-Cover Texts, + and with no Back-Cover Texts. A copy of the license is included in + the section entitled "GNU Free Documentation + License". + + + + + + + + Source and pre-formatted versions available + The source code and other machine readable formats + of this book can be found on the Internet via anonymous FTP at the + Linux Documentation Project home page http://www.linuxdoc.org/, or + at the home page of this book at http://people.debian.org/~ba + gpuss/. + Available are at least Postscript and TeX .DVI formats. + + + + +Introduction +
In the beginning, the file was without + form, and void; and emptiness was upon the face of the bits. + And the Fingers of the Author moved upon the face of the + keyboard. And the Author said, Let there be words, and there + were words.
+ + The Linux System Administrator's Guide, + describes the system administration aspects of using Linux. + It is intended for people who know next to nothing about system + administration (those saying ``what is it?''), but who have already + mastered at least the basics of normal usage. This manual + doesn't tell you how to install Linux; that is described in the + Installation and Getting Started document. See below for more + information about Linux manuals. + + System administration covers all the things that you have to + do to keep a computer system in usable order. It includes + things like backing up files (and restoring them if necessary), + installing new programs, creating accounts for users (and deleting + them when no longer needed), making certain that the filesystem + is not corrupted, and so on. If a computer were, say, a house, + system administration would be called maintenance, and would + include cleaning, fixing broken windows, and other such things. + + + The structure of this manual is such that many of the + chapters should be usable independently, so if you need information + about backups, for example, you can read just that chapter. However, + this manual is first and foremost a tutorial and can be read + sequentially or as a whole. + + This manual is not intended to be used completely + independently. Plenty of the rest of the Linux documentation is also + important for system administrators. After all, a system + administrator is just a user with special privileges and duties. + Very useful resources are the manual pages, which should always be + consulted when you are not familiar with a command. If you do not + know which command you need, then the apropos + command can be used. Consult its manual page for more details. + + While this manual is targeted at Linux, a general principle + has been that it should be useful with other UNIX based operating + systems as well. Unfortunately, since there is so much variance + between different versions of UNIX in general, and in system + administration in particular, there is little hope to cover + all variants. Even covering all possibilities for Linux is + difficult, due to the nature of its development. + + There is no one official Linux distribution, so different + people have different setups and many people have a setup they + have built up themselves. This book is not targeted at any + one distribution. Distributions can and do vary considerably. + When possible, differences have been noted and alternatives + given. + + In trying to describe how things work, rather than just + listing ``five easy steps'' for each task, there is much information + here that is not necessary for everyone, but those parts are marked + as such and can be skipped if you use a preconfigured system. + Reading everything will, naturally, increase your understanding of + the system and should make using and administering it more + productive. + + Understanding is the key to success with + Linux. This book could just provide recipes, but what + would you do when confronted by a problem this book had + no recipe for? If the book can provide understanding + then recipes are not required, they will be self evident + + + + Like all other Linux related development, the work + to write this manual was done on a volunteer basis: I did it because + I thought it might be fun and because I felt it should be done. + However, like all volunteer work, there is a limit to how much time, + knowledge and experience people have. This means that the manual is + not necessarily as good as it would be if a wizard had been paid + handsomely to write it + and had spent millennia to perfect it. Be warned. + + One particular point where corners have been cut is that + many things that are already well documented in other freely + available manuals and so are mostly not covered here. This applies + especially to program specific documentation, such as all the + details of using mkfs. Only the purpose of the + program and as much of its usage as is necessary for the purposes of + this manual is described. For further information, consult these + other manuals. Usually, all of the referred to documentation is + part of the full Linux + documentation set. + + + +About This Book + +Acknowledgements +Joanna's acknowledgements + + Lars has tried to make this manual as good as possible + and I would like, as a current maintainer, to keep up the good + work. I would really like to hear from you if you have any + ideas on how to make it better. Bad language, factual errors, + ideas for new areas to cover, rewritten sections, information + about how various UNIX versions do things, I am interested in + all of it. My contact information is available via the World + Wide Web at + http://www.iki.fi/viu/. + + + Many people have helped me with this book, directly or + indirectly. I would like to especially thank Matt Welsh for + inspiration and LDP leadership, Andy Oram for getting me to work + again with much-valued feedback, Olaf Kirch for showing me that it + can be done, and Adam Richter at Yggdrasil and others for showing + me that other people can find it interesting as well. + + Stephen Tweedie, H. Peter Anvin, Remy Card, Theodore + Ts'o, and Stephen Tweedie have let me borrow their work (and + thus make the book look thicker and much more impressive): + a comparison between the xia and ext2 filesystems, the device + list and a description of the ext2 filesystem. These aren't + part of the book any more. I am most grateful for this, and + very apologetic for the earlier versions that sometimes lacked + proper attribution. + + In addition, I would like to thank Mark Komarinski for + sending his material in 1993 and the many system administration + columns in Linux Journal. They are quite informative and + inspirational. + + Many useful comments have been sent by a large number + of people. My miniature black hole of an archive doesn't let + me find all their names, but some of them are, in alphabetical + order: Paul Caprioli, Ales Cepek, Marie-France Declerfayt, + Dave Dobson, Olaf Flebbe, Helmut Geyer, Larry Greenfield and + his father, Stephen Harris, Jyrki Havia, Jim Haynes, York Lam, + Timothy Andrew Lister, Jim Lynch, Michael J. Micek, Jacob Navia, + Dan Poirier, Daniel Quinlan, Jouni K Seppänen, Philippe Steindl, + G.B. Stotte. My apologies to anyone I have forgotten. + + +Stephen's acknowledgements + As the newest maintainer I would like to thank Lars and + Joanna for their hard work on the guide. + + In a guide like this one there are likely to be at least + some minor inaccuracies. And there are almost certainly going to + be sections that become out of date from time to time. If you + notice any of this then please let me know by sending me an email + to: bagpuss@debian.org. I will take virtually + any form of input (diffs, just plain text, html, whatever), I am + in no way above allowing others to help me maintain such a large + text as this :) + + Many thanks to Helen Topping Shaw for getting the red pen out + and making the text far better than it would otherwise have been. + Also thanks are due just for being wonderful. + + The current web home of the guide is + + http://people.debian.org/~bagpuss + + + + +Typographical Conventions + + Throughout this book, I have tried to use uniform + typographical conventions. Hopefully they aid readability. If + you can suggest any improvements please contact me. + + Filenames are expressed as: + /usr/share/doc/foo. + + Command names are expressed as: fsck + + Email addresses are expressed as: + stephen@clothcat.demon.co.uk + + URLs are expressed as: http://www.linuxdoc.org + + + I will add to this section as things come up whilst + editing. If you notice anything that should be added then + please let me know. + + + + +Overview of a Linux System + +
God saw everything that he + had made, and saw that it was very good. -- Bible + King James Version. Genesis 1:31
+ + This chapter gives an overview of a Linux system. First, + the major services provided by the operating system are described. + Then, the programs that implement these services are described + with a considerable lack of detail. The purpose of this chapter + is to give an understanding of the system as a whole, so that + each part is described in detail elsewhere. + + +Various parts of an operating system + + A UNIX operating system consists + of a kernel and some + system programs. There are also some + application programs for doing work. + The kernel is the heart of the operating system. + + In fact, it is often mistakenly considered + to be the operating system itself, but it is not. + An operating system provides many more services than a + plain kernel. + + It keeps track of files on the disk, starts programs and runs them + concurrently, assigns memory and other resources to various + processes, receives packets from and sends packets to the network, + and so on. The kernel does very little by itself, but it provides + tools with which all services can be built. It also prevents anyone + from accessing the hardware directly, forcing everyone to use the + tools it provides. + I always think of this as a form of encapsulation + which may help those of you with an object oriented programming + background to visualise it better. + This way the kernel provides some protection for users from each + other. The tools provided by the kernel are used via + system calls. See manual page section 2 for more + information on these. + + The system programs use the tools provided by the kernel to + implement the various services required from an operating system. + System programs, and all other programs, run `on top of the + kernel', in what is called the user mode. + The difference between system and application programs is + one of intent: applications are intended for getting useful + things done (or for playing, if it happens to be a game), + whereas system programs are needed to get the system working. + A word processor is an application; mount + is a system program. The difference is often somewhat blurry, + however, and is important only to compulsive categorisers. + + An operating system can also contain compilers and their + corresponding libraries (GCC and the C library in particular under + Linux), although not all programming languages need be part of + the operating system. Documentation, and sometimes even games, + can also be part of it. Traditionally, the operating system has + been defined by the contents of the installation tape or disks; + with Linux it is not as clear since it is spread all over the + FTP sites of the world. + + + + +Important parts of the kernel + + The Linux kernel consists of several important parts: process + management, memory management, hardware device drivers, filesystem + drivers, network management, and various other bits and pieces. + + shows some of them. + +
+ Some of the more important parts of the Linux kernel + +
+ + Probably the most important parts of the kernel (nothing else + works without them) are memory management and + process management. Memory management takes care of assigning + memory areas and swap space areas to processes, parts of the + kernel, and for the buffer cache. Process management creates + processes, and implements multitasking by switching the + active process on the processor. + + At the lowest level, the kernel contains a hardware device + driver for each kind of hardware it supports. Since the world is + full of different kinds of hardware, the number of hardware device + drivers is large. There are often many otherwise similar pieces + of hardware that differ in how they are controlled by software. + The similarities make it possible to have general classes of + drivers that support similar operations; each member of the class + has the same interface to the rest of the kernel but differs in + what it needs to do to implement them. For example, all disk + drivers look alike to the rest of the kernel, i.e., they all + have operations like `initialise the drive', `read sector N', + and `write sector N'. + + Some software services provided by the kernel itself have + similar properties, and can therefore be abstracted into classes. + For example, the various network protocols have been abstracted + into one programming interface, the BSD socket library. Another + example is the virtual filesystem (VFS) + layer that abstracts the filesystem operations away from their + implementation. Each filesystem type provides an implementation + of each filesystem operation. When some entity tries to use + a filesystem, the request goes via the VFS, which routes the + request to the proper filesystem driver. + + + + +Major services in a UNIX system + + This section describes some of the more important UNIX + services, but without much detail. They are described more + thoroughly in later chapters. + + +<command>init</command> + + The single most important service in a UNIX system is + provided by init. init + is started as the first process of every UNIX system, as the last + thing the kernel does when it boots. When init + starts, it continues the boot process by doing various startup + chores (checking and mounting filesystems, starting daemons, + etc). + + The exact list of things that init + does depends on which flavour it is; there are several to choose + from. init usually provides the concept of + single user mode, in which no one can + log in and root uses a shell at the console; the usual mode is + called multiuser mode. Some flavours + generalise this as run levels; single + and multiuser modes are considered to be two run levels, and + there can be additional ones as well, for example, to run X on + the console. + + Linux allows for up to 10 + runlevels, 0-9, but usually only some of + these are defined by default. Runlevel 0 is defined as ``system + halt''. Runlevel 1 is defined as ``single user mode''. + Runlevel 6 is defined as ``system reboot''. Other runlevels are + dependent on how your particular distribution has defined them, + and they vary significantly between distributions. Looking at + the contents of /etc/inittab usually will + give some hint what the predefined runlevels are and what they + have been defined as. + + In normal operation, init makes sure + getty is working (to allow users to log in), + and to adopt orphan processes (processes whose parent has died; in + UNIX all processes must + be in a single tree, so orphans must be adopted). + + When the system is shut down, it is init + that is in charge of killing all other processes, unmounting all + filesystems and stopping the processor, along with anything else + it has been configured to do. + + + + +Logins from terminals + + Logins from terminals (via serial lines) and the console + (when not running X) are provided by the getty + program. init starts a separate instance of + getty for each terminal upon which logins are to + be allowed. getty reads the username and runs + the login program, which reads the password. If + the username and password are correct, login runs + the shell. When the shell terminates, i.e., the user logs out, or + when login terminated because the username and + password didn't match, init notices this and + starts a new instance of getty. The kernel has no + notion of logins, this is all handled by the + system programs. + + + + +Syslog + + The kernel and many system programs + produce error, warning, and other messages. It is often important + that these messages can be viewed later, even much later, so they + should be written to a file. The program doing this is + syslog. It can be configured to sort the + messages to different files according to writer or degree of + importance. For example, kernel messages are often directed to a + separate file from the others, since kernel messages are often more + important and need to be read + regularly to spot problems. + + + + +Periodic command execution: <command>cron</command> and +<command>at</command> + + Both users and system administrators often need + to run commands periodically. For example, the system administrator + might want to run a command to clean the directories with temporary + files (/tmp and /var/tmp) + from old files, to keep the disks from filling up, since not all + programs clean up after + themselves correctly. + + The cron service is set up to do this. + Each user can have a crontab file, where she + lists the commands she wishes to execute and the times they should + be executed. The cron daemon takes care of + starting the commands when specified. + + The at service is similar to + cron, but it is once only: the command is + executed at the given time, but it is not repeated. + + See the manual pages cron(1), crontab(1), crontab(5), at(1) and + atd(8) for more in depth information. + + + +Graphical user interface + + UNIX and Linux don't incorporate the user interface + into the kernel; instead, they let it be implemented by user level + programs. This applies for both text mode and graphical + environments. + + This arrangement makes the system more flexible, but has + the disadvantage that it is simple to implement a different user + interface for each program, making the system harder to + learn. + + The graphical environment primarily used with Linux + is called the X Window System (X for short). X also does + not implement a user interface; it only implements a window + system, i.e., tools with which a graphical user interface can + be implemented. Some popular window managers are: fvwm, icewm, + blackbox and windowmaker. There are also two popular desktop + managers, KDE and Gnome. + + + + +Networking + + Networking is the act of connecting two or more computers + so that they can communicate with each other. The actual methods + of connecting and communicating are slightly complicated, but + the end result is very useful. + + UNIX operating systems have many networking features. + Most basic services (filesystems, printing, backups, etc) can + be done over the network. This can make system administration + easier, since it allows centralised administration, while + still reaping in the benefits of microcomputing and distributed + computing, such as lower costs and better fault tolerance. + + However, this book merely glances at networking; see the + Linux Network Administrators' Guide + + http://www.linuxdoc.org/LDP/nag2/index.html for + more information, including a basic description of how networks + operate. + + + + +Network logins + + Network logins work a little differently than normal logins. + There is a separate physical serial line for each terminal via + which it is possible to log in. For each person logging in via + the network, there is a separate virtual network connection, + and there can be any number of these. + + Well, at least there can be many. Network + bandwidth still being a scarce resource, there is still + some practical upper limit to the number of concurrent + logins via one network connection. + + It is therefore not possible to run a separate + getty for each possible virtual connection. + There are also several different ways to log in via a network, + telnet and rlogin being + the major ones in TCP/IP networks. + These days many Linux system administrators + consider telnet and rlogin + to be insecure and prefer ssh + , the ``secure shell'', which encrypts traffic + going over the network, thereby making it far less likely + that the malicious can ``sniff'' your connection and gain + sensitive data like usernames and passwords. It is + highly recommended you use ssh rather than + telnet or rlogin. + + + + Network logins have, instead of a herd of + gettys, a single daemon per way of logging in + (telnet and rlogin have + separate daemons) that listens for all incoming login attempts. + When it notices one, it starts a new instance of itself to + handle that single attempt; the original instance continues to + listen for other attempts. The new instance works similarly + to getty. + + + + +Network file systems + One of the more useful things that can be done with + networking services is sharing files via a network + file system. The one usually used is called the + Network File System, or NFS, developed by Sun. + + With a network file system any file operations done by + a program on one machine are sent over the network to another + computer. This fools the program to think that all the files + on the other computer are actually on the computer the program + is running on. This makes information sharing extremely simple, + since it requires no modifications to programs. + + Another popular way of sharing files is Samba http://www.samba.org. This + protocol allows the sharing of files with MS Windows machines + (via Network Neighbourhood). It also allows the sharing of + printers across machines. + + + +Mail + Electronic mail is the most popularly used method for + communicating via computer. An electronic letter is stored in a + file using a special format, and special mail programs are used + to send and read the letters. + + Each user has an incoming mailbox + (a file in the special format), where all new mail is stored. + When someone sends mail, the mail program locates the receiver's + mailbox and appends the letter to the mailbox file. If the + receiver's mailbox is in another machine, the letter is sent to + the other machine, which delivers it to the mailbox as it best + sees fit. + + The mail system consists of many programs. The + delivery of mail to local or remote mailboxes is done by one + program (the mail transfer agent (MTA), + e.g., sendmail + or smail), while the programs users use + are many and varied (mail user agent (MUA), + e.g., pine, mutt + or elm). The mailboxes are usually stored + in /var/spool/mail. + + + + +Printing + Only one person can use a printer at one time, but it is + uneconomical not to share printers between users. The printer is + therefore managed by software that implements a print + queue: all print jobs are put into a queue and + whenever the printer is done with one job, the next one is sent + to it automatically. This relieves the users from organising + the print queue and fighting over control of the printer. + Instead, they form a new queue + at the printer, waiting for their + printouts, since no one ever seems to be able to get the + queue software to know exactly when anyone's printout is + really finished. This is a great boost to intra-office + social relations. + + + The print queue software also spools + the printouts on disk, i.e., the text is kept in a file while + the job is in the queue. This allows an application program + to spit out the print jobs quickly to the print queue software; + the application does not have to wait until the job is actually + printed to continue. This is really convenient, since it + allows one to print out one version, and not have to wait for + it to be printed before one can make a completely revised new + version. + + + + +The filesystem layout + The filesystem is divided into many parts; + usually along the lines of a root filesystem with + /bin, /lib, + /etc, /dev, and + a few others; a /usr filesystem with + programs and unchanging data; a /var + filesystem with changing data (such as log files); and a + /home filesystem for everyone's personal + files. Depending on the hardware configuration and the decisions + of the system administrator, the division can be different; + it can even be all in one filesystem. + + describes the filesystem + layout in some little detail; the Filesystem Hierarchy Standard covers it + in somewhat more detail. + + http://www.pathname.com/fhs/ + + + + + + + +Overview of the Directory Tree + +
Two days later, there was Pooh, sitting + on his branch, dangling his legs, and there, beside him, were + four pots of honey... (A.A. Milne)
+ + This chapter describes the important parts of a standard Linux + directory tree, based on the Filesystem Hierarchy Standard. It + outlines the normal way of breaking the directory tree into separate + filesystems with different purposes and gives the motivation behind + this particular split. Not all Linux distributions follow this + standard slavishly, but it is generic enough to give you an + overview. + + +Background + + This chapter is loosely based on the Filesystems + Hierarchy Standard (FHS) + + http://www.pathname.com/fhs/ + version 2.1, which attempts to + set a standard for how the directory tree in a Linux + + Or any Unix like system. For example the BSD + derivatives. + + system is organised. Such a standard has the advantage that it will + be easier to write or port software for Linux, and to administer + Linux machines, since everything should be in standardised places. + There is no authority behind the standard that forces anyone to + comply with it, but it has gained the support of many Linux + distributions. It is not a good idea to break with the FHS without + very compelling reasons. The FHS attempts to follow Unix tradition + and current trends, making Linux systems familiar to those with + experience with other Unix systems, and vice versa. + + This chapter is not as detailed as the FHS. A system + administrator should also read the full FHS for a complete + understanding. + + This chapter does not explain all files in detail. The + intention is not to describe every file, but to give an overview of + the system from a filesystem point of view. Further information on + each file is available elsewhere in this manual or in the Linux + manual pages. + + The full directory tree is intended to be breakable into + smaller parts, each capable of being on its own disk or partition, + to accommodate to disk size limits and to ease backup and other + system administration tasks. The major parts are the root + (/), /usr, + /var, and /home + filesystems (see ). Each part has a + different purpose. The directory tree has been designed so that it + works well in a network of Linux machines which may share some parts + of the filesystems over a read-only device (e.g., a CD-ROM), or over + the network with NFS. + +
Parts of a Unix + directory tree. Dashed lines indicate partition + limits. +
+ + The roles of the different parts of the directory tree are + described below. + + + + The root filesystem is specific for + each machine (it is generally stored on a local disk, + although it could be a ramdisk or network drive as well) and + contains the files that are necessary for booting the system + up, and to bring it up to such a state that the other + filesystems may be mounted. The contents of the root + filesystem will therefore be sufficient for the single user + state. It will also contain tools for fixing a broken + system, and for recovering lost files + from backups. + + The /usr filesystem + contains all commands, libraries, manual pages, and other + unchanging files needed during normal operation. No files in + /usr should be specific for any given + machine, nor should they be modified during normal use. This + allows the files to be shared over the network, which can be + cost-effective since it saves disk space (there can easily + be hundreds of megabytes, increasingly multiple gigabytes in + /usr). It can make administration + easier (only the master /usr needs to + be changed when updating an application, not each machine + separately) to have /usr network mounted. Even if the + filesystem is on a local disk, it could be mounted + read-only, to lessen the chance of filesystem corruption + during a crash. + + The /var + filesystem contains files that change, such as spool + directories (for mail, news, printers, etc), log files, + formatted manual pages, and temporary files. Traditionally + everything in /var has been somewhere + below /usr, but that made it impossible + to mount /usr + read-only. + + The /home + filesystem contains the users' home directories, i.e., all + the real data on the system. Separating home directories to + their own directory tree or filesystem makes backups easier; + the other parts often do not have to be backed up, or at + least not as often as they seldom change. A big + /home might have to be broken across + several filesystems, which requires adding an extra naming + level below /home, for example + /home/students and + /home/staff. + + + + Although the different parts have been called filesystems + above, there is no requirement that they actually be on separate + filesystems. They could easily be kept in a single one if the + system is a small single-user system and the user wants to keep + things simple. The directory tree might also be divided into + filesystems differently, depending on how large the disks are, and + how space is allocated for various purposes. The important part, + though, is that all the standard names work; + even if, say, /var and + /usr are actually on the same partition, the + names /usr/lib/libc.a and + /var/log/messages must work, for example by + moving files below /var into + /usr/var, and making /var + a symlink to + /usr/var. + + The Unix filesystem structure groups files according to + purpose, i.e., all commands are in one place, all data files in + another, documentation in a third, and so on. An alternative would + be to group files files according to the program they belong to, + i.e., all Emacs files would be in one directory, all TeX in another, + and so on. The problem with the latter approach is that it makes it + difficult to share files (the program directory often contains both + static and sharable and changing and non-sharable files), and + sometimes to even find the files (e.g., manual pages in a huge + number of places, and making the manual page programs find all of + them is a maintenance + nightmare). + + + + +The root filesystem + + The root filesystem should generally be small, since + it contains very critical files and a small, infrequently + modified filesystem has a better chance of not getting corrupted. + A corrupted root filesystem will generally mean that the system + becomes unbootable except with special measures (e.g., from a + floppy), so you don't want to risk it. + + The root directory generally doesn't contain any files, except + perhaps the standard boot image for the system, usually called + /vmlinuz. All other files are in + subdirectories in the root filesystems: + + + + + /bin + + Commands needed during bootup + that might be used by normal users (probably after + bootup). + + + /sbin + + Like /bin, but the + commands are not intended for normal users, although they + may use them if necessary and allowed. + /sbin is not usually in the default + path of normal users, but will be in root's default + path. + + + /etc + + Configuration files specific to the + machine. + + + /root + + The home directory for user root. This is + usually not accessible to other users on the + system + + + /lib + + Shared libraries needed by the programs on + the root filesystem. + + + /lib/modules + + Loadable kernel modules, especially those + that are needed to boot the system when recovering from + disasters (e.g., network and filesystem + drivers). + + + /dev + + Device files. Some of the more commonly + used device files are examined in + + + + /tmp + + Temporary files. Programs running after + bootup should use /var/tmp, not + /tmp, since the former is probably on a + disk with more space. Often /tmp will be a symbolic link to + /var/tmp. + + + /boot + Files used by the bootstrap loader, + e.g., LILO. Kernel images are often kept here instead + of in the root directory. If there are many kernel + images, the directory can easily grow rather big, and it + might be better to keep it in a separate filesystem. + Another reason would be to make sure the kernel + images are within the first 1024 cylinders of an IDE + disk. + + This 1024 cylinder limit is no + longer true in most cases. With modern BIOSes and + later versions of LILO (the LInux LOader) the 1024 + cylinder limit can be passed with logical block + addressing (LBA). See the lilo + manual page for more details. + + + + + /mnt + Mount point for temporary mounts by + the system administrator. Programs aren't supposed to mount + on /mnt automatically. + /mnt might be divided into + subdirectories (e.g., /mnt/dosa might + be the floppy drive using an MS-DOS filesystem, and + /mnt/exta might be the same + with an ext2 filesystem). + + /proc, + /usr, /var, + /home + + Mount points + for the other filesystems. + + Although /proc does not + reside on any disk in reality. See the section about + /proc later in the + chapter. + + + + + + + + + +The <filename>/etc</filename> directory + + The /etc directory contains a lot + of files. Some of them are described below. For others, you + should determine which program they belong to and read the manual + page for that program. Many networking configuration files are + in /etc as well, and are described in the + Networking Administrators' Guide. + + + + + /etc/rc or +/etc/rc.d or +/etc/rc?.d + Scripts or directories of scripts + to run at startup or when changing the run level. + See for further + information. + + + /etc/passwd + + The user database, with fields giving the + username, real name, home directory, encrypted password, and + other information about each user. The format is documented + in the passwd manual page. The encrypted + passwords are much more commonly found in the + /etc/shadow these days. This means + that almost everything about the user + except the password is stored in the + passwd file. History and convention + make a name change undesirable. + + + + /etc/fdprm + Floppy disk parameter table. + Describes what different floppy disk formats look + like. Used by setfdprm. See the + setfdprm manual page for more + information. + + + /etc/fstab + + Lists the filesystems mounted automatically + at startup by the mount -a command (in + /etc/rc or equivalent startup file). + Under Linux, also contains information about swap areas used + automatically by swapon -a. See and the mount + manual page for more information. Also + fstab usually has its own manual page in + section 5. + + + /etc/group + + Similar to /etc/passwd, + but describes groups instead of users. See the + group manual page in section 5 for more + information. + + + /etc/inittab + Configuration file for + init. + + + /etc/issue + Output by getty before + the login prompt. Usually contains a short description or + welcoming message to the system. The contents are up to + the system administrator. + + + /etc/magic + The configuration file + for file. Contains the + descriptions of various file formats based on + which file guesses the type of + the file. See the magic and + file manual pages for more information. + + + + /etc/motd + + The message of the day, automatically + output after a successful login. Contents are up to the + system administrator. Often used for getting information + to every user, such as warnings about planned downtimes. + + + + /etc/mtab + + List of currently mounted filesystems. + Initially set up by the bootup scripts, and updated + automatically by the mount + command. Used when a list of mounted filesystems is + needed, e.g., by the df command. + + + + /etc/shadow + + Shadow password file on systems with shadow + password software installed. Shadow passwords move the + encrypted password from /etc/passwd + into /etc/shadow; the latter is not + readable by anyone except root. This makes it harder to + crack passwords. If your distribution gives you a choice + (many do) of whether or not to use shadow passwords then you + are highly recommended to do + so. + + + /etc/login.defs + + Configuration file for the + login command. The + login.defs file usually has a manual + page in section 5. + + + /etc/printcap + + Like /etc/termcap, but + intended for printers. However it uses different syntax. + The printcap has a manual page in + section 5. + + + /etc/profile, +/etc/csh.login, +/etc/csh.cshrc + Files executed at login or startup time + by the Bourne or C shells. These allow the system + administrator to set global defaults for all users. + See the manual pages for the respective shells. + + + + /etc/securetty + + Identifies secure terminals, i.e., the + terminals from which root is allowed to log in. Typically + only the virtual consoles are listed, so that it becomes + impossible (or at least harder) to gain superuser privileges + by breaking into a system over a modem or a network. Do not + allow root logins over a network. Prefer to log in as an + unprivileged user and use su or + sudo to gain root + privileges. + + + /etc/shells + Lists trusted shells. The + chsh command allows users to change + their login shell only to shells listed in this file. + ftpd, the server process that provides + FTP services for a machine, will check that the user's + shell is listed in /etc/shells + and will not let people log in unless the shell is + listed there. + + + /etc/termcap + The terminal capability database. + Describes by what ``escape sequences'' various terminals + can be controlled. Programs are written so that instead + of directly outputting an escape sequence that only + works on a particular brand of terminal, they look up + the correct sequence to do whatever it is they want to + do in /etc/termcap. As a result + most programs work with most kinds of terminals. + See the termcap, curs_termcap, + and terminfo manual pages for + more information. + + + + + + + +The <filename>/dev</filename> directory + + The /dev directory contains + the special device files for all the devices. The device files are + named using special conventions; these are described in . The device files are created during + installation, and later with the /dev/MAKEDEV + script. The /dev/MAKEDEV.local is a script + written by the system administrator that creates local-only device + files or links (i.e. those that are not part of the standard + MAKEDEV, such as device files for some + non-standard device driver). + + + + +The <filename>/usr</filename> filesystem + + The /usr filesystem is often + large, since all programs are installed there. All files + in /usr usually come from a Linux + distribution; locally installed programs and other stuff goes + below /usr/local. This makes it possible + to update the system from a new version of the distribution, + or even a completely new distribution, without having to + install all programs again. Some of the subdirectories of + /usr are listed below (some of the less + important directories have been dropped; see the FSSTND for + more information). + + + + + /usr/X11R6 + + The X Window System, all files. To simplify + the development and installation of X, the X files have not + been integrated into the rest of the system. There is a + directory tree below /usr/X11R6 similar + to that below /usr itself. + + + + /usr/bin + + Almost all user commands. Some commands are + in /bin or in + /usr/local/bin. + + + + /usr/sbin + + System administration commands that are not + needed on the root filesystem, e.g., most server programs. + + + + /usr/share/man, +/usr/share/info, +/usr/share/doc + + Manual pages, GNU Info documents, and + miscellaneous other documentation files, respectively. + + + + /usr/include + Header files for the C + programming language. This should actually be below + /usr/lib for consistency, but the + tradition is overwhelmingly in support for this name. + + + + /usr/lib + Unchanging data files for programs and + subsystems, including some site-wide configuration + files. The name lib comes from library; + originally libraries of programming subroutines + were stored in /usr/lib. + + + + /usr/local + + The place for locally installed software and + other files. Distributions may not install anything in + here. It is reserved solely for the use of the local + administrator. This way he can be absolutely certain that + no updates or upgrades to his distribution will overwrite + any extra software he has installed + locally. + + + + + + +The <filename>/var</filename> filesystem + + The /var contains data that is changed + when the system is running normally. It is specific for each + system, i.e., not shared over the network with other computers. + + + + + /var/cache/man + + A cache for man pages that are formatted on + demand. The source for manual pages is usually stored in + /usr/share/man/man?/ (where ? is the + manual section. See the manual page for + man in section 7); some manual pages + might come with a pre-formatted version, which might be + stored in /usr/share/man/cat*. Other + manual pages need to be formatted when they are first + viewed; the formatted version is then stored in + /var/cache/man so that the next person + to view the same page won't have to wait for it to be + formatted. + + + /var/games + + Any variable data belonging to games in + /usr should be placed here. This is in + case /usr is mounted read only. + + + + + /var/lib + + Files that change while the system is + running normally. + + + /var/local + Variable data for programs that are + installed in /usr/local (i.e., + programs that have been installed by the system + administrator). Note that even locally installed + programs should use the other /var + directories if they are appropriate, e.g., + /var/lock. + + + /var/lock + Lock files. Many programs + follow a convention to create a lock file in + /var/lock to indicate that they + are using a particular device or file. Other programs + will notice the lock file and won't attempt to use the + device or file. + + + /var/log + + Log files from various programs, especially + login + (/var/log/wtmp, which logs all logins + and logouts into the system) and syslog + (/var/log/messages, where all kernel + and system program message are usually stored). Files in + /var/log can often grow indefinitely, + and may require cleaning at regular + intervals. + + + /var/mail + + This is the FHS approved location for user + mailbox files. Depending on how far your distribution has + gone towards FHS compliance, these files may still be held + in /var/spool/mail. + + + + /var/run + Files that contain information about the + system that is valid until the system is next booted. + For example, /var/run/utmp + contains information about people currently logged + in. + + + /var/spool + + Directories for news, printer queues, and + other queued work. Each different spool has its own + subdirectory below /var/spool, e.g., + the news spool is in /var/spool/news. + Note that some installations which are not fully compliant + with the latest version of the FHS may have user mailboxes + under /var/spool/mail. + + + + /var/tmp + Temporary files that are large + or that need to exist for a longer time than + what is allowed for /tmp. + (Although the system administrator might not allow + very old files in /var/tmp + either.) + + + + + + +The <filename>/proc</filename> filesystem + + The /proc filesystem contains a + illusionary filesystem. It does not exist on a disk. Instead, the + kernel creates it in memory. It is used to provide information + about the system (originally about processes, hence the name). Some + of the more important files and directories are explained below. + The /proc filesystem is described in more + detail in the proc manual page. + + + + + /proc/1 + A directory with information about + process number 1. Each process has a directory below + /proc with the name being its process + identification number. + + + /proc/cpuinfo + Information about the processor, + such as its type, make, model, and performance. + + + + /proc/devices + List of device drivers configured into the + currently running kernel. + + + /proc/dma + Shows which DMA channels are being used + at the moment. + + + /proc/filesystems + Filesystems configured into the kernel. + + + + /proc/interrupts + Shows which interrupts are + in use, and how many of each there have been. + + + + /proc/ioports + Which I/O ports are in use at the moment. + + + + /proc/kcore + An image of the physical memory of + the system. This is exactly the same size as your + physical memory, but does not really take up that much + memory; it is generated on the fly as programs access it. + (Remember: unless you copy it elsewhere, nothing under + /proc takes up any disk space + at all.) + + + /proc/kmsg + Messages output by the kernel. + These are also routed to syslog. + + + + /proc/ksyms + Symbol table for the kernel. + + + + /proc/loadavg + The `load average' of the system; three + meaningless indicators of how much work the system has + to do at the moment. + + + /proc/meminfo + Information about memory usage, both + physical and swap. + + + /proc/modules + Which kernel modules are loaded at + the moment. + + + /proc/net + Status information about network + protocols. + + + /proc/self + A symbolic link to the process + directory of the program that is looking at + /proc. When two processes look at + /proc, they get different links. + This is mainly a convenience to make it easier + for programs to get at their process directory. + + + + /proc/stat + Various statistics about the system, such + as the number of page faults since the system was booted. + + + + /proc/uptime + The time the system has been up. + + + + /proc/version + The kernel version. + + + + + Note that while the above files tend to be easily readable + text files, they can sometimes be formatted in a way that is not + easily digestible. There are many commands that do little more than + read the above files and format them for easier understanding. For + example, the free program reads + /proc/meminfo and converts the amounts given in + bytes to kilobytes (and adds a little more information, as + well). + + + + + + +Device Files + +This chapter gives an overview of what a device file is, and how to +create one. It also lists some of the more common device files. The +canonical list of device files is +/usr/src/linux/Documentation/devices.txt if you have +the Linux kernel source code installed on your system. The devices listed +here are correct as of kernel version 2.2.17. + + +The <command>MAKEDEV</command> Script + + Most device files will already be created and will be there + ready to use after you install your Linux system. If by some chance + you need to create one which is not provided then you should first + try to use the MAKEDEV script. This script is + usually located in /dev/MAKEDEV but might also + have a copy (or a symbolic link) in + /sbin/MAKEDEV. If it turns out not to be in + your path then you will need to specify the path to it + explicitly. + + In general the command is used as: + + + # /dev/MAKEDEV -v ttyS0 + create ttyS0 c 4 64 root:dialout 0660 + + + This will create the device file /dev/ttyS0 + with major node 4 and minor node 64 as a character device with + access permissions 0660 with owner root and group dialout. + + ttyS0 is a serial port. The major and + minor node numbers are numbers understood by the kernel. The kernel + refers to hardware devices as numbers, this would be very difficult + for us to remember, so we use filenames. Access permissions of 0660 + means read and write permission for the owner (root in this case) + and read and write permission for members of the group (dialout in + this case) with no access for anyone else. + + + +The <command>mknod</command> command + + MAKEDEV is the preferred way of creating + device files which are not present. However sometimes the + MAKEDEV script will not know about the device + file you wish to create. This is where the mknod + command comes in. In order to use mknod you need + to know the major and minor node numbers for the device you wish to + create. The devices.txt file in the kernel + source documentation is the canonical source of this + information. + + To take an example, let us suppose that our version of the + MAKEDEV script does not know how to create the + /dev/ttyS0 device file. We need to use + mknod to create it. We know from looking at the + devices.txt file that it should be a character + device with major number 4 and minor number 64. So we now know all + we need to create the file. + + + # mknod /dev/ttyS0 c 4 64 + # chown root.dialout /dev/ttyS0 + # chmod 0644 /dev/ttyS0 + # ls -l /dev/ttyS0 +crw-rw---- 1 root dialout 4, 64 Oct 23 18:23 /dev/ttyS0 + + + As you can see, many more steps are required to create the file. In + this example you can see the process required however. It is + unlikely in the extreme that the ttyS0 file would not be provided by + the MAKEDEV script, but it suffices to illustrate + the point. + + + +Device List + + This list which follows is by no means exhaustive or as + detailed as it could be. Many of these device files will need + support compiled into your kernel for the hardware. Read the kernel + documentation to find details of any particular device. + + If you think there are other devices which should be included here but + aren't then let me know. I will try to include them in the next revision. + + + + /dev/dsp + + Digital Signal Processor. Basically this forms + the interface between software which produces sound and your + soundcard. It is a character device on major node 14 and minor + 3. + + + /dev/fd0 + + The first floppy drive. If you are lucky enough + to have several drives then they will be numbered sequentially. + It is a character device on major node 2 and minor + 0. + + + /dev/fb0 + + The first framebuffer device. A framebuffer is + an abstraction layer between software and graphics hardware. + This means that applications do not need to know about what kind + of hardware you have but merely how to communicate with the + framebuffer driver's API (Application Programming Interface) + which is well defined and standardised. The framebuffer is a + character device and is on major node 29 and minor + 0. + + + /dev/hda + + /dev/hda is the master IDE + drive on the primary IDE controller. + /dev/hdb is the slave drive on the primary + controller. /dev/hdc and + /dev/hdd are the master and slave devices + on the secondary controller respectively. Each disk is divided + into partitions. Partitions 1-4 are primary partitions and + partitions 5 and above are logical partitions inside extended + partitions. Therefore the device file which references each + partition is made up of several parts. For example + /dev/hdc9 references partition 9 (a logical + partition inside an extended partition type) on the master IDE + drive on the secondary IDE controller. The major and minor node + numbers are somewhat complex. For the first IDE controller all + partitions are block devices on major node 3. The master drive + hda is at minor 0 and the slave drive + hdb is at minor 64. For each partition + inside the drive add the partition number to the minor node + number for the drive. For example + /dev/hdb5 is major 3, minor 69 (64 + 5 = + 69). Drives on the secondary interface are handled the same way, + but with major node 22. + + + /dev/ht0 + + The first IDE tape drive. Subsequent drives are + numbered ht1 etc. They are character + devices on major node 37 and start at minor node 0 for + ht0 1 for ht1 + etc. + + + /dev/js0 + + The first analogue joystick. Subsequent joysticks + are numbered js1, js2 + etc. Digital joysticks are called djs0, + djs1 and so on. They are character devices + on major node 15. The analogue joysticks start at minor node 0 + and go up to 127 (more than enough for even the most fanatic + gamer). Digital joysticks start at minor node + 128. + + + /dev/lp0 + + The first parallel printer device. Subsequent + printers are numbered lp1, + lp2 etc. They are character devices on + major mode 6 and minor nodes starting at 0 and numbered + sequentially. + + + /dev/loop0 + + The first loopback device. Loopback devices are + used for mounting filesystems which are not located on other + block devices such as disks. For example if you wish to mount + an iso9660 CD ROM image without burning it to CD then you need + to use a loopback device to do so. This is usually transparent + to the user and is handled by the mount + command. Refer to the manual pages for mount + and losetup. The loopback devices are block + devices on major node 7 and with minor nodes starting at 0 and + numbered sequentially. + + + /dev/md0 + + First metadisk group. Metadisks are related to + RAID (Redundant Array of Independent Disks) devices. Please + refer to the various RAID HOWTOs at the LDP for more details. + Metadisk devices are block devices on major node 9 with minor + nodes starting at 0 and numbered + sequentially. + + + /dev/mixer + + This is part of the OSS (Open Sound System) + driver. Refer to the OSS documentation at http://www.opensound.com + for more details. It is a character device on major node 14, + minor node 0. + + + /dev/null + + The bit bucket. A black hole where you can send + data for it never to be seen again. Anything sent to + /dev/null will disappear. This can be + useful if, for example, you wish to run a command but not have + any feedback appear on the terminal. It is a character device + on major node 1 and minor node 3. + + + /dev/psaux + + The PS/2 mouse port. This is a character device + on major node 10, minor node 1. + + + /dev/pda + + Parallel port IDE disks. These are named + similarly to disks on the internal IDE controllers + (/dev/hd*). They are block devices on major + node 45. Minor nodes need slightly more explanation here. The + first device is /dev/pda and it is on minor + node 0. Partitions on this device are found by adding the + partition number to the minor number for the device. Each + device is limited to 15 partitions each rather than 63 (the + limit for internal IDE disks). /dev/pdb + minor nodes start at 16, /dev/pdc at 32 and + /dev/pdd at 48. So for example the minor + node number for /dev/pdc6 would be 38 (32 + + 6 = 38). This scheme limits you to 4 parallel disks of 15 + partitions each. + + + /dev/pcd0 + + Parallel port CD ROM drives. These are numbered + from 0 onwards. All are block devices on major node 46. + /dev/pcd0 is on minor node 0 with + subsequent drives being on minor nodes 1, 2, 3 + etc. + + + + /dev/pt0 + + Parallel port tape devices. Tapes do not have + partitions so these are just numbered sequentially. They are + character devices on major node 96. The minor node numbers + start from 0 for /dev/pt0, 1 for + /dev/pt1, and so on. + + + /dev/parport0 + + The raw parallel ports. Most devices which are + attached to parallel ports have their own drivers. This is a + device to access the port directly. It is a character device on + major node 99 with minor node 0. Subsequent devices after the + first are numbered sequentially incrementing the minor + node. + + + /dev/random or /dev/urandom + + These are kernel random number generators. + /dev/random is a non-deterministic + generator which means that the value of the next number cannot + be guessed from the preceding ones. It uses the entropy of the + system hardware to generate numbers. When it has no more + entropy to use then it must wait until it has collected more + before it will allow any more numbers to be read from it. + /dev/urandom works similarly. Initially it + also uses the entropy of the system hardware, but when there is + no more entropy to use it will continue to return numbers using + a pseudo random number generating formula. This is considered + to be less secure for vital purposes such as cryptographic key + pair generation. If security is your overriding concern then + use /dev/random, if speed is more important + then /dev/urandom works fine. They are + character devices on major node 1 with minor nodes 8 for + /dev/random and 9 for + /dev/urandom. + + + /dev/zero + + This is a simple way of getting many 0s. Every + time you read from this device it will return 0. This can be + useful sometimes, for example when you want a file of fixed + length but don't really care what it contains. It is a + character device on major node 1 and minor node + 5. + + + + + + + + + + +Using Disks and Other Storage Media + +
On a clear disk you can seek forever. +
+ + + + When you install or upgrade your system, you need to do a + fair amount of work on your disks. You have to make filesystems on + your disks so that files can be stored on them and reserve + space for the different parts of your system. + + This chapter explains all these initial activities. Usually, + once you get your system set up, you won't have to go through the + work again, except for using floppies. You'll need to come back to + this chapter if you add a new disk or want to fine-tune your disk usage. + + The basic tasks in administering disks are: + + + + + Format your disk. This does various things to prepare it for use, + such as checking for bad sectors. (Formatting is nowadays + not necessary for most hard disks.) + + + Partition a hard disk, if you want to use it for several activities + that aren't supposed to interfere with one another. One reason for + partitioning is to store different operating systems on the same + disk. Another reason is to keep user files separate from system + files, which simplifies back-ups and helps protect the system files + from corruption. + + + + Make a filesystem (of a suitable type) on each disk or partition. + The disk means nothing to Linux until you make a filesystem; then + files can be created and accessed on it. + + + + Mount different filesystems to form a single tree structure, either + automatically, or manually as needed. (Manually mounted filesystems + usually need to be unmounted manually as well.) + + + + + contains information + about virtual memory and disk caching, of which you also need + to be aware when using disks. + + +Two kinds of devices + + UNIX, and therefore Linux, recognises two different + kinds of device: random-access block devices (such as disks), and + character devices (such as tapes and serial lines), some of which + may be serial, and some random-access. Each supported device is + represented in the filesystem as a device + file. When you read or write a device file, the data + comes from or goes to the device it represents. This way no special + programs (and no special application programming methodology, such + as catching interrupts or polling a serial port) are necessary to + access devices; for example, to send a file to the printer, one + could just say + + +$ cat filename > /dev/lp1 +$ + + + and the contents of the file are printed (the file must, of course, + be in a form that the printer understands). However, since it is + not a good idea to have several people cat their files to the + printer at the same time, one usually uses a special program to send + the files to be printed (usually lpr). This + program makes sure that only one file is being printed at a time, + and will automatically send files to the printer as soon as it + finishes with the previous file. Something similar is needed for + most devices. In fact, one seldom needs to worry + about device files at all. + + Since devices show up as files in the filesystem (in the + /dev directory), it is easy to see just what + device files exist, using ls or another suitable + command. In the output of ls -l, the first + column contains the type of the file and its permissions. For + example, inspecting a serial device might give + + +$ ls -l /dev/ttyS0 +crw-rw-r-- 1 root dialout 4, 64 Aug 19 18:56 /dev/ttyS0 +$ + + + The first character in the first column, i.e., + `c' in crw-rw-rw- above, tells + an informed user the type of the file, in this case a character + device. For ordinary files, the first character is + `-', for directories it is + `d', and for block devices + `b'; see the ls man page + for further information. + + Note that usually all device files exist even though the + device itself might be not be installed. So just because you have a + file /dev/sda, it doesn't mean that you really + do have an SCSI hard disk. Having all the device files makes the + installation programs simpler, and makes it easier to add new + hardware (there is no need to find out the correct parameters + for and create the device files for the new device). + + + + +Hard disks + + This subsection introduces terminology related to hard + disks. If you already know the terms and concepts, you can skip + this subsection. + + See for a schematic picture + of the important parts in a hard disk. A hard disk consists of one + or more circular platters, + + The platters are made of a hard + substance, e.g., aluminium, which gives the hard disk + its name. + + of which either or both surfaces are coated + with a magnetic substance used for recording the data. For each + surface, there is a read-write head that + examines or alters the recorded data. The platters rotate on a + common axis; typical rotation speed is 5400 or 7200 rotations per + minute, although high-performance hard disks have higher speeds and + older disks may have lower speeds. The heads move along the radius + of the platters; this movement combined with the rotation of the + platters allows the head to access all parts of the surfaces. + + The processor (CPU) and the actual disk communicate through + a disk controller. This relieves the rest of + the computer from knowing how to use the drive, since the + controllers for different types of disks can be made to use the same + interface towards the rest of the computer. Therefore, the computer + can say just ``hey disk, give me what I want'', instead of a long + and complex series of electric signals to move the head to the + proper location and waiting for the correct position to come under + the head and doing all the other unpleasant stuff necessary. (In + reality, the interface to the controller is still complex, but much + less so than it would otherwise be.) The controller may also do + other things, such as caching, or automatic bad sector + replacement. + + The above is usually all one needs to understand about the + hardware. There are also other things, such as the motor that + rotates the platters and moves the heads, and the electronics that + control the operation of the mechanical parts, but they are mostly + not relevant for understanding the working principles of a hard + disk. + + The surfaces are usually divided into concentric rings, + called tracks, and these in turn are divided + into sectors. This division is used to + specify locations on the hard disk and to allocate disk space to + files. To find a given place on the hard disk, one might say + ``surface 3, track 5, sector 7''. Usually the number of sectors is + the same for all tracks, but some hard disks put more sectors in + outer tracks (all sectors are of the same physical size, so more of + them fit in the longer outer tracks). Typically, a sector will hold + 512 bytes of data. The disk itself + can't handle smaller amounts of data than one sector. + +
+ A schematic picture of a hard disk. + +
+ + Each surface is divided into tracks (and sectors) in + the same way. This means that when the head for one surface is on a + track, the heads for the other surfaces are also on the + corresponding tracks. All the corresponding tracks taken together + are called a cylinder. It takes time to + move the heads from one track (cylinder) to another, so by placing + the data that is often accessed together (say, a file) so that it is + within one cylinder, it is not necessary to move the heads to read + all of it. This improves performance. It is not always possible to + place files like this; files that are stored in several places on + the disk are called + fragmented. + + The number of surfaces (or heads, which is the same thing), + cylinders, and sectors vary a lot; the specification of the number + of each is called the geometry of a hard + disk. The geometry is usually stored in a special, battery-powered + memory location called the CMOS RAM, from + where the operating system can fetch it during bootup or driver + initialisation. + + Unfortunately, the BIOS + + The BIOS is some built-in software stored on + ROM chips. It takes care, among other things, of the + initial stages of booting. + + has a design limitation, which makes it impossible to specify a + track number that is larger than 1024 in the CMOS RAM, which is too + little for a large hard disk. To overcome this, the hard disk + controller lies about the geometry, and translates the + addresses given by the computer into something that fits + reality. For example, a hard disk might have 8 heads, 2048 tracks, + and 35 sectors per track. + + The numbers are completely + imaginary. + + Its controller could lie to the computer and claim that it has 16 + heads, 1024 tracks, and 35 sectors per track, thus not exceeding the + limit on tracks, and translates the address that the computer gives + it by halving the head number, and doubling the track number. The + mathematics can be more complicated in reality, because the numbers + are not as nice as here (but again, the details are not relevant for + understanding the principle). This translation distorts the + operating system's view of how the disk is organised, thus making it + impractical to use the all-data-on-one-cylinder trick to boost + performance. + + The translation is only a problem for IDE disks. SCSI disks + use a sequential sector number (i.e., the controller translates a + sequential sector number to a head, cylinder, and sector triplet), + and a completely different method for the CPU to talk with the + controller, so they are insulated from the problem. Note, however, + that the computer might not know the real geometry of an SCSI disk + either. + + Since Linux often will not know the real geometry of a disk, + its filesystems don't even try to keep files within a single + cylinder. Instead, it tries to assign sequentially numbered sectors + to files, which almost always gives similar performance. The issue + is further complicated by on-controller caches, and automatic + prefetches done by the controller. + + Each hard disk is represented by a separate device + file. There can (usually) be only two or four IDE hard disks. These + are known as /dev/hda, + /dev/hdb, /dev/hdc, and + /dev/hdd, respectively. SCSI hard disks are + known as /dev/sda, + /dev/sdb, and so on. Similar naming + conventions exist for other hard disk types; see for more information. Note that the device + files for the hard disks give access to the entire disk, with no + regard to partitions (which will be discussed below), and it's easy + to mess up the partitions or the data in them if you aren't careful. + The disks' device files are usually used only to get access to the + master boot record (which will also be discussed below). + + + + +Floppies + + A floppy disk consists of a flexible membrane covered on one + or both sides with similar magnetic substance as a hard disk. The + floppy disk itself doesn't have a read-write head, that is included + in the drive. A floppy corresponds to one platter in a hard disk, + but is removable and one drive can be used to access different + floppies, and the same floppy can be read by many drives, whereas + the hard disk is one indivisible unit. + + Like a hard disk, a floppy is divided into tracks and sectors + (and the two corresponding tracks on either side of a floppy + form a cylinder), but there are many fewer of them than on a + hard disk. + + A floppy drive can usually use several different types of disks; + for example, a 3.5 inch drive can use both 720 kB and 1.44 MB disks. + Since the drive has to operate a bit differently and the operating + system must know how big the disk is, there are many device files + for floppy drives, one per combination of drive and disk type. + Therefore, /dev/fd0H1440 is the first floppy + drive (fd0), which must be a 3.5 inch drive, using a 3.5 inch, high + density disk (H) of size 1440 kB (1440), i.e., a normal 3.5 inch HD + floppy. + + + The names for floppy drives are complex, however, and Linux + therefore has a special floppy device type that automatically + detects the type of the disk in the drive. It works by trying to + read the first sector of a newly inserted floppy using different + floppy types until it finds the correct one. This naturally requires + that the floppy is formatted first. The automatic devices are called + /dev/fd0, /dev/fd1, and so + on. + + The parameters the automatic device uses to access a disk can + also be set using the program setfdprm. This can + be useful if you need to use disks that do not follow any usual + floppy sizes, e.g., if they have an unusual number of sectors, or if + the autodetecting for some reason fails and the proper device file is + missing. + + Linux can handle many nonstandard floppy disk formats + in addition to all the standard ones. Some of these require using + special formatting programs. We'll skip these disk types for now, + but in the mean time you can examine the + /etc/fdprm file. It specifies the settings + that setfdprm recognises. + + The operating system must know when a disk has been changed in + a floppy drive, for example, in order to avoid using cached data + from the previous disk. Unfortunately, the signal line that is used + for this is sometimes broken, and worse, this won't always be + noticeable when using the drive from within MS-DOS. If you are + experiencing weird problems using floppies, this might be the + reason. The only way to correct it is to repair the floppy drive. + + + + +CD-ROMs + + A CD-ROM drive uses an optically read, plastic coated disk. + The information is recorded on the surface of the disk + + That is, the surface inside the disk, on + the metal disk inside the plastic coating. + + in small `holes' aligned along a spiral from the centre to the edge. + The drive directs a laser beam along the spiral to read the disk. + When the laser hits a hole, the laser is reflected in one way; when + it hits smooth surface, it is reflected in another way. This makes + it easy to code bits, and therefore information. The rest is easy, + mere mechanics. + + CD-ROM drives are slow compared to hard disks. Whereas a + typical hard disk will have an average seek time less than 15 + milliseconds, a fast CD-ROM drive can use tenths of a second for + seeks. The actual data transfer rate is fairly high at hundreds of + kilobytes per second. The slowness means that CD-ROM drives are not + as pleasant to use as hard disks (some Linux distributions provide + `live' filesystems on CD-ROMs, making it unnecessary to copy the + files to the hard disk, making installation easier and saving a lot + of hard disk space), although it is still possible. For installing + new software, CD-ROMs are very good, since maximum speed is not + essential during installation. + + There are several ways to arrange data on a CD-ROM. The most + popular one is specified by the international standard ISO 9660. + This standard specifies a very minimal filesystem, which is even + more crude than the one MS-DOS uses. On the other hand, it is so + minimal that every operating system should be able to map it to its + native system. + + For normal UNIX use, the ISO 9660 filesystem is not usable, so + an extension to the standard has been developed, called the Rock + Ridge extension. Rock Ridge allows longer filenames, symbolic + links, and a lot of other goodies, making a CD-ROM look more or less + like any contemporary UNIX filesystem. Even better, a Rock Ridge + filesystem is still a valid ISO 9660 filesystem, making it usable by + non-UNIX systems as well. Linux supports both ISO 9660 and the Rock + Ridge extensions; the extensions are recognised and used + automatically. + + The filesystem is only half the battle, however. Most CD-ROMs + contain data that requires a special program to access, and most of + these programs do not run under Linux (except, possibly, under + dosemu, the Linux MS-DOS emulator, or wine, the Windows emulator. + + Ironically perhaps, wine actually stands + for ``Wine Is Not an Emulator''. Wine, more strictly, is an + API (Application Program Interface) replacement. Please see + the wine documentation at http://www.winehq.com + for more information. + + There is also VMWare, a commercial product which emulates an + entire x86 machine in software + + See the VMWare website, http://www.vmware.com + for more information.) + + . + + A CD-ROM drive is accessed via the corresponding device file. + There are several ways to connect a CD-ROM drive to the computer: + via SCSI, via a sound card, or via EIDE. The hardware hacking + needed to do this is outside the scope of this book, but the + type of connection decides the device file. + + + + +Tapes + + A tape drive uses a tape, similar + + But completely + different, of course. + + to cassettes used for music. A tape is serial in nature, which + means that in order to get to any given part of it, you first have + to go through all the parts in between. A disk can be accessed + randomly, i.e., you can jump directly to any place on the disk. + The serial access of tapes makes them slow. + + On the other hand, tapes are relatively cheap to make, + since they do not need to be fast. They can also easily be made + quite long, and can therefore contain a large amount of data. This + makes tapes very suitable for things like archiving and backups, + which do not require large speeds, but benefit from + low costs and large storage capacities. + + + + +Formatting + + Formatting is the process of writing marks + on the magnetic media that are used to mark tracks and sectors. + Before a disk is formatted, its magnetic surface is a complete mess + of magnetic signals. When it is formatted, some order is brought + into the chaos by essentially drawing lines where the tracks go, and + where they are divided into sectors. The actual details are not + quite exactly like this, but that is irrelevant. What is important + is that a disk cannot be used unless it has been formatted. + + The terminology is a bit confusing here: in MS-DOS and MS + Windows, the word formatting is used to cover also the process of + creating a filesystem (which will be discussed below). There, the + two processes are often combined, especially for floppies. When the + distinction needs to be made, the real formatting is called + low-level formatting, while making the + filesystem is called high-level formatting. + In UNIX circles, the two are called formatting and making a + filesystem, so that's what is used in this book as well. + + For IDE and some SCSI disks the formatting is actually + done at the factory and doesn't need to be repeated; hence most + people rarely need to worry about it. In fact, formatting a hard + disk can cause it to work less well, for example because a disk + might need to be formatted in some very special way to + allow automatic bad sector replacement to work. + + Disks that need to be or can be formatted often require a + special program anyway, because the interface to the formatting + logic inside the drive is different from drive to drive. The + formatting program is often either on the controller BIOS, or is + supplied as an MS-DOS program; neither of these can easily + be used from within Linux. + + During formatting one might encounter bad spots on the + disk, called bad blocks or bad + sectors. These are sometimes handled by the drive + itself, but even then, if more of them develop, something needs to + be done to avoid using those parts of the disk. The logic to do + this is built into the filesystem; how to add the information into + the filesystem is described below. Alternatively, one might create + a small partition that covers just the bad part of the disk; this + approach might be a good idea if the bad spot is very large, since + filesystems can sometimes have trouble with very large bad areas. + + Floppies are formatted with fdformat. The + floppy device file to use is given as the parameter. For example, + the following command would format a high density, 3.5 inch floppy + in the first floppy drive: + + + $ fdformat /dev/fd0H1440 + Double-sided, 80 tracks, 18 sec/track. Total capacity + 1440 kB. + Formatting ... done + Verifying ... done + $ + + + Note that if you want to use an autodetecting device (e.g., + /dev/fd0), you must set + the parameters of the device with setfdprm first. + To achieve the same effect as above, one would have to do the + following: + + + $ setfdprm /dev/fd0 1440/1440 + $ fdformat /dev/fd0 + Double-sided, 80 tracks, 18 sec/track. Total capacity + 1440 kB. + Formatting ... done + Verifying ... done + $ + + + It is usually more convenient to choose the correct device file that + matches the type of the floppy. Note that it is unwise to format + floppies to contain more information than what they are + designed for. + + fdformat will also validate the floppy, + i.e., check it for bad blocks. It will try a bad block several + times (you can usually hear this, the drive noise changes + dramatically). If the floppy is only marginally bad (due to dirt on + the read/write head, some errors are false signals), + fdformat won't complain, but a real error will + abort the validation process. The kernel will print log messages for + each I/O error it finds; these will go to the console or, if + syslog is being used, to the file + /usr/log/messages. fdformat + itself won't tell where the error is (one usually doesn't care, + floppies are cheap enough that a bad one is automatically thrown + away). + + + $ fdformat /dev/fd0H1440 + Double-sided, 80 tracks, 18 sec/track. Total capacity + 1440 kB. + Formatting ... done + Verifying ... read: Unknown error + $ + + + The badblocks command can be used to search any + disk or partition for bad blocks (including a floppy). It does not + format the disk, so it can be used to check even existing + filesystems. The example below checks a 3.5 inch floppy with two + bad blocks. + + + $ badblocks /dev/fd0H1440 1440 + 718 + 719 + $ + + + badblocks outputs the block numbers of the bad + blocks it finds. Most filesystems can avoid such bad blocks. They + maintain a list of known bad blocks, which is initialised when the + filesystem is made, and can be modified later. The initial search + for bad blocks can be done by the mkfs command + (which initialises the filesystem), but later checks should be done + with badblocks and the new blocks should be added + with fsck. We'll describe + mkfs + and fsck later. + + Many modern disks automatically notice bad blocks, and attempt + to fix them by using a special, reserved good block instead. This is + invisible to the operating system. This feature should be + documented in the disk's manual, if you're curious if it is + happening. Even such disks can fail, if the number of bad blocks + grows too large, although chances are that by then the disk + will be so rotten as to be unusable. + + + + +Partitions + + A hard disk can be divided into several + partitions. Each partition functions as if + it were a separate hard disk. The idea is that if you have one hard + disk, and want to have, say, two operating systems on it, you can + divide the disk into two partitions. Each operating system uses its + partition as it wishes and doesn't touch the other ones. This way + the two operating systems can co-exist peacefully on the same hard + disk. Without partitions one would have to buy a hard disk for each + operating system. + + Floppies are not usually partitioned. There is no technical reason + against this, but since they're so small, partitions would be useful + only very rarely. CD-ROMs are usually also not partitioned, since + it's easier to use them as one big disk, and there is seldom a need + to have several operating systems on one. + + +The MBR, boot sectors and partition table + + The information about how a hard disk has been partitioned + is stored in its first sector (that is, the first sector of the + first track on the first disk surface). The first sector is the + master boot record (MBR) of the disk; this is + the sector that the BIOS reads in and starts when the machine is + first booted. The master boot record contains a small program that + reads the partition table, checks which partition is active (that + is, marked bootable), and reads the first sector of that partition, + the partition's boot sector (the MBR is also + a boot sector, but it has a special status and therefore a special + name). This boot sector contains another small program that reads + the first part of the operating system stored on that partition + (assuming it is bootable), and then starts it. + + The partitioning scheme is not built into the hardware, or + even into the BIOS. It is only a convention that many operating + systems follow. Not all operating systems do follow it, but they + are the exceptions. Some operating systems support partitions, but + they occupy one partition on the hard disk, and use their internal + partitioning method within that partition. The latter type exists + peacefully with other operating systems (including Linux), and does + not require any special measures, but an operating system that + doesn't support partitions cannot co-exist on the same disk with any + other operating system. + + As a safety precaution, it is a good idea to write down the + partition table on a piece of paper, so that if it ever corrupts you + don't have to lose all your files. (A bad partition table can be + fixed with fdisk). The relevant information is + given by the fdisk -l command: + + + $ fdisk -l /dev/hda + + Disk /dev/hda: 15 heads, 57 sectors, 790 cylinders + Units = cylinders of 855 * 512 bytes + + Device Boot Begin Start End Blocks Id System + /dev/hda1 1 1 24 10231+ 82 Linux swap + /dev/hda2 25 25 48 10260 83 Linux native + /dev/hda3 49 49 408 153900 83 Linux native + /dev/hda4 409 409 790 163305 5 Extended + /dev/hda5 409 409 744 143611+ 83 Linux native + /dev/hda6 745 745 790 19636+ 83 Linux native + $ + + + + + +Extended and logical partitions + + The original partitioning scheme for PC hard disks allowed + only four partitions. This quickly turned out to be too little in + real life, partly because some people want more than four operating + systems (Linux, MS-DOS, OS/2, Minix, FreeBSD, NetBSD, or Windows/NT, + to name a few), but primarily because sometimes it is a good idea to + have several partitions for one operating system. For example, swap + space is usually best put in its own partition for Linux instead of + in the main Linux partition for reasons of speed (see below). + + To overcome this design problem, extended + partitions were invented. This trick allows + partitioning a primary partition into + sub-partitions. The primary partition thus subdivided is the + extended partition; the sub-partitions are + logical partitions. They behave like primary + partitions, but are created differently. There is no speed + difference between them. + + The partition structure of a hard disk might look like that + in . The disk is divided into + three primary partitions, the second of which is divided into two + logical partitions. Part of the disk is not partitioned at all. + The disk as a whole and each primary partition has a boot sector. + +
+ A sample hard disk partitioning. + +
+ + + + +Partition types + + The partition tables (the one in the MBR, and the ones for + extended partitions) contain one byte per partition that identifies + the type of that partition. This attempts to identify the operating + system that uses the partition, or what it uses it for. The purpose + is to make it possible to avoid having two operating systems + accidentally using the same partition. However, in reality, + operating systems do not really care about the partition type byte; + e.g., Linux doesn't care at all what it is. Worse, some of them use + it incorrectly; e.g., at least some versions of DR-DOS ignore the + most significant bit of the byte, while others don't. + + There is no standardisation agency to specify what each byte + value means, but some commonly accepted ones are included in in + . A more complete list is available + in the Linux fdisk program. + + + Partition types (from the Linux <command>fdisk</command> program). + + + + + + 0 Empty + 40 Venix 80286 + 94 Amoeba BBT + + + + 1 DOS 12-bit FAT + 51 Novell? + a5 BSD/386 + + + + 2 XENIX root + 52 Microport + b7 BSDI fs + + + + 3 XENIX usr + 63 GNU HURD + b8 BSDI swap + + + + 4 DOS 16-bit FAT <32M + 64 Novell + c7 Syrinx + + + + 5 Extended + 75 PC/IX + db CP/M + + + + 6 DOS 16-bit >=32M + 80 Old MINIX + e1 DOS access + + + + 7 OS/2 HPFS + 81 Linux/MINIX + e3 DOS R/O + + + + 8 AIX + 82 Linux swap + f2 DOS secondary + + + + 9 AIX bootable + 83 Linux native + ff BBT + + + + a OS/2 Boot Manager + 93 Amoeba + + + + + +
+ + + + +Partitioning a hard disk + + There are many programs for creating and removing + partitions. Most operating systems have their own, and it can be a + good idea to use each operating system's own, just in case it does + something unusual that the others can't. Many of the programs are + called fdisk, including the Linux one, or + variations thereof. Details on using the Linux + fdisk are given on its man page. The + cfdisk command is similar to + fdisk, but has a nicer (full screen) user + interface. + + When using IDE disks, the boot partition (the partition + with the bootable kernel image files) must be completely within the + first 1024 cylinders. This is because the disk is used via the BIOS + during boot (before the system goes into protected mode), and BIOS + can't handle more than 1024 cylinders. It is sometimes possible to + use a boot partition that is only partly within the first 1024 + cylinders. This works as long as all the files that are read with + the BIOS are within the first 1024 cylinders. Since this is + difficult to arrange, it is a very bad idea to + do it; you never know when a kernel update or disk defragmentation + will result in an unbootable system. Therefore, make sure your boot + partition is completely within the first 1024 cylinders + + This may no longer be true with newer + versions of LILO that support LBA (Logical Block + Addressing). Consult the documentation for your + distribution to see if it has a version of LILO where + LBA is supported. + + . + + Some newer versions of the BIOS and IDE disks can, in fact, + handle disks with more than 1024 cylinders. If you have such a + system, you can forget about the problem; if you aren't quite + sure of it, put it within the first 1024 cylinders. + + Each partition should have an even number of sectors, + since the Linux filesystems use a 1 kilobyte block size, i.e., two + sectors. An odd number of sectors will result in the last sector + being unused. This won't result in any problems, but it is ugly, + and some versions of fdisk will warn about it. + + Changing a partition's size usually requires first backing up + everything you want to save from that partition (preferably the + whole disk, just in case), deleting the partition, creating new + partition, then restoring everything to the new partition. If the + partition is growing, you may need to adjust the sizes (and backup and + restore) of the adjoining partitions as well. + + Since changing partition sizes is painful, it is preferable to + get the partitions right the first time, or have an effective and + easy to use backup system. If you're installing from a media that + does not require much human intervention (say, from CD-ROM, as + opposed to floppies), it is often easy to play with different + configuration at first. Since you don't already have data to back + up, it is not so painful to modify partition sizes several times. + + There is a program for MS-DOS, called + fips + + The fips program is + included in most Linux distributions. The commercial + partition manager ``Partition Magic'' also has a similar + facility but with a nicer interface. Please do remember + that partitioning is dangerous. Make + sure you have a recent backup of + any important data before you try changing partition + sizes ``on the fly''. The GNU program + parted can resize other types of + partitions as well as MS-DOS, but sometimes in a limited + manner. Consult the parted documentation + before using it, better safe than sorry. + + + , which resizes an MS-DOS partition without requiring the backup and + restore, but for other filesystems it is still necessary. + + + + +Device files and partitions + + Each partition and extended partition has its own + device file. The naming convention for these files is that a + partition's number is appended after the name of the whole disk, + with the convention that 1-4 are primary partitions (regardless of + how many primary partitions there are) and number greater than 5 are + logical partitions (regardless of within which primary partition + they reside). For example, /dev/hda1 is the + first primary partition on the first IDE hard disk, and + /dev/sdb7 is the third extended partition on + the second SCSI hard disk. + + + + + + +Filesystems + + +What are filesystems? + + A filesystem is the methods and + data structures that an operating system uses to keep track of files + on a disk or partition; that is, the way the files are organised on + the disk. The word is also used to refer to a partition or disk + that is used to store the files or the type of the filesystem. + Thus, one might say ``I have two filesystems'' meaning one has two + partitions on which one stores files, or that one is using the + ``extended filesystem'', meaning the type of the filesystem. + + The difference between a disk or partition and the + filesystem it contains is important. A few programs (including, + reasonably enough, programs that create filesystems) operate + directly on the raw sectors of a disk or partition; if there is an + existing file system there it will be destroyed or seriously + corrupted. Most programs operate on a filesystem, and therefore + won't work on a partition that doesn't contain one (or that contains + one of the wrong type). + + Before a partition or disk can be used as a filesystem, it + needs to be initialised, and the bookkeeping data structures need to + be written to the disk. This process is called + making a filesystem. + + Most UNIX filesystem types have a similar general + structure, although the exact details vary quite a bit. The central + concepts are superblock, + inode, data block, + directory block, and indirection + block. The superblock contains information about the + filesystem as a whole, such as its size (the exact information here + depends on the filesystem). An inode contains all information about + a file, except its name. The name is stored in the directory, + together with the number of the inode. A directory entry consists of + a filename and the number of the inode which represents the file. + The inode contains the numbers of several data blocks, which are + used to store the data in the file. There is space only for a few + data block numbers in the inode, however, and if more are needed, + more space for pointers to the data blocks is allocated dynamically. + These dynamically allocated blocks are indirect blocks; the name + indicates that in order to find the data block, one has to find + its number in the indirect block first. + + UNIX filesystems usually allow one to create a + hole in a file (this is done with the + lseek() system call; check the manual page), + which means that the filesystem just pretends that at a particular + place in the file there is just zero bytes, but no actual disk + sectors are reserved for that place in the file (this means that the + file will use a bit less disk space). This happens especially often + for small binaries, Linux shared libraries, some databases, and a + few other special cases. (Holes are implemented by storing a + special value as the address of the data block in the indirect block + or inode. This special address means that no data block is + allocated for that part of the file, ergo, there is a hole in the + file.) + + + + +Filesystems galore + + Linux supports several types of filesystems. As of this + writing the most important ones are: + + + + minix + The oldest, presumed to be the most + reliable, but quite limited in features (some time stamps + are missing, at most 30 character filenames) and restricted + in capabilities (at most 64 MB per filesystem). + + + + xia + A modified version of the minix filesystem + that lifts the limits on the filenames and filesystem sizes, + but does not otherwise introduce new features. It is not + very popular, but is reported to work very well. + + + + ext2 + The most featureful of the native Linux + filesystems, currently also the most popular one. It is + designed to be easily upwards compatible, so that new + versions of the filesystem code do not require re-making the + existing filesystems. + + + ext + An older version of ext2 that wasn't upwards + compatible. It is hardly ever used in new installations any + more, and most people have converted to ext2. + + + + reiserfs + A more robust filesystem. Journalling is + used which makes data loss less likely. Journalling is a + mechanism whereby a record is kept of transaction which are + to be performed, or which have been performed. This allows + the filesystem to reconstruct itself fairly easily after + damage caused by, for example, improper + shutdowns./glossentry> + + + + + In addition, support for several foreign filesystem exists, + to make it easier to exchange files with other operating systems. + These foreign filesystems work just like native ones, except that + they may be lacking in some usual UNIX features, or have curious + limitations, or other oddities. + + + + + msdos + Compatibility with MS-DOS (and OS/2 and + Windows NT) FAT filesystems. + + + umsdos + Extends the msdos filesystem driver under + Linux to get long filenames, owners, permissions, links, and + device files. This allows a normal msdos filesystem to be + used as if it were a Linux one, thus removing the need for a + separate partition for Linux. + + + vfat + This is an extension of the FAT filesystem + known as FAT32. It supports larger disk sizes than FAT. + Most MS Windows disks are vfat. + + + + iso9660 + The standard CD-ROM filesystem; the popular + Rock Ridge extension to the CD-ROM standard that allows + longer file names is supported automatically. + + + + nfs + A networked filesystem that allows sharing a + filesystem between many computers to allow easy access to + the files from all of them. + + + smbfs + A networks filesystem which allows sharing + of a filesystem with an MS Windows computer. It is + compatible with the Windows file sharing protocols. + + + + hpfs + The OS/2 filesystem. + + + + sysv + SystemV/386, Coherent, and Xenix filesystems. + + + + + + The choice of filesystem to use depends on the situation. If + compatibility or other reasons make one of the non-native + filesystems necessary, then that one must be used. If one can + choose freely, then it is probably wisest to use ext2, since it has + all the features but does not suffer from lack of performance. + + There is also the proc filesystem, usually accessible as + the /proc directory, which is not really a + filesystem at all, even though it looks like one. The proc + filesystem makes it easy to access certain kernel data structures, + such as the process list (hence the name). It makes these data + structures look like a filesystem, and that filesystem can be + manipulated with all the usual file tools. For example, to get a + listing of all processes one might use the command + + +$ ls -l /proc +total 0 +dr-xr-xr-x 4 root root 0 Jan 31 20:37 1 +dr-xr-xr-x 4 liw users 0 Jan 31 20:37 63 +dr-xr-xr-x 4 liw users 0 Jan 31 20:37 94 +dr-xr-xr-x 4 liw users 0 Jan 31 20:37 95 +dr-xr-xr-x 4 root users 0 Jan 31 20:37 98 +dr-xr-xr-x 4 liw users 0 Jan 31 20:37 99 +-r--r--r-- 1 root root 0 Jan 31 20:37 devices +-r--r--r-- 1 root root 0 Jan 31 20:37 dma +-r--r--r-- 1 root root 0 Jan 31 20:37 filesystems +-r--r--r-- 1 root root 0 Jan 31 20:37 interrupts +-r-------- 1 root root 8654848 Jan 31 20:37 kcore +-r--r--r-- 1 root root 0 Jan 31 11:50 kmsg +-r--r--r-- 1 root root 0 Jan 31 20:37 ksyms +-r--r--r-- 1 root root 0 Jan 31 11:51 loadavg +-r--r--r-- 1 root root 0 Jan 31 20:37 meminfo +-r--r--r-- 1 root root 0 Jan 31 20:37 modules +dr-xr-xr-x 2 root root 0 Jan 31 20:37 net +dr-xr-xr-x 4 root root 0 Jan 31 20:37 self +-r--r--r-- 1 root root 0 Jan 31 20:37 stat +-r--r--r-- 1 root root 0 Jan 31 20:37 uptime +-r--r--r-- 1 root root 0 Jan 31 20:37 +version +$ + + + (There will be a few extra files that don't correspond to + processes, though. The above example has been shortened.) + + Note that even though it is called a filesystem, no part of + the proc filesystem touches any disk. It exists only in the + kernel's imagination. Whenever anyone tries to look at any part of + the proc filesystem, the kernel makes it look as if the part existed + somewhere, even though it doesn't. So, even though there is a + multi-megabyte /proc/kcore file, it doesn't + take any disk space. + + +Which filesystem should be used? + + There is usually little point in using many different + filesystems. Currently, ext2fs is the most popular one, and it is + probably the wisest choice. Depending on the overhead for + bookkeeping structures, speed, (perceived) reliability, + compatibility, and various other reasons, it may be advisable to use + another file system. This needs to be decided on a case-by-case + basis. + Currently there are several filesystems vying + for replacement of ext2, these include reiserfs and ext3. + They include ``journalling''. A definition and explanation + of journalling is outside the (current) scope of this book, + but put very simply it is a mechanism whereby the filesystem + is more robust against power failure, or other inelegant + shutdowns. This makes data loss far less likely and so not + surprisingly it is looking like it will be the standard + in Linux filesystems eventually. + + + + + +Creating a filesystem + + Filesystems are created, i.e., initialised, with the + mkfs command. There is actually a separate + program for each filesystem type. mkfs is just a + front end that runs the appropriate program depending on the desired + filesystem type. The type is selected with the + option. + + The programs called by mkfs have slightly + different command line interfaces. The common and most important + options are summarised below; see the manual pages for more. + + + + + + Select the type of the filesystem. + + + + + + Search for bad blocks and initialise the bad + block list accordingly. + + + + -l filename + + Read the initial bad block list from the name file. + + + + + To create an ext2 filesystem on a floppy, one would give the + following commands: + + +$ fdformat -n /dev/fd0H1440 +Double-sided, 80 tracks, 18 sec/track. Total capacity +1440 kB. +Formatting ... done +$ badblocks /dev/fd0H1440 1440 $>$ +bad-blocks +$ mkfs -t ext2 -l bad-blocks +/dev/fd0H1440 +mke2fs 0.5a, 5-Apr-94 for EXT2 FS 0.5, 94/03/10 +360 inodes, 1440 blocks +72 blocks (5.00%) reserved for the super user +First data block=1 +Block size=1024 (log=0) +Fragment size=1024 (log=0) +1 block group +8192 blocks per group, 8192 fragments per group +360 inodes per group + +Writing inode tables: done +Writing superblocks and filesystem accounting information: +done +$ + + + First, the floppy was formatted (the option + prevents validation, i.e., bad block checking). Then bad blocks + were searched with badblocks, with the output + redirected to a file, bad-blocks. Finally, the + filesystem was created, with the bad block list initialised + by whatever badblocks found. + + The option could have been used with + mkfs instead of badblocks + and a separate file. The example below does that. + + +$ mkfs -t ext2 -c +/dev/fd0H1440 +mke2fs 0.5a, 5-Apr-94 for EXT2 FS 0.5, 94/03/10 +360 inodes, 1440 blocks +72 blocks (5.00%) reserved for the super user +First data block=1 +Block size=1024 (log=0) +Fragment size=1024 (log=0) +1 block group +8192 blocks per group, 8192 fragments per group +360 inodes per group + +Checking for bad blocks (read-only test): done +Writing inode tables: done +Writing superblocks and filesystem accounting information: +done +$ + + + The option is more convenient than a separate + use of badblocks, but + badblocks is necessary for checking + after the filesystem has been created. + + The process to prepare filesystems on hard disks or + partitions is the same as for floppies, except that the formatting + isn't needed. + + + + +Mounting and unmounting + + Before one can use a filesystem, it has to be + mounted. The operating system then does + various bookkeeping things to make sure that everything works. Since + all files in UNIX are in a single directory tree, the mount + operation will make it look like the contents of the new filesystem + are the contents of an existing subdirectory in some already mounted + filesystem. + + For example, shows three + separate filesystems, each with their own root directory. When the + last two filesystems are mounted below /home + and /usr, respectively, on the first + filesystem, we can get a single directory tree, as in + . + +
+ Three separate filesystems. + +
+ +
+ <filename>/home</filename> and <filename>/usr</filename> + have been + mounted. + +
+ + The mounts could be done as in the following example: + + +$ mount /dev/hda2 /home +$ mount /dev/hda3 /usr +$ + + + The mount command takes two arguments. The first + one is the device file corresponding to the disk or partition + containing the filesystem. The second one is the directory below + which it will be mounted. After these commands the contents of the + two filesystems look just like the contents of the + /home and /usr + directories, respectively. One would then say that + ``/dev/hda2 is mounted + on /home'', and similarly for + /usr. To look at either filesystem, one would + look at the contents of the directory on which it has been mounted, + just as if it were any other directory. Note the difference between + the device file, /dev/hda2, and the mounted-on + directory, /home. The device file gives access + to the raw contents of the disk, the mounted-on directory gives + access to the files on the disk. The mounted-on directory is called + the mount point. + + Linux supports many filesystem types. + mount tries to guess the type of the filesystem. + You can also use the option to specify + the type directly; this is sometimes necessary, since the heuristics + mount uses do not always work. For example, to + mount an MS-DOS floppy, you could use the following command: + + + $ mount -t msdos /dev/fd0 + /floppy + $ + + + + + The mounted-on directory need not be empty, although it + must exist. Any files in it, however, will be inaccessible by name + while the filesystem is mounted. (Any files that have already been + opened will still be accessible. Files that have hard links from + other directories can be accessed using those names.) There is no + harm done with this, and it can even be useful. For instance, some + people like to have /tmp and + /var/tmp synonymous, and make + /tmp be a symbolic link to + /var/tmp. When the system is booted, before + the /var filesystem is mounted, a + /var/tmp directory residing on the root + filesystem is used instead. When /var is + mounted, it will make the /var/tmp directory + on the root filesystem inaccessible. If + /var/tmp didn't exist on the root filesystem, + it would be impossible to use temporary files + before mounting /var. + + If you don't intend to write anything to the filesystem, use + the switch for mount to do a + read-only mount. This will make the kernel + stop any attempts at writing to the filesystem, and will also stop + the kernel from updating file access times in the inodes. Read-only + mounts are necessary for unwritable media, e.g., CD-ROMs. + + The alert reader has already noticed a slight + logistical problem. How is the first filesystem (called the + root filesystem, because it contains the root + directory) mounted, since it obviously can't be mounted on another + filesystem? Well, the answer is that it is done by magic. + + For more + information, see the kernel source or the Kernel Hackers' + Guide. + + The root filesystem is magically mounted at boot time, and one can + rely on it to always be mounted. If the root filesystem can't be + mounted, the system does not boot. The name of the filesystem that + is magically mounted as root is either compiled into the kernel, or + set using LILO or rdev. + + The root filesystem is usually first mounted read-only. + The startup scripts will then run fsck to verify + its validity, and if there are no problems, they will + re-mount it so that writes will also be + allowed. fsck must not be run on a mounted + filesystem, since any changes to the filesystem while + fsck is running will cause + trouble. Since the root filesystem is mounted read-only while + it is being checked, fsck can fix any problems + without worry, since the remount operation will flush + any metadata that the filesystem keeps in memory. + + On many systems there are other filesystems that should + also be mounted automatically at boot time. These are specified + in the /etc/fstab file; see the fstab man + page for details on the format. The details of exactly when the + extra filesystems are mounted depend on many factors, and can be + configured by each administrator if need be; see + . + + When a filesystem no longer needs to be mounted, it can be + unmounted with umount. + + It should of course be + unmount, but the n mysteriously disappeared in + the 70s, and hasn't been seen since. Please return it to Bell + Labs, NJ, if you find it. + + umount takes one argument: + either the device file or the mount point. + For example, to unmount the directories of + the previous example, one could use the commands + + +$ umount /dev/hda2 +$ umount /usr +$ + + + + See the man page for further instructions on how to + use the command. It is imperative that you always unmount a mounted + floppy. Don't just pop the floppy out of the + drive! Because of disk caching, the data is not + necessarily written to the floppy until you unmount it, so removing + the floppy from the drive too early might cause the contents to + become garbled. If you only read from the floppy, this is not very + likely, but if you write, even accidentally, + the result may be catastrophic. + + Mounting and unmounting requires super user privileges, i.e., + only root can do it. The reason for this is that if any user can + mount a floppy on any directory, then it is rather easy to create a + floppy with, say, a Trojan horse disguised as + /bin/sh, or any other often used program. + However, it is often necessary to allow users to use floppies, and + there are several ways to do this: + + + + Give the users the root password. This is + obviously bad security, but is the easiest solution. It works well + if there is no need for security anyway, which is the case + on many non-networked, personal systems. + + Use a program such as sudo to + allow users to use mount. This is still bad security, but doesn't + directly give super user privileges to everyone. + + It requires several seconds of hard + thinking on the users' behalf. Furthermore + sudo can be configured to only allow + users to execute certain commands. See the sudo(8), + sudoers(5), and visudo(8) manual pages. + + + + + Make the users use mtools, a + package for manipulating MS-DOS filesystems, without mounting them. + This works well if MS-DOS floppies are all that is needed, but is + rather awkward otherwise. + + + List the floppy devices and their allowable mount + points together with the suitable options in + /etc/fstab. + + + + The last alternative can be implemented by adding a line like the + following to the /etc/fstab file: + + + /dev/fd0 /floppy msdos user,noauto 0 0 + + + The columns are: device file to mount, directory to mount on, + filesystem type, options, backup frequency (used by + dump), and fsck pass number + (to specify the order in which filesystems should be checked + upon boot; 0 means no check). + + The option stops this mount to be done + automatically when the system is started (i.e., it stops + mount -a from mounting it). The + option allows any user to mount the + filesystem, and, because of security reasons, disallows execution of + programs (normal or setuid) and interpretation of device files from + the mounted filesystem. After this, any user can mount a floppy with + an msdos filesystem with the following command: + + + $ mount /floppy + $ + + + The floppy can (and needs to, of course) be unmounted with + the corresponding umount command. + + If you want to provide access to several types of floppies, + you need to give several mount points. The settings can be + different for each mount point. For example, to give access to both + MS-DOS and ext2 floppies, you could have the following to lines in + /etc/fstab: + + + /dev/fd0 /dosfloppy msdos user,noauto 0 0 + /dev/fd0 /ext2floppy ext2 user,noauto 0 0 + + + For MS-DOS filesystems (not just floppies), you probably want to + restrict access to it by using the , + , and filesystem options, + described in detail on the mount manual page. If + you aren't careful, mounting an MS-DOS filesystem gives everyone at + least read access to the files in it, which + is not a good idea. + + + + +Checking filesystem integrity with +<command>fsck</command> + + Filesystems are complex creatures, and as such, they + tend to be somewhat error-prone. A filesystem's correctness and + validity can be checked using the fsck command. + It can be instructed to repair any minor problems it finds, and to + alert the user if there any unrepairable problems. Fortunately, the + code to implement filesystems is debugged quite effectively, so + there are seldom any problems at all, and they are usually caused by + power failures, failing hardware, or operator errors; + for example, by not shutting down the system properly. + + Most systems are setup to run fsck + automatically at boot time, so that any errors are detected (and + hopefully corrected) before the system is used. Use of a corrupted + filesystem tends to make things worse: if the data structures are + messed up, using the filesystem will probably mess them up even + more, resulting in more data loss. However, fsck + can take a while to run on big filesystems, and since errors almost + never occur if the system has been shut down properly, a couple of + tricks are used to avoid doing the checks in such cases. The first + is that if the file /etc/fastboot exists, no + checks are made. The second is that the ext2 filesystem has a + special marker in its superblock that tells whether the filesystem + was unmounted properly after the previous mount. This allows + e2fsck (the version of fsck + for the ext2 filesystem) to avoid checking the filesystem if the + flag indicates that the unmount was done (the assumption being that + a proper unmount indicates no problems). Whether the + /etc/fastboot trick works on your system + depends on your startup scripts, but the ext2 trick works every time + you use e2fsck. It has to be explicitly bypassed + with an option to e2fsck to be avoided. (See + the e2fsck man page for + details on how.) + + The automatic checking only works for the + filesystems that are mounted automatically at boot time. Use + fsck manually to check other filesystems, + e.g., floppies. + + If fsck finds unrepairable problems, + you need either in-depth knowledge of how filesystems work in + general, and the type of the corrupt filesystem in particular, or + good backups. The latter is easy (although sometimes tedious) to + arrange, the former can sometimes be arranged via a friend, the + Linux newsgroups and mailing lists, or some other source of support, + if you don't have the know-how yourself. I'd like to tell you more + about it, but my lack of education and experience in this regard + hinders me. The debugfs + program by Theodore Ts'o should be useful. + + fsck must only be run on unmounted + filesystems, never on mounted filesystems (with the exception of the + read-only root during startup). This is because it accesses the raw + disk, and can therefore modify the filesystem without the operating + system realizing it. There will + be trouble, if the operating system is confused. + + + + +Checking for disk errors with <command>badblocks</command> + + It can be a good idea to periodically check for bad blocks. + This is done with the badblocks command. It + outputs a list of the numbers of all bad blocks it can find. This + list can be fed to fsck to be recorded in the + filesystem data structures so that the operating system won't try to + use the bad blocks for storing data. The following example will show + how this could be done. + + + $ badblocks /dev/fd0H1440 1440 > + bad-blocks + $ fsck -t ext2 -l bad-blocks + /dev/fd0H1440 + Parallelising fsck version 0.5a (5-Apr-94) + e2fsck 0.5a, 5-Apr-94 for EXT2 FS 0.5, 94/03/10 + Pass 1: Checking inodes, blocks, and sizes + Pass 2: Checking directory structure + Pass 3: Checking directory connectivity + Pass 4: Check reference counts. + Pass 5: Checking group summary information. + + /dev/fd0H1440: ***** FILE SYSTEM WAS MODIFIED ***** + /dev/fd0H1440: 11/360 files, 63/1440 blocks + $ + + + If badblocks reports a block that was already used, + e2fsck will try to move the block to another + place. If the block was really bad, not just marginal, the + contents of the file may be corrupted. + + + + +Fighting fragmentation + + When a file is written to disk, it can't always be written + in consecutive blocks. A file that is not stored in consecutive + blocks is fragmented. It takes longer to + read a fragmented file, since the disk's read-write head will have + to move more. It is desirable to avoid fragmentation, although it + is less of a problem in a system with a good buffer + cache with read-ahead. + + The ext2 filesystem attempts to keep fragmentation at a + minimum, by keeping all blocks in a file close together, even if + they can't be stored in consecutive sectors. Ext2 effectively + always allocates the free block that is nearest to other blocks in a + file. For ext2, it is therefore seldom necessary to worry about + fragmentation. There is a program for defragmenting an ext2 + filesystem called, strangely enough, defrag + + + + http://www.go.dlr.de/linux/src/defrag-0.73.tar.gz + + . + + There are many MS-DOS defragmentation programs that + move blocks around in the filesystem to remove fragmentation. For + other filesystems, defragmentation must be done by backing up the + filesystem, re-creating it, and restoring the files from backups. + Backing up a filesystem before defragmenting is a good idea for all + filesystems, since many things can go wrong + during the defragmentation. + + + + +Other tools for all filesystems + + Some other tools are also useful for managing filesystems. + df shows the free disk space on one or more + filesystems; du shows how much disk space a + directory and all its files contain. These can be used to hunt down + disk space wasters. Both have manual pages which detail + the (many) options which can be used. + + sync forces all unwritten blocks + in the buffer cache (see ) to be + written to disk. It is seldom necessary to do this by hand; the + daemon process update does this automatically. + It can be useful in catastrophes, for example if + update or its helper process + bdflush dies, or if you must turn off power + now and can't wait for + update to run. Again, there are manual pages. + The man is your very best friend in linux. Its + cousin apropos is also very useful when you don't + know what the name of the command you want + is. + + + + +Other tools for the ext2 filesystem + + In addition to the filesystem creator + (mke2fs) and checker (e2fsck) + accessible directly or via the filesystem type independent front + ends, the ext2 + filesystem has some additional tools that can be useful. + + tune2fs adjusts filesystem parameters. + Some of the more interesting parameters are: + + + + + A maximal mount count. e2fsck enforces a check + when filesystem has been mounted too many times, even if the clean + flag is set. For a system that is used for developing or testing + the system, it might be a good idea to reduce this limit. + + + + A maximal time between checks. e2fsck can also + enforce a maximal time between two checks, even if the clean flag is + set, and the filesystem hasn't been mounted very often. This can be + disabled, however. + + + + Number of blocks reserved for root. Ext2 reserves some blocks for + root so that if the filesystem fills up, it is still possible to do + system administration without having to delete anything. The + reserved amount is by default 5 percent, which on most disks isn't + enough to be wasteful. However, for floppies there is no point in + reserving any blocks. + + + + + See the tune2fs manual page for more + information. + + dumpe2fs shows information about an ext2 + filesystem, mostly from the superblock. shows a sample output. Some of the + information in the output is technical and requires understanding of + how the filesystem works (see appendix XXX ext2fspaper), but much of + it is readily understandable even for layadmins. + +
+Sample output from <command>dumpe2fs</command> + + +dumpe2fs 0.5b, 11-Mar-95 for EXT2 FS 0.5a, 94/10/23 +Filesystem magic number: 0xEF53 +Filesystem state: clean +Errors behavior: Continue +Inode count: 360 +Block count: 1440 +Reserved block count: 72 +Free blocks: 1133 +Free inodes: 326 +First block: 1 +Block size: 1024 +Fragment size: 1024 +Blocks per group: 8192 +Fragments per group: 8192 +Inodes per group: 360 +Last mount time: Tue Aug 8 01:52:52 1995 +Last write time: Tue Aug 8 01:53:28 1995 +Mount count: 3 +Maximum mount count: 20 +Last checked: Tue Aug 8 01:06:31 1995 +Check interval: 0 +Reserved blocks uid: 0 (user root) +Reserved blocks gid: 0 (group root) + +Group 0: + Block bitmap at 3, Inode bitmap at 4, Inode table at 5 + 1133 free blocks, 326 free inodes, 2 directories + Free blocks: 307-1439 + Free inodes: 35-360 + +
+ + debugfs is a filesystem debugger. + It allows direct access to the filesystem data structures stored on + disk and can thus be used to repair a disk that is so broken that + fsck can't fix it automatically. It has also been + known to be used to recover deleted files. However, + debugfs very much requires that you understand + what you're doing; a failure to understand can + destroy all your data. + + dump and restore can be + used to back up an ext2 filesystem. They are ext2 specific versions + of the traditional UNIX backup tools. See + for more information on backups. + + + + + + +Disks without filesystems + + Not all disks or partitions are used as filesystems. + A swap partition, for example, will not have a filesystem on it. + Many floppies are used in a tape-drive emulating fashion, so that a + tar (tape archive) or other file is written + directly on the raw disk, without a filesystem. Linux boot floppies + don't + contain a filesystem, only the raw kernel. + + Avoiding a filesystem has the advantage of making more of + the disk usable, since a filesystem always has some bookkeeping + overhead. It also makes the disks more easily compatible with other + systems: for example, the tar file format is the + same on all systems, while filesystems are different on most + systems. You will quickly get used to disks without filesystems if + you need them. Bootable Linux floppies + also do not necessarily have a filesystem, although they may. + + One reason to use raw disks is to make image copies of them. + For instance, if the disk contains a partially damaged filesystem, + it is a good idea to make an exact copy of it before trying to fix + it, since then you can start again if your fixing breaks things even + more. One way to do this is to use dd: + + + $ dd if=/dev/fd0H1440 + of=floppy-image + 2880+0 records in + 2880+0 records out + $ dd if=floppy-image + of=/dev/fd0H1440 + 2880+0 records in + 2880+0 records out + $ + + + The first dd makes an exact image of the floppy + to the file floppy-image, the second one writes + the image to the floppy. (The user has presumably switched the + floppy before the second command. Otherwise the + command pair is of doubtful usefulness.) + + + + +Allocating disk space + + +Partitioning schemes + + It is not easy to partition a disk in the best possible way. + Worse, there is no universally correct way to do it; there are + too many factors involved. + + The traditional way is to have a (relatively) small + root filesystem, which contains /bin, + /etc, /dev, + /lib, /tmp, and other + stuff that is needed to get the system up and running. This way, + the root filesystem (in its own partition or on its own disk) is all + that is needed to bring up the system. The reasoning is that if the + root filesystem is small and is not heavily used, it is less likely + to become corrupt when the system crashes, and you will therefore + find it easier to fix any problems caused by the crash. Then you + create separate partitions or use separate disks for the directory + tree below /usr, the users' home directories + (often under /home), and the swap space. + Separating the home directories (with the users' files) in their own + partition makes backups easier, since it is usually not necessary to + backup programs (which reside below /usr). In + a networked environment it is also possible to share + /usr among several machines (e.g., by using + NFS), thereby reducing the total disk space required by several tens + or hundreds of megabytes times + the number of machines. + + The problem with having many partitions is that it splits + the total amount of free disk space into many small pieces. + Nowadays, when disks and (hopefully) operating systems are more + reliable, many people prefer to have just one partition that holds + all their files. On the other hand, it can be less + painful to back up (and restore) a small partition. + + + + For a small hard disk (assuming you don't do kernel + development), the best way to go is probably to have just one + partition. For large hard disks, it is probably better to have a + few large partitions, just in case something does go wrong. (Note + that `small' and `large' are used in a relative sense here; your + needs for disk space + decide what the threshold is.) + + If you have several disks, you might wish to have the + root filesystem (including /usr) on one, + and the users' home directories on another. + + It is a good idea to be prepared to experiment a bit + with different partitioning schemes (over time, not just while first + installing the system). This is a bit of work, since it essentially + requires you to install the system from scratch several times + + This is not actually true, it is possible to move partitions + and mountpoints without reinstalling, but it is (currently) + beyond the scope of this book to explain how. It is on the + TODO list to write a section on this. If you have + experience and knowledge in this area then perhaps you could + write it for me and save me the bother? :) + + , but it is the only way to be sure you do + it right. + + + + +Space requirements + + The Linux distribution you install will give some indication + of how much disk space you need for various configurations. Programs + installed separately may also do the same. This will help you plan + your disk space usage, but you should prepare for the future and + reserve some extra space for things you will + notice later that you need. + + The amount you need for user files depends on what your + users wish to do. Most people seem to need as much space for their + files as possible, but the amount they will live happily with varies + a lot. Some people do only light text processing and will survive + nicely with a few megabytes, others do heavy + image processing and will need gigabytes. + + By the way, when comparing file sizes given in + kilobytes or megabytes and disk space given in megabytes, it can be + important to know that the two units can be different. Some disk + manufacturers like to pretend that a kilobyte is 1000 bytes and a + megabyte is 1000 kilobytes, while all the rest of the computing + world uses 1024 for both factors. Therefore, my 345 MB hard disk + was really a 330 MB hard disk. + + Swap space allocation is discussed in . + + + + +Examples of hard disk allocation + + I used to have a 109 MB hard disk. Now I am using a 330 MB + hard disk. I'll explain how and why I partitioned those + disks. + + The 109 MB disk I partitioned in a lot of ways, when my + needs and the operating systems I used changed; I'll explain + two typical scenarios. First, I used to run MS-DOS together + with Linux. For that, I needed about 20 MB of hard disk, or + just enough to have MS-DOS, a C compiler, an editor, a few other + utilities, the program I was working on, and enough free disk + space to not feel claustrophobic. For Linux, I had a 10 MB swap + partition, and the rest, or 79 MB, was a single partition with all + the files I had under Linux. I experimented with having separate + root, /usr, and /home + partitions, but there was never enough free disk space in one + piece to do much interesting. + + When I didn't need MS-DOS anymore, I repartitioned the + disk so that I had a 12 MB swap partition, and again had the + rest as a single filesystem. + + The 330 MB disk is partitioned into several partitions, like + this: + + + + + 5 MB root filesystem + 10 MB swap partition + 180 MB /usr + filesystem + + 120 MB /home + filesystem + + 15 MB scratch partition + + + + + The scratch partition is for playing around with things that + require their own partition, e.g., trying different Linux + distributions, or comparing speeds of filesystems. When not + needed for anything else, it is used as swap space (I like to + have a lot of open windows). + + This section is somewhat out of date. Most people + these days have disks that stretch into the multiple + Gigabytes. It is still quite scalable (just multiply by + some factor to make it fit your hardware) for the moment + though, updating it to take + account of larger disks is planned. + + + + +Adding more disk space for Linux + + Adding more disk space for Linux is easy, at least after the + hardware has been properly installed (the hardware installation + is outside the scope of this book). You format it if necessary, + then create the partitions and filesystem as described above, + and add the proper lines to /etc/fstab + so that it is mounted automatically. + + + + +Tips for saving disk space + + The best tip for saving disk space is to avoid installing + unnecessary programs. Most Linux distributions have an + option to install only part of the packages they contain, + and by analysing your needs you might notice that you don't + need most of them. This will help save a lot of disk space, + since many programs are quite large. Even if you do need a + particular package or program, you might not need all of it. + For example, some on-line documentation might be unnecessary, + as might some of the Elisp files for GNU Emacs, some of the + fonts for X11, or some of the libraries for programming. + + If you cannot uninstall packages, you might look into + compression. Compression programs such as gzip + or zip will compress (and uncompress) + individual files or groups of files. The gzexe + system will compress and uncompress programs invisibly to the + user (unused programs are compressed, then uncompressed as they + are used). The experimental DouBle system will compress all + files in a filesystem, invisibly to the programs that use them. + (If you are familiar with products such as Stacker for MS-DOS + or DriveSpace for Windows, the principle is the same.) + + + + + + + + + +Memory Management + +
Minnet, jag har tappat mitt minne, + är jag svensk eller finne, kommer inte ihåg... + (Bosse Österberg) + + A Swedish drinking song, (rough) translation: ``Memory, I + have lost my memory. Am I Swedish or Finnish? I can't + remember'' +
+ + This section describes the Linux memory management + features, i.e., virtual memory and the disk buffer cache. + The purpose and workings and the things the system administrator + needs to take into consideration are described. + + +What is virtual memory? + + Linux supports virtual memory, that + is, using a disk as an extension of RAM so that the effective + size of usable memory grows correspondingly. The kernel will + write the contents of a currently unused block of memory to the + hard disk so that the memory can be used for another purpose. + When the original contents are needed again, they are read back + into memory. This is all made completely transparent to the + user; programs running under Linux only see the larger amount of + memory available and don't notice that parts of them reside on + the disk from time to time. Of course, reading and writing the + hard disk is slower (on the order of a thousand times slower) + than using real memory, so the programs don't run as fast. + The part of the hard disk that is used as virtual memory is + called the swap space. + + Linux can use either a normal file in the filesystem or a + separate partition for swap space. A swap partition is + faster, but it is easier to change the size of a swap file + (there's no need to repartition the whole hard disk, and + possibly install everything from scratch). When you know how + much swap space you need, you should go for a swap partition, + but if you are uncertain, you can use a swap file first, use + the system for a while so that you can get a feel for how much + swap you need, and then make a swap partition when you're + confident about its size. + + You should also know that Linux allows one to use several swap + partitions and/or swap files at the same time. This means + that if you only occasionally need an unusual amount of swap space, + you can set up an extra swap file at such times, instead of + keeping the whole amount allocated all the time. + + A note on operating system terminology: computer science + usually distinguishes between swapping (writing the whole process + out to swap space) and paging (writing only fixed size parts, + usually a few kilobytes, at a time). Paging is usually more + efficient, and that's what Linux does, but traditional Linux + terminology talks about swapping anyway. + + Thus quite needlessly annoying a + number of computer scientists greatly. + + + + + + +Creating a swap space + + A swap file is an ordinary file; it is in no way special + to the kernel. The only thing that matters to the kernel is that it + has no holes, and that it is prepared for use with + mkswap. It must reside on a local disk, however; + it can't reside in a filesystem that has been mounted + over NFS due to implementation reasons. + + The bit about holes is important. The swap file reserves + the disk space so that the kernel can quickly swap out a page + without having to go through all the things that are necessary + when allocating a disk sector to a file. The kernel merely + uses any sectors that have already been allocated to the file. + Because a hole in a file means that there are no disk sectors + allocated (for that place in the file), it is not good for the + kernel to try to use them. + + One good way to create the swap file without holes is through + the following command: + + + $ dd if=/dev/zero of=/extra-swap bs=1024 + count=1024 + 1024+0 records in + 1024+0 records out + $ + + + where /extra-swap is the name of the swap + file and the size of is given after the count=. + It is best for the size to be a multiple of 4, because the + kernel writes out memory pages, which + are 4 kilobytes in size. If the size is not a multiple of 4, + the last couple of kilobytes may be unused. + + A swap partition is also not special in any way. You create + it just like any other partition; the only difference is that + it is used as a raw partition, that is, it will not contain any + filesystem at all. It is a good idea to mark swap partitions + as type 82 (Linux swap); this will the make partition listings + clearer, even though it is not strictly necessary to the + kernel. + + After you have created a swap file or a swap partition, you + need to write a signature to its beginning; this contains some + administrative information and is used by the kernel. The + command to do this is mkswap, used like this: + + + $ mkswap /extra-swap 1024 + Setting up swapspace, size = 1044480 + bytes + $ + + + Note that the swap space is still not in use yet: it exists, + but the kernel does not use it to provide virtual memory. + + You should be very careful when using + mkswap, since it does not check that the + file or partition isn't used for anything else. You + can easily overwrite important files and partitions with + mkswap! Fortunately, you should + only need to use mkswap when you install + your system. + + The Linux memory manager limits the size of each swap space to + about 127 MB (for various technical reasons, the actual limit + is (4096-10) * 8 * 4096 = 133890048$ bytes, or + 127.6875 megabytes). You can, however, use up to + 8 swap spaces simultaneously, for a total of almost + 1 GB. + + A gigabyte here, a gigabyte there, pretty + soon we start talking about real memory. + + This is actually no longer true, this section is slated + for a rewrite Real Soon Now (tm). With newer kernels and versions + of the mkswap command the actual limit depends on + architecture. For i386 and compatibles it is 2Gigabytes, other + architectures vary. Consult the mkswap(8) manual page for more + details. + + + + +Using a swap space + + An initialised swap space is taken into use with + swapon. This command tells the kernel that + the swap space can be used. The path to the swap space is given + as the argument, so to start swapping on a temporary swap file + one might use the following command. + + +$ swapon /extra-swap +$ + + + Swap spaces can be used automatically by listing them in + the /etc/fstab file. + + +/dev/hda8 none swap sw 0 0 +/swapfile none swap sw 0 0 + + + The startup scripts will run the command swapon + -a, which will start swapping on all the swap + spaces listed in /etc/fstab. Therefore, + the swapon command is usually used only when + extra swap is needed. + + You can monitor the use of swap spaces with + free. It will tell the total amount of swap + space used. + + +$ free + total used free shared + buffers +Mem: 15152 14896 256 12404 2528 +-/+ buffers: 12368 2784 +Swap: 32452 6684 25768 +$ + + + The first line of output (Mem:) shows the + physical memory. The total column does not show the physical + memory used by the kernel, which is usually about a megabyte. + The used column shows the amount of memory used (the second + line does not count buffers). The free column shows completely + unused memory. The shared column shows the amount of memory + shared by several processes; the more, the merrier. The buffers + column shows the current size of the disk buffer cache. + + That last line (Swap:) shows similar + information for the swap spaces. If this line is all zeroes, + your swap space is not activated. + + The same information is available via + top, or using the proc filesystem in file + /proc/meminfo. It is currently difficult + to get information on the use of a specific swap space. + + A swap space can be removed from use with + swapoff. It is usually not necessary to do it, + except for temporary swap spaces. Any pages in use in the swap + space are swapped in first; if there is not sufficient physical + memory to hold them, they will then be swapped out (to some other + swap space). If there is not enough virtual memory to hold all + of the pages Linux will start to thrash; after a long while it + should recover, but meanwhile the system is unusable. You should + check (e.g., with free) that there is enough + free memory before removing a swap space from use. + + All the swap spaces that are used automatically + with swapon -a can be removed from use + with swapoff -a; it looks at the file + /etc/fstab to find what to remove. + Any manually used swap spaces will remain in use. + + Sometimes a lot of swap space can be in use even though + there is a lot of free physical memory. This can happen for + instance if at one point there is need to swap, but later a big + process that occupied much of the physical memory terminates + and frees the memory. The swapped-out data is not automatically + swapped in until it is needed, so the physical memory may remain + free for a long time. There is no need to worry about this, + but it can be comforting to know what is happening. + + + + +Sharing swap spaces with other operating systems + + Virtual memory is built into many operating systems. + Since they each need it only when they are running, i.e., never at + the same time, the swap spaces of all but the currently running + one are being wasted. It would be more efficient for them to + share a single swap space. This is possible, but can require a + bit of hacking. The Tips-HOWTO contains some advice on how to + implement this. + + + + +Allocating swap space + + Some people will tell you that you should allocate twice as + much swap space as you have physical memory, but this is a bogus + rule. Here's how to do it properly: + + + + + + Estimate your total memory needs. This is the largest + amount of memory you'll probably need at a time, that is the + sum of the memory requirements of all the programs you want to + run at the same time. This can be done by running at the same + time all the programs you are likely to ever be running at the + same time. + + For instance, if you want to run X, you should allocate + about 8 MB for it, gcc wants several megabytes (some + files need an unusually large amount, up to tens of + megabytes, but usually about four should do), and so on. + The kernel will use about a megabyte by itself, and the + usual shells and other small utilities perhaps a few + hundred kilobytes (say a megabyte together). There is + no need to try to be exact, rough estimates are fine, + but you might want to be on the pessimistic side. + + Remember that if there are going to be several people + using the system at the same time, they are all going + to consume memory. However, if two people run the same + program at the same time, the total memory consumption + is usually not double, since code pages and shared + libraries exist only once. + + The free and ps + commands are useful for estimating the memory needs. + + + + + + Add some security to the estimate in step 1. This is because + estimates of program sizes will probably be wrong, because + you'll probably forget some programs you want to run, and to + make certain that you have some extra space just in case. A + couple of megabytes should be fine. (It is better to allocate + too much than too little swap space, but there's no need to + over-do it and allocate the whole disk, since unused swap space + is wasted space; see later about adding more swap.) Also, + since it is nicer to deal with even numbers, you can round the + value up to the next full megabyte. + + + + + + Based on the computations above, you know how much memory + you'll be needing in total. So, in order to allocate swap + space, you just need to subtract the size of your physical + memory from the total memory needed, and you know how much + swap space you need. (On some versions of UNIX, you need to + allocate space for an image of the physical memory as well, so + the amount computed in step 2 is what you need and you shouldn't + do the subtraction.) + + + + + + If your calculated swap space is very much larger than your + physical memory (more than a couple times larger), you should + probably invest in more physical memory, otherwise performance + will be too low. + + + + It's a good idea to have at least some swap space, even if + your calculations indicate that you need none. Linux uses + swap space somewhat aggressively, so that as much physical + memory as possible can be kept free. Linux will swap out + memory pages that have not been used, even if the memory + is not yet needed for anything. This avoids waiting for + swapping when it is needed: the swapping can be done + earlier, when the disk is otherwise idle. + + Swap space can be divided among several disks. This + can sometimes improve performance, depending on the + relative speeds of the disks and the access patterns + of the disks. You might want to experiment with a few + schemes, but be aware that doing the experiments + properly is quite difficult. You should not believe + claims that any one scheme is superior to any other, + since it won't always be true. + + + + + +The buffer cache + + Reading from a disk + + Except a RAM disk, for obvious + reasons. + + is very slow compared to accessing (real) memory. In addition, + it is common to read the same part of a disk several times + during relatively short periods of time. For example, one + might first read an e-mail message, then read the letter into + an editor when replying to it, then make the mail program read + it again when copying it to a folder. Or, consider how often + the command ls might be run on a system with + many users. By reading the information from disk only once + and then keeping it in memory until no longer needed, one can + speed up all but the first read. This is called disk + buffering, and the memory used for the purpose is + called the buffer cache. + + Since memory is, unfortunately, a finite, nay, scarce + resource, the buffer cache usually cannot be big enough (it + can't hold all the data one ever wants to use). When the cache + fills up, the data that has been unused for the longest time + is discarded and the memory thus freed is used for the new + data. + + Disk buffering works for writes as well. On the one hand, + data that is written is often soon read again (e.g., a source + code file is saved to a file, then read by the compiler), + so putting data that is written in the cache is a good idea. + On the other hand, by only putting the data into the cache, not + writing it to disk at once, the program that writes runs quicker. + The writes can then be done in the background, without slowing + down the other programs. + + Most operating systems have buffer caches (although + they might be called something else), but not all of + them work according to the above principles. Some are + write-through: the data is written to disk + at once (it is kept in the cache as well, of course). The cache + is called write-back if the writes are done + at a later time. Write-back is more efficient than write-through, + but also a bit more prone to errors: if the machine crashes, + or the power is cut at a bad moment, or the floppy is removed + from the disk drive before the data in the cache waiting to be + written gets written, the changes in the cache are usually lost. + This might even mean that the filesystem (if there is one) is + not in full working order, perhaps because the unwritten data + held important changes to the bookkeeping information. + + Because of this, you should never turn off the + power without using a proper shutdown procedure (see ), or remove a floppy from the + disk drive until it has been unmounted (if it was mounted) + or after whatever program is using it has signalled that it + is finished and the floppy drive light doesn't shine anymore. + The sync command flushes + the buffer, i.e., forces all unwritten data to be written to disk, + and can be used when one wants to be sure that everything is + safely written. In traditional UNIX systems, there is a program + called update running in the background + which does a sync every 30 seconds, so + it is usually not necessary to use sync. + Linux has an additional daemon, bdflush, + which does a more imperfect sync more frequently to avoid the + sudden freeze due to heavy disk I/O that sync + sometimes causes. + + Under Linux, bdflush is started by + update. There is usually no reason to worry + about it, but if bdflush happens to die for + some reason, the kernel will warn about this, and you should + start it by hand (/sbin/update). + + The cache does not actually buffer files, but blocks, which + are the smallest units of disk I/O (under Linux, they are usually + 1 kB). This way, also directories, super blocks, other filesystem + bookkeeping data, and non-filesystem disks are cached. + + The effectiveness of a cache is primarily decided by its + size. A small cache is next to useless: it will hold so little + data that all cached data is flushed from the cache before it + is reused. The critical size depends on how much data is read + and written, and how often the same data is accessed. The only + way to know is to experiment. + + If the cache is of a fixed size, it is not very good to have + it too big, either, because that might make the free memory too + small and cause swapping (which is also slow). To make the most + efficient use of real memory, Linux automatically uses all free + RAM for buffer cache, but also automatically makes the cache + smaller when programs need more memory. + + Under Linux, you do not need to do anything to make use + of the cache, it happens completely automatically. Except for + following the proper procedures for shutdown and removing + floppies, you do not need to worry about it. + + + + +Boots And Shutdowns + +
+Start me up +Ah... you've got to... you've got to +Never, never never stop +Start it up +Ah... start it up, never, never, never + You make a grown man cry, + you make a grown man cry +(Rolling Stones) +
+ + This section explains what goes on when a Linux system is + brought up and taken down, and how it should be done properly. + If proper procedures are not followed, files might be corrupted + or lost. + + +An overview of boots and shutdowns + + The act of turning on a computer system and causing its + operating system to be loaded + + On early computers, it wasn't enough + to merely turn on the computer, you had to manually load the + operating system as well. These new-fangled thing-a-ma-jigs do + it all by themselves. + + is called booting. The name comes from + an image of the computer pulling itself up from its bootstraps, + but the act itself slightly more realistic. + + During bootstrapping, the computer first loads a small piece + of code called the bootstrap loader, which + in turn loads and starts the operating system. The bootstrap + loader is usually stored in a fixed location on a hard disk + or a floppy. The reason for this two step process is that + the operating system is big and complicated, but the first + piece of code that the computer loads must be very small (a + few hundred bytes), to avoid making the firmware unnecessarily + complicated. + + Different computers do the bootstrapping differently. + For PCs, the computer (its BIOS) reads in the first sector + (called the boot sector) of a floppy or + hard disk. The bootstrap loader is contained within this sector. + It loads the operating system from elsewhere on the disk (or + from some other place). + + After Linux has been loaded, it initialises the hardware and + device drivers, and then runs init. +init + starts other processes to allow users to log in, and do things. + The details of this part will be discussed below. + + In order to shut down a Linux system, first all processes + are told to terminate (this makes them close any files and + do other necessary things to keep things tidy), then filesystems + and swap areas are unmounted, and finally a message is printed + to the console that the power can be turned off. If the proper + procedure is not followed, terrible things can and will happen; + most importantly, the filesystem buffer cache might not be flushed, + which means that all data in it is lost and the filesystem on + disk is inconsistent, and therefore possibly unusable. + + + + + +The boot process in closer look + + You can boot Linux either from a floppy or from the hard + disk. The installation section in the Installation and + Getting Started guide (XXX citation) + tells you how to install Linux so you can boot it the way + you want to. + + When a PC is booted, the BIOS will do various tests to + check that everything looks all right, + + This is called + the power on self test, or + POST for short. + + and will then start the actual booting. It will choose a disk + drive (typically the first floppy drive, if there is a floppy + inserted, otherwise the first hard disk, if one is installed + in the computer; the order might be configurable, however) + and will then read its very first sector. This is called the + boot sector; for a hard disk, it is also + called the master boot record, since a + hard disk can contain several partitions, each with their own + boot sectors. + + The boot sector contains a small program (small enough to + fit into one sector) whose responsibility is to read the actual + operating system from the disk and start it. When booting Linux + from a floppy disk, the boot sector contains code that just reads + the first few hundred blocks (depending on the actual kernel + size, of course) to a predetermined place in memory. On a Linux + boot floppy, there is no filesystem, the kernel is just stored + in consecutive sectors, since this simplifies the boot process. + It is possible, however, to boot from a floppy with a filesystem, + by using LILO, the LInux LOader. + + When booting from the hard disk, the code in the master + boot record will examine the partition table (also in the master + boot record), identify the active partition (the partition that is + marked to be bootable), read the boot sector from that partition, + and then start the code in that boot sector. The code in the + partition's boot sector does what a floppy disk's boot sector + does: it will read in the kernel from the partition and start it. + The details vary, however, since it is generally not useful to + have a separate partition for just the kernel image, so the + code in the partition's boot sector can't just read the disk + in sequential order, it has to find the sectors wherever the + filesystem has put them. There are several ways around this + problem, but the most common way is to use LILO. (The details + about how to do this are irrelevant for this discussion, however; + see the LILO documentation for more information; it is most + thorough.) + + When booting with LILO, it will normally go right ahead + and read in and boot the default kernel. It is also possible + to configure LILO to be able to boot one of several kernels, + or even other operating systems than Linux, and it is possible + for the user to choose which kernel or operating system is to + be booted at boot time. LILO can be configured so that if one + holds down the alt, shift, or + ctrl key at boot time (when LILO is loaded), + LILO will ask what is to be booted and not boot the default + right away. Alternatively, LILO can be configured so that it + will always ask, with an optional timeout that will cause the + default kernel to be booted. + + With LILO, it is also possible to give a kernel + command line argument, after the name of the kernel + or operating system. + + Booting from floppy and from hard disk have both their + advantages, but generally booting from the hard disk is nicer, + since it avoids the hassle of playing around with floppies. + It is also faster. However, it can be more troublesome to install + the system to boot from the hard disk, so many people will first + boot from floppy, then, when the system is otherwise installed + and working well, will install LILO and start booting from the + hard disk. + + After the Linux kernel has been read into the memory, by + whatever means, and is started for real, roughly the following + things happen: + + + + + The Linux kernel is installed compressed, so it will first + uncompress itself. The beginning of the kernel image + contains a small program that does this. + + + + If you have a super-VGA card that Linux + recognises and that has some special text modes (such as 100 + columns by 40 rows), Linux asks you which mode + you want to use. During the kernel compilation, it is + possible to preset a video mode, so that this is never asked. + This can also be done with LILO or rdev. + + + + After this, the kernel checks what other hardware there is + (hard disks, floppies, network adapters, etc), and configures + some of its device drivers appropriately; while it does this, + it outputs messages about its findings. For example, when I + boot, I it looks like this: + + + +LILO boot: +Loading linux. +Console: colour EGA+ 80x25, 8 virtual consoles +Serial driver version 3.94 with no serial options enabled +tty00 at 0x03f8 (irq = 4) is a 16450 +tty01 at 0x02f8 (irq = 3) is a 16450 +lp_init: lp1 exists (0), using polling driver +Memory: 7332k/8192k available (300k kernel code, 384k reserved, 176k +data) +Floppy drive(s): fd0 is 1.44M, fd1 is 1.2M +Loopback device init +Warning WD8013 board not found at i/o = 280. +Math coprocessor using irq13 error reporting. +Partition check: + hda: hda1 hda2 hda3 +VFS: Mounted root (ext filesystem). +Linux version 0.99.pl9-1 (root@haven) 05/01/93 14:12:20 + + + + The exact texts are different on different systems, depending + on the hardware, the version of Linux being used, and how + it has been configured. + + + Then the kernel will try to mount the root + filesystem. The place is configurable at compilation time, or + any time with rdev or LILO. The filesystem + type is detected automatically. If the mounting of the root + filesystem fails, for example because you didn't remember to + include the corresponding filesystem driver in the kernel, the + kernel panics and halts the system (there isn't much it can do, + anyway). + + The root filesystem is usually mounted read-only (this can + be set in the same way as the place). This makes it possible + to check the filesystem while it is mounted; it is not a good + idea to check a filesystem that is mounted read-write. + + + After this, the kernel starts + the program init (located in + /sbin/init) in the background (this will + always become process number 1). init does + various startup chores. The exact things it does depends on how + it is configured; see for more information + (not yet written). It will at least start some essential + background daemons. + + init then switches to + multi-user mode, and starts a getty for virtual + consoles and serial lines. getty is the + program which lets people log in via virtual consoles and serial + terminals. init may also start some other + programs, depending on how it is configured. + + After this, the boot is complete, and the system + is up and running normally. + + + + + + + +More about shutdowns + + It is important to follow the correct procedures when you shut + down a Linux system. If you fail do so, your filesystems probably + will become trashed and the files probably will become scrambled. + This is because Linux has a disk cache that won't write things + to disk at once, but only at intervals. This greatly improves + performance but also means that if you just turn off the power + at a whim the cache may hold a lot of data and that what is on + the disk may not be a fully working filesystem (because only + some things have been written to the disk). + + Another reason against just flipping the power switch is that + in a multi-tasking system there can be lots of things going on + in the background, and shutting the power can be quite + disastrous. By using the proper shutdown sequence, you ensure + that all background processes can save their data. + + The command for properly shutting down a Linux system + is shutdown. It is usually used in one of + two ways. + + If you are running a system where you are the only user, + the usual way of using shutdown is to quit + all running programs, log out on all virtual consoles, log + in as root on one of them (or stay logged in as root if you + already are, but you should change to root's home directory or + the root directory, to avoid problems with unmounting), then + give the command shutdown -h now (substitute + now with a plus sign and a number in minutes + if you want a delay, though you usually don't on a single user + system). + + Alternatively, if your system has many users, use the command + shutdown -h +time message, where +time + is the + time in minutes until the system is halted, and +message + is a short explanation of why the system is shutting down. + + +# shutdown -h +10 'We will install a new +disk. System should +> be back on-line in three hours.' +# + + + This will warn everybody that the system will shut down in + ten minutes, and that they'd better get lost or lose data. + The warning is printed to every terminal on which someone is + logged in, including all xterms: + + + +Broadcast message from root (ttyp0) Wed Aug 2 01:03:25 1995... + +We will install a new disk. System should +be back on-line in three hours. +The system is going DOWN for system halt in 10 minutes !! + + + + The warning is automatically repeated a few times before the boot, + with shorter and shorter intervals as the time runs out. + + When the real shutting down starts after any delays, all + filesystems (except the root one) are unmounted, user processes + (if anybody is still logged in) are killed, daemons are shut down, + all filesystem are unmounted, and generally everything settles + down. When that is done, init prints out a + message that you can power down the machine. Then, and only then, + should you move your fingers towards the power switch. + + Sometimes, although rarely on any good system, it is + impossible to shut down properly. For instance, if the kernel + panics and crashes and burns and generally misbehaves, it might + be completely impossible to give any new commands, hence shutting + down properly is somewhat difficult, and just about everything + you can do is hope that nothing has been too severely damaged + and turn off the power. If the troubles are a bit less severe + (say, somebody hit your keyboard with an axe), and the kernel + and the update program still run normally, + it is probably a good idea to wait a couple of minutes to give + update a chance to flush the buffer cache, + and only cut the power after that. + + Some people like to shut down using the command + sync + + sync flushes the + buffer cache. + + three times, waiting for the disk I/O to stop, then turn off + the power. If there are no running programs, this is about + equivalent to using shutdown. However, it + does not unmount any filesystems and this can lead to problems + with the ext2fs ``clean filesystem'' flag. The triple-sync + method is not recommended. + + (In case you're wondering: the reason for three syncs is + that in the early days of UNIX, when the commands were + typed separately, that usually gave sufficient time for most + disk I/O to be finished.) + + + + + +Rebooting + + Rebooting means booting the system again. This can be + accomplished by first shutting it down completely, turning + power off, and then turning it back on. A simpler way is to + ask shutdown to reboot the system, instead + of merely halting it. This is accomplished by using the + option to shutdown, + for example, by giving the command shutdown -r + now. + + Most Linux systems run shutdown -r now + when ctrl-alt-del is pressed on the keyboard. This reboots the + system. The action on ctrl-alt-del is configurable, however, and + it might be better to allow for some delay before the reboot on + a multiuser machine. Systems that are physically accessible to + anyone might even be configured to do nothing when ctrl-alt-del + is pressed. + + + + +Single user mode + + The shutdown command can also be used + to bring the system down to single user mode, in which no one + can log in, but root can use the console. This is useful for + system administration tasks that can't be done while the system is + running normally. + + + + +Emergency boot floppies + + It is not always possible to boot a computer from the hard +disk. + For example, if you make a mistake in configuring LILO, you might + make your system unbootable. For these situations, you need an + alternative way of booting that will always work (as long as the + hardware works). For typical PCs, this means booting from the + floppy drive. + + Most Linux distributions allow one to create an + emergency boot floppy during installation. + It is a good idea to do this. However, some such boot disks + contain only the kernel, and assume you will be using the programs + on the distribution's installation disks to fix whatever problem + you have. Sometimes those programs aren't enough; for example, + you might have to restore some files from backups made with + software not on the installation disks. + + Thus, it might be necessary to create a custom root floppy + as well. The Bootdisk HOWTO by Graham + Chapman (XXX citation) contains instructions for doing this. + You must, of course, remember to keep your emergency boot and + root floppies up to date. + + You can't use the floppy drive you use to mount the root + floppy for anything else. This can be inconvenient if you only + have one floppy drive. However, if you have enough memory, you + can configure your boot floppy to load the root disk to a ramdisk + (the boot floppy's kernel needs to be specially configured for + this). Once the root floppy has been loaded into the ramdisk, + the floppy drive is free to mount other disks. + + + + +<command>init</command> + + + +
Uuno on numero yksi + (Slogan for a series of Finnish movies.)
+ + This chapter describes the init process, + which is the first user level process started by the kernel. + init has many important duties, such as + starting getty (so that users can log in), + implementing run levels, and taking care of orphaned processes. + This chapter explains how init is configured + and how you can make use of the different run levels. + + +<command>init</command> comes first + + init is one of those programs that + are absolutely essential to the operation of a Linux system, + but that you still can mostly ignore. A good Linux distribution + will come with a configuration for init + that will work for most systems, and on these systems there is + nothing you need to do about init. Usually, + you only need to worry about init if you hook + up serial terminals, dial-in (not dial-out) modems, or if you + want to change the default run level. + + When the kernel has started itself (has been loaded + into memory, has started running, and has initialised all + device drivers and data structures and such), it finishes its + own part of the boot process by starting a user level program, + init. Thus, init is always + the first process (its process number is always 1). + + The kernel looks for init + in a few locations that have been historically used + for it, but the proper location for it (on a Linux + system) is /sbin/init. If the + kernel can't find init, it tries to run + /bin/sh, and if that also fails, the startup + of the system fails. + + When init starts, it finishes the + boot process by doing a number of administrative tasks, such + as checking filesystems, cleaning up /tmp, + starting various services, and starting a getty + for each terminal and virtual console where users should be able + to log in (see ). + + After the system is properly up, init + restarts getty for each terminal + after a user has logged out (so that the next user can log + in). init also adopts orphan processes: when + a process starts a child process and dies before its child, the + child immediately becomes a child of init. + This is important for various technical reasons, but it is good + to know it, since it makes it easier to understand process lists + and process tree graphs. + + init itself is not + allowed to die. You can't kill init + even with SIGKILL. + + There are a few variants of init + available. Most Linux distributions + use sysvinit (written by Miquel + van Smoorenburg), which is based on the System V + init design. The BSD versions of Unix have + a different init. The primary difference + is run levels: System V has them, BSD does not (at least + traditionally). This difference is not essential. We'll look + at sysvinit only. + + + + +Configuring <command>init</command> to start +<command>getty</command>: the +<filename>/etc/inittab</filename> file + + When it starts up, init reads the +/etc/inittab + configuration file. While the system is running, it will + re-read it, if sent the HUP signal; + + Using the command kill -HUP + 1 as root, for example + + this feature makes it unnecessary to boot the system to make + changes to the init configuration take + effect. + + The /etc/inittab file is + a bit complicated. We'll start with the simple case + of configuring getty lines. Lines in + /etc/inittab consist of four colon-delimited + fields: + + +id:runlevels:action:process + + + The fields are described below. In addition, + /etc/inittab can contain empty lines, and + lines that begin with a number sign (`#'); + these are both ignored. + + + id + + This identifies the line in the file. For + getty lines, it specifies the terminal + it runs on (the characters after /dev/tty + in the device file name). For other lines, + it doesn't matter (except for length restrictions), + but it should be unique. + + + runlevels + + The run levels the line should be considered + for. The run levels are given as single digits, + without delimiters. (Run levels are described + in the next section.) + + + action + + What action should be taken by the line, e.g., + respawn to run the command in the + next field again, when it exits, or once + to run it just once. + + + process + + The command to run. + + + + + To start a getty on the first virtual terminal + (/dev/tty1), in all the normal multi-user + run levels (2-5), one would write the following line: + + +1:2345:respawn:/sbin/getty 9600 tty1 + + + The first field says that this is the line for +/dev/tty1. + The second field says that it applies to run levels 2, 3, 4, + and 5. The third field means that the command should be run + again, after it exits (so that one can log in, log out, and + then log in again). The last field is the command that runs + getty on the first virtual terminal. + + Different versions of + getty are run differently. Consult + your manual page, and make sure it is the correct + manual page. + + + If you wanted to add terminals or dial-in modem lines to a + system, you'd add more lines to /etc/inittab, + one for each terminal or dial-in line. For more details, see the + manual pages init, inittab, + and getty. + + If a command fails when it starts, + and init is configured to + restart it, it will use a lot of + system resources: init starts it, + it fails, init starts it, it fails, + init starts it, it fails, and so on, ad + infinitum. To prevent this, init will keep + track of how often it restarts a command, and if the frequency + grows to high, it will delay for five minutes before restarting + again. + + + + +Run levels + + A run level is a state of + init and the whole system that defines what + system services are operating. Run levels are identified by + numbers, see . There is no consensus of +how to use the + user defined run levels (2 through 5). Some system administrators + use run levels to define which subsystems are working, e.g., + whether X is running, whether the network is operational, and + so on. Others have all subsystems always running or start and + stop them individually, without changing run levels, since run + levels are too coarse for controlling their systems. You need + to decide for yourself, but it might be easiest to follow the + way your Linux distribution does things. + + + Run level numbers + + + 0 Halt the system. + 1 Single-user mode (for special +administration). + 2-5 Normal operation (user +defined). + 6 Reboot. + + +
+ + Run levels are configured in /etc/inittab +by lines like + the following: + + +l2:2:wait:/etc/init.d/rc 2 + + + The first field is an arbitrary label, the second one means + that this applies for run level 2. The third field means + that init should run the command in the + fourth field once, when the run level is entered, and that + init should wait for it to complete. The + /etc/init.d/rc command runs whatever + commands are necessary to start and stop services to enter run + level 2. + + The command in the fourth field does all the hard work of + setting up a run level. It starts services that aren't already + running, and stops services that shouldn't be running in the + new run level any more. Exactly what the command is, and how run + levels are configured, depends on the Linux distribution. + + When init starts, it looks for a line + in /etc/inittab that specifies the default + run level: + + +id:2:initdefault: + + + You can ask init to go to a non-default run + level at startup by giving the kernel a command line argument + of single or emergency. + Kernel command line arguments can be given via LILO, for example. + This allows you to choose the single user mode (run level 1). + + While the system is running, the telinit + command can change the run level. When the run level is + changed, init runs the relevant command from + /etc/inittab. + + + + +Special configuration in +<filename>/etc/inittab</filename> + + The /etc/inittab has some special + features that allow init to react to special + circumstances. These special features are marked by special + keywords in the third field. Some examples: + + + + powerwait + + Allows init to shut the system + down, when the power fails. This assumes the use of + a UPS, and software that watches the UPS and informs + init that the power is off. + + + ctrlaltdel + + Allows init to reboot the system, when + the user presses ctrl-alt-del on the console keyboard. + Note that the system administrator can configure the + reaction to ctrl-alt-del to be something else instead, + e.g., to be ignored, if the system is in a public + location. (Or to start nethack.) + + + sysinit + + Command to be run when the system is booted. This command + usually cleans up /tmp, for example. + + + + + The list above is not exhaustive. See your + inittab manual page for all possibilities, + and for details on how to use the above ones. + + + + +Booting in single user mode + + An important run level is single user +mode (run level 1), + in which only the system administrator is using the machine + and as few system services, including logins, as possible are + running. Single user mode is necessary for a few administrative + tasks, + + It probably shouldn't be used for playing + nethack. + + such as running fsck on a + /usr partition, since this requires that + the partition be unmounted, and that can't happen, unless just + about all system services are killed. + + A running system can be taken to single user mode by using + telinit to request run level 1. At bootup, + it can be entered by giving the word single + or emergency on the kernel command line: the + kernel gives the command line to init as well, + and init understands from that word that it + shouldn't use the default run level. (The kernel command line is + entered in a way that depends on how you boot the system.) + + Booting into single user mode is sometimes necessary so + that one can run fsck by hand, before anything + mounts or otherwise touches a broken /usr + partition (any activity on a broken filesystem is likely to + break it more, so fsck should be run as soon + as possible). + + The bootup scripts init runs + will automatically enter single user mode, if the automatic + fsck at bootup fails. This is an attempt to + prevent the system from using a filesystem that is so broken that + fsck can't fix it automatically. Such breakage + is relatively rare, and usually involves a broken hard disk or an + experimental kernel release, but it's good to be prepared. + + As a security measure, a properly configured system + will ask for the root password before starting the shell in + single user mode. Otherwise, it would be simple to just enter + a suitable line to LILO to get in as root. (This will break if + /etc/passwd has been broken by filesystem + problems, of course, and in that case you'd better have a boot + floppy handy.) + + + + +Logging In And Out + +
I don't care to belong to a club + that accepts people like me as a member. + (Groucho Marx)
+ + + This section describes what happens when a user logs + in or out. The various interactions of background processes, + log files, configuration files, and so on are described in + some detail. + + + +Logins via terminals + + shows how logins happen via + terminals. First, init makes sure there is + a getty program for the terminal connection + (or console). getty listens at the terminal + and waits for the user to notify that he is ready to login in + (this usually means that the user must type something). When it + notices a user, getty outputs a welcome message + (stored in /etc/issue), and prompts for + the username, and finally runs the login + program. login gets the username as a + parameter, and prompts the user for the password. If these + match, login starts the shell configured + for the user; else it just exits and terminates the process + (perhaps after giving the user another chance at entering the + username and password). init notices that + the process terminated, and starts a new getty + for the terminal. + + +
+ Logins via terminals: the interaction of +<command>init</command>, +<command>getty</command>, <command>login</command>, and the +shell. + +
+ + Note that the only new process is the + one created by init (using the + fork system call); getty + and login only replace the program running in + the process (using the exec system call). + + + A separate program, for noticing the user, is needed + for serial lines, since it can be (and traditionally was) + complicated to notice when a terminal becomes active. + getty also adapts to the speed and other + settings of the connection, which is important especially for + dial-in connections, where these parameters may change from call + to call. + + There are several versions of getty + and init in use, all with their good and + bad points. It is a good idea to learn about the versions on + your system, and also about the other versions (you could use the + Linux Software Map to search them). If you don't have dial-ins, + you probably don't have to worry about getty, + but init is still important. + + + + +Logins via the network + + Two computers in the same network are usually linked via a + single physical cable. When they communicate over the network, + the programs in each computer that take part in the communication + are linked via a virtual connection, a sort + of imaginary cable. As far as the programs at either end of the + virtual connection are concerned, they have a monopoly on their + own cable. However, since the cable is not real, only imaginary, + the operating systems of both computers can have several virtual + connections share the same physical cable. This way, using just + a single cable, several programs can communicate without having + to know of or care about the other communications. It is even + possible to have several computers use the same cable; the virtual + connections exist between two computers, and the other computers + ignore those connections that they don't take part in. + + That's a complicated and over-abstracted description of + the reality. It might, however, be good enough to understand + the important reason why network logins are somewhat different + from normal logins. The virtual connections are established + when there are two programs on different computers that wish + to communicate. Since it is in principle possible to login + from any computer in a network to any other computer, there is + a huge number of potential virtual communications. Because of + this, it is not practical to start a getty + for each potential login. + + There is a single process inetd (corresponding to + getty) that handles all network logins. + When it notices an incoming network login (i.e., it notices + that it gets a new virtual connection to some other computer), + it starts a new process to handle that single login. The original + process remains and continues to listen for new logins. + + To make things a bit more complicated, there is + more than one communication protocol for network logins. + The two most important ones are telnet and + rlogin. In addition to logins, there are many + other virtual connections that may be made (for FTP, Gopher, HTTP, + and other network services). It would be ineffective to have a + separate process listening for a particular type of connection, + so instead there is only one listener that can recognise the type + of the connection and can start the correct type of program to + provide the service. This single listener is called +inetd; + see the Linux Network Administrators' Guide + for more information. + + + + +What <command>login</command> does + + The login program takes care of + authenticating the user (making sure that the username and + password match), and of setting up an initial environment for + the user by setting permissions for the serial line and starting + the shell. + + Part of the initial setup is outputting the contents of + the file /etc/motd (short for message of the + day) and checking for electronic mail. These can be disabled + by creating a file called .hushlogin in + the user's home directory. + + If the file /etc/nologin + exists, logins are disabled. That file is typically + created by shutdown and relatives. + login checks for this file, and will + refuse to accept a login if it exists. If it does exist, + login outputs its contents to the terminal + before it quits. + + login logs all failed login attempts in + a system log file (via syslog). It also logs + all logins by root. Both of these can be useful when tracking + down intruders. + + Currently logged in people are listed in + /var/run/utmp. This file is valid only + until the system is next rebooted or shut down; it is cleared + when the system is booted. It lists each user and the terminal + (or network connection) he is using, along with some other useful + information. The who, w, + and other similar commands look in utmp + to see who are logged in. + + All successful logins are recorded into + /var/log/wtmp. This file will grow without + limit, so it must be cleaned regularly, for example by having + a weekly cron job to clear it. + + Good Linux distributions do this out + of the box. + + The last command browses + wtmp. + + Both utmp and + wtmp are in a binary format (see the + utmp manual page); it is unfortunately not + convenient to examine them without special programs. + + + + +X and xdm + + XXX X implements logins via xdm; also: xterm -ls + + + + +Access control + + The user database is traditionally contained in the + /etc/passwd file. Some systems use + shadow passwords, and have moved the + passwords to /etc/shadow. Sites with many + computers that share the accounts use NIS or some other method + to store the user database; they might also automatically copy + the database from one central location to all other computers. + + + The user database contains not only the passwords, but + also some additional information about the users, such as their + real names, home directories, and login shells. This other + information needs to be public, so that anyone can read it. + Therefore the password is stored encrypted. This does have + the drawback that anyone with access to the encrypted password + can use various cryptographic methods to guess it, without + trying to actually log into the computer. Shadow passwords try + to avoid this by moving the password into another file, which + only root can read (the password is still stored encrypted). + However, installing shadow passwords later onto a system that + did not support them can be difficult. + + With or without passwords, it is important to make + sure that all passwords in a system are good, i.e., not easily + guessed. The crack program can be used + to crack passwords; any password it can find is by definition + not a good one. While crack can be run + by intruders, it can also be run by the system administrator + to avoid bad passwords. Good passwords can also be enforced + by the passwd program; this is in fact more + effective in CPU cycles, since cracking passwords requires quite + a lot of computation. + + The user group database is kept in + /etc/group; for systems with shadow + passwords, there can be a /etc/shadow.group. + + + root usually can't login via most terminals + or the network, only via terminals listed in the + /etc/securetty file. This makes it necessary + to get physical access to one of these terminals. It is, however, + possible to log in via any terminal as any other user, and use + the su command to become root. + + + + +Shell startup + + When an interactive login shell starts, it automatically + executes one or more pre-defined files. Different shells execute + different files; see the documentation of each shell for further + information. + + Most shells first run some global file, for example, the + Bourne shell (/bin/sh) and its derivatives + execute /etc/profile; in addition, + they execute .profile in the user's + home directory. /etc/profile allows the + system administrator to have set up a common user environment, + especially by setting the PATH to include local + command directories in addition to the normal ones. On the other + hand, .profile allows the user to customise + the environment to his own tastes by overriding, if necessary, + the default environment. + + + + + +Managing user accounts + +
The similarities of sysadmins and drug + dealers: both measure stuff in Ks, and both have users. + (Old, tired computer joke.)
+ + This chapter explains how to create new user accounts, + how to modify the properties of those accounts, and how to remove + the accounts. Different Linux systems have different tools for + doing this. + + +What's an account? + + When a computer is used by many people it is usually + necessary to differentiate between the users, for example, so that + their private files can be kept private. This is important even + if the computer can only be used by a single person at a time, + as with most microcomputers. + + It might be quite embarrassing if my + sister could read my love letters. + + Thus, each user is given a unique username, and that name is + used to log in. + + There's more to a user than just a name, however. An + account is all the files, resources, + and information belonging to one user. The term hints at banks, + and in a commercial system each account usually has some money + attached to it, and that money vanishes at different speeds + depending on how much the user stresses the system. For example, + disk space might have a price per megabyte and day, and processing + time might have a price per second. + + + + +Creating a user + + The Linux kernel itself treats users are mere numbers. + Each user is identified by a unique integer, the user + id or uid, because numbers are + faster and easier for a computer to process than textual names. + A separate database outside the kernel assigns a textual name, + the username, to each user id. The database + contains additional information as well. + + To create a user, you need to add information about + the user to the user database, and create a home directory for + him. It may also be necessary to educate the user, and set up + a suitable initial environment for him. + + Most Linux distributions come with a program for + creating accounts. There are several such programs available. + Two command line alternatives are adduser + and useradd; there may be a GUI tool as well. + Whatever the program, the result is that there is little if + any manual work to be done. Even if the details are many and + intricate, these programs make everything seem trivial. However, + describes how to do it by hand. + + + +<filename>/etc/passwd</filename> and other informative +files + + The basic user database in a Unix system is the text file, + /etc/passwd (called the password + file), which lists all valid usernames and their + associated information. The file has one line per username, + and is divided into seven colon-delimited fields: + + + + Username. + Password, in an encrypted form. + Numeric user id. + Numeric group id. + Full name or other description of +account. + Home directory. + Login shell (program to run at +login). + + + + The format is explained in more detail on the + passwd manual page. + + Any user on the system may read the password file, + so that they can, for example, learn the name of another user. + This means that the password (the second field) is also available + to everyone. The password file encrypts the password, so in + theory there is no problem. However, the encryption is breakable, + especially if the password is weak (e.g., it is short or it can + be found in a dictionary). Therefore it is not a good idea to + have the password in the password file. + + + Many Linux systems have shadow passwords. +This is + an alternative way of storing the password: the encrypted + password is stored in a separate file, +/etc/shadow, + which only root can read. The /etc/passwd + file only contains a special marker in the second field. + Any program that needs to verify a user is setuid, and + can therefore access the shadow password file. Normal + programs, which only use the other fields in the password + file, can't get at the password. + + Yes, this means that the + password file has all the information about a user + except his password. The wonder + of development. + + + + + + +Picking numeric user and group ids + + On most systems it doesn't matter what the numeric user + and group ids are, but if you use the Network filesystem (NFS), + you need to have the same uid and gid on all systems. This + is because NFS also identifies users with the numeric uids. + If you aren't using NFS, you can let your account creation tool + pick them automatically. + + If you are using NFS, you'll have to be invent a mechanism + for synchronising account information. One alternative is to + the NIS system (see XXX network-admin-guide). + + However, you should try to avoid re-using numeric uids + (and textual usernames), because the new owner of the uid (or + username) may get access to the old owner's files (or mail, + or whatever). + + + + + + +Initial environment: <filename>/etc/skel</filename> + + When the home directory for a new user is created, it is + initialised with files from the /etc/skel + directory. The system administrator can create files in + /etc/skel that will provide a nice + default environment for users. For example, he might create a + /etc/skel/.profile that sets the EDITOR + environment variable to some editor that is friendly towards + new users. + + However, it is usually best to try to keep + /etc/skel as small as possible, since it + will be next to impossible to update existing users' files. For + example, if the name of the friendly editor changes, all existing + users would have to edit their .profile. The + system administrator could try to do it automatically, with a + script, but that is almost certain going to break someone's file. + + + Whenever possible, it is better to put global configuration + into global files, such as /etc/profile. This + way it is possible to update it without breaking users' + own setups. + + + + +Creating a user by hand + + To create a new account manually, follow these steps: + + + + + Edit /etc/passwd with + vipw and add a new line for the new account. Be + careful with the syntax. Do not edit directly with an + editor! vipw locks the file, so + that other commands won't try to update it at the same time. You + should make the password field be `*', so + that it is impossible to log in. + + Similarly, edit /etc/group + with vigr, if you need to create a new group + as well. + + Create the home directory of the user with + mkdir. + + Copy the files from + /etc/skel to the new home directory. + + + Fix ownerships and permissions with + chown and chmod. The + option is most useful. The correct + permissions vary a little from one site to another, but usually + the following commands do the right thing: + + +cd /home/newusername +chown -R username.group . +chmod -R go=u,go-w . +chmod go= . + + + + + Set the password with passwd. + + + + + + After you set the password in the last step, the account + will work. You shouldn't set it until everything else has been + done, otherwise the user may inadvertently log in while you're + still copying the files. + + + It is sometimes necessary to create dummy + accounts + + Surreal users? + + that are not used by people. For example, to set up an anonymous + FTP server (so that anyone can download files from it, without + having to get an account first), you need to create an account + called ftp. In such cases, it is usually not necessary to set + the password (last step above). Indeed, it is better not to, so + that no-one can use the account, unless they first become root, + since root can become any user. + + + + + + + + +Changing user properties + + + There are a few commands for changing various + properties of an account (i.e., the relevant field + in /etc/passwd): + + + chfn + Change the full name field. + + chsh + Change the login shell. + + passwd + Change the password. + + + + The super-user may use these commands to change the properties + of any account. Normal users can only change the properties + of their own account. It may sometimes be necessary to disable + these commands (with chmod) for normal users, + for example in an environment with many novice users. + + + Other tasks need to be done by hand. For example, to + change the username, you need to edit +/etc/passwd + directly (with vipw, remember). Likewise, to add + or remove the user to more groups, you need to edit + /etc/group (with vigr). Such +tasks tend to + be rare, however, and should be done with caution: for + example, if + you change the username, e-mail will no longer reach the + user, unless you also create a mail alias. + + The user's name might change due to + marriage, for example, and he might want to have his + username reflect his new name. + + + + + + +Removing a user + + To remove a user, you first remove all + his files, mailboxes, mail aliases, print jobs, + cron and at jobs, + and all other references to the user. Then you remove the + relevant lines from /etc/passwd and + /etc/group (remember to remove the username + from all groups it's been added to). It may be a good idea to + first disable the account (see below), before you start removing + stuff, to prevent the user from using the account while it is + being removed. + + + Remember that users may have files outside their home + directory. The find command can find them: + + +find / -user username + + + However, note that the above command will take a + long time, if you have large disks. If you + mount network disks, you need to be careful so that you won't + trash the network or the server. + + Some Linux distributions come with special + commands to do this; look for deluser or + userdel. However, it is easy to do it by + hand as well, and the commands might not do everything. + + + + +Disabling a user temporarily + + It is sometimes necessary to temporarily disable an + account, without removing it. For example, the user might not + have paid his fees, or the system administrator may suspect that + a cracker has got the password of that account. + + The best way to disable an account is to change its shell + into a special program that just prints a message. This way, + whoever tries to log into the account, will fail, and will + know why. The message can tell the user to contact the system + administrator so that any problems may be dealt with. + + + It would also be possible to change the username + or password to something else, but then the user + won't know what is going on. Confused users mean more + work. + + But they can be so + fun, if you're a BOFH. + + + + A simple way to create the special programs is to write + `tail scripts': + + +#!/usr/bin/tail +2 +This account has been closed due to a security breach. +Please call 555-1234 and wait for the men in black to arrive. + + + The first two characters (`#!') tell the + kernel that the rest of the line is a command that needs to be + run to interpret this file. The tail command + in this case outputs everything except the first line to the + standard output. + + + If user billg is suspected of a security breach, + the system administrator would do something like this: + + +# chsh -s +/usr/local/lib/no-login/security billg +# su - tester +This account has been closed due to a security breach. +Please call 555-1234 and wait for the men in black to arrive. +# + + + The purpose of the su is to test that the + change worked, of course. + + Tail scripts should be kept in a separate directory, + so that their names don't interfere with normal user commands. + + + + + + + + + +Backups + +
+Hardware is indeterministically reliable. +Software is deterministically unreliable. +People are indeterministically unreliable. +Nature is deterministically reliable. +
+ + This chapter explains about why, how, and when to make + backups, and how to restore things from backups. + + +On the importance of being backed up + + Your data is valuable. It will cost you time and effort + re-create it, and that costs money or at least personal grief + and tears; sometimes it can't even be re-created, e.g., if it + is the results of some experiments. Since it is an investment, + you should protect it and take steps to avoid losing it. + + There are basically four reasons why you might lose data: + hardware failures, software bugs, human action, or natural + disasters. + + The fifth reason is ``something + else''. + + Although modern hardware tends to be quite reliable, it can + still break seemingly spontaneously. The most critical piece + of hardware for storing data is the hard disk, which relies on + tiny magnetic fields remaining intact in a world filled with + electromagnetic noise. Modern software doesn't even tend to + be reliable; a rock solid program is an exception, not a rule. + Humans are quite unreliable, they will either make a mistake, or + they will be malicious and destroy data on purpose. Nature might + not be evil, but it can wreak havoc even when being good. All in + all, it is a small miracle that anything works at all. + + Backups are a way to protect the investment in data. + By having several copies of the data, it does not matter as much + if one is destroyed (the cost is only that of the restoration + of the lost data from the backup). + + It is important to do backups properly. Like everything + else that is related to the physical world, backups will fail + sooner or later. Part of doing backups well is to make sure + they work; you don't want to notice that your backups didn't work. + + Don't laugh. This has happened to + several people. + + Adding insult to injury, you might have a bad crash just as + you're making the backup; if you have only one backup medium, + it might destroyed as well, leaving you with the smoking ashes + of hard work. + + Been there, done that... + + Or you might notice, when trying to restore, that you forgot to + back up something important, like the user database on a 15000 + user site. Best of all, all your backups might be working + perfectly, but the last known tape drive reading the kind of + tapes you used was the one that now has a bucketful of water + in it. + + When it comes to backups, paranoia is in the job + description. + + + + +Selecting the backup medium + + The most important decision regarding backups is the choice + of backup medium. You need to consider cost, reliability, speed, + availability, and usability. + + Cost is important, since you should preferably have + several times more backup storage than what you need for the data. + A cheap medium is usually a must. + + Reliability is extremely important, since a broken + backup can make a grown man cry. A backup medium must be able + to hold data without corruption for years. The way you use the + medium affects it reliability as a backup medium. A hard disk + is typically very reliable, but as a backup medium it is not + very reliable, if it is in the same computer as the disk you + are backing up. + + Speed is usually not very important, if backups can be done + without interaction. It doesn't matter if a backup takes two + hours, as long as it needs no supervision. On the other hand, + if the backup can't be done when the computer would otherwise + be idle, then speed is an issue. + + Availability is obviously necessary, since you can't + use a backup medium if it doesn't exist. Less obvious is the + need for the medium to be available even in the future, and on + computers other than your own. Otherwise you may not be able + to restore your backups after a disaster. + + Usability is a large factor in how often backups are made. + The easier it is to make backups, the better. A backup medium + mustn't be hard or boring to use. + + The typical alternatives are floppies and tapes. + Floppies are very cheap, fairly reliable, not very fast, + very available, but not very usable for large amounts of data. + Tapes are cheap to somewhat expensive, fairly reliable, fairly + fast, quite available, and, depending on the size of the tape, + quite comfortable. + + There are other alternatives. They are usually not very + good on availability, but if that is not a problem, they can + be better in other ways. For example, magneto-optical disks + can have good sides of both floppies (they're random access, + making restoration of a single file quick) and tapes (contain + a lot of data). + + + + +Selecting the backup tool + + There are many tools that can be used to make + backups. The traditional UNIX tools used for backups + are tar, cpio, and + dump. In addition, there are large number + of third party packages (both freeware and commercial) that + can be used. The choice of backup medium can affect the choice + of tool. + + tar and cpio are + similar, and mostly equivalent from a backup point of view. + Both are capable of storing files on tapes, and retrieving + files from them. Both are capable of using almost any media, + since the kernel device drivers take care of the low level + device handling and the devices all tend to look alike to user + level programs. Some UNIX versions of tar + and cpio may have problems with unusual files + (symbolic links, device files, files with very long pathnames, and + so on), but the Linux versions should handle all files correctly. + + + dump is different in that it reads + the filesystem directly and not via the filesystem. It is + also written specifically for backups; tar + and cpio are really for archiving files, + although they work for backups as well. + + Reading the filesystem directly has some advantages. + It makes it possible to back files up without affecting their time + stamps; for tar and cpio, + you would have to mount the filesystem read-only first. + Directly reading the filesystem is also more effective, if + everything needs to be backed up, since it can be done with + much less disk head movement. The major disadvantage is that + it makes the backup program specific to one filesystem type; + the Linux dump program understands the ext2 + filesystem only. + + dump also directly supports + backup levels (which we'll be discussing below); with + tar and cpio this has to + be implemented with other tools. + + A comparison of the third party backup tools is beyond + the scope of this book. The Linux Software Map lists many of + the freeware ones. + + + + +Simple backups + + A simple backup scheme is to back up everything once, + then back up everything that has been modified since the + previous backup. The first backup is called a full + backup, the subsequent ones are incremental + backups. A full backup is often more labourious + than incremental ones, since there is more data to write to the + tape and a full backup might not fit onto one tape (or floppy). + Restoring from incremental backups can be many times more work + than from a full one. Restoration can be optimised so that + you always back up everything since the previous full backup; + this way, backups are a bit more work, but there should never + be a need to restore more than a full backup and an incremental + backup. + + If you want to make backups every day and have six + tapes, you could use tape 1 for the first full backup (say, on + a Friday), and tapes 2 to 5 for the incremental backups (Monday + through Thursday). Then you make a new full backup on tape 6 + (second Friday), and start doing incremental ones with tapes 2 + to 5 again. You don't want to overwrite tape 1 until you've got + a new full backup, lest something happens while you're making + the full backup. After you've made a full backup to tape 6, + you want to keep tape 1 somewhere else, so that when your other + backup tapes are destroyed in the fire, you still have at least + something left. When you need to make the next full backup, + you fetch tape 1 and leave tape 6 in its place. + + If you have more than six tapes, you can use the extra + ones for full backups. Each time you make a full backup, you + use the oldest tape. This way you can have full backups from + several previous weeks, which is good if you want to find an old, + now deleted file, or an old version of a file. + + +Making backups with <command>tar</command> + + + A full backup can easily be made with tar: + + +# tar --create --file /dev/ftape +/usr/src +tar: Removing leading / from absolute path names in +the archive +# + + + The example above uses the GNU version of tar + and its long option names. The traditional version of + tar only understands single character + options. The GNU version can also handle backups that don't + fit on one tape or floppy, and also very long paths; not all + traditional versions can do these things. (Linux only uses + GNU tar.) + + If your backup doesn't fit on one tape, you need to use + the () option: + + +# tar -cMf /dev/fd0H1440 +/usr/src +tar: Removing leading / from absolute path names in +the archive +Prepare volume #2 for /dev/fd0H1440 and hit return: +# + + + Note that you should format the floppies before you begin the + backup, or else use another window or virtual terminal and do + it when tar asks for a new floppy. + + After you've made a backup, you should check that it is OK, + using the () option: + + +# tar --compare --verbose -f +/dev/ftape +usr/src/ +usr/src/linux +usr/src/linux-1.2.10-includes/ +.... +# + + + Failing to check a backup means that you will not notice that your + backups aren't working until after you've lost the original data. + + + An incremental backup can be done with + tar using the + () option: + + +# tar --create --newer '8 Sep 1995' +--file /dev/ftape /usr/src +--verbose +tar: Removing leading / from absolute path names in +the archive +usr/src/ +usr/src/linux-1.2.10-includes/ +usr/src/linux-1.2.10-includes/include/ +usr/src/linux-1.2.10-includes/include/linux/ +usr/src/linux-1.2.10-includes/include/linux/modules/ +usr/src/linux-1.2.10-includes/include/asm-generic/ +usr/src/linux-1.2.10-includes/include/asm-i386/ +usr/src/linux-1.2.10-includes/include/asm-mips/ +usr/src/linux-1.2.10-includes/include/asm-alpha/ +usr/src/linux-1.2.10-includes/include/asm-m68k/ +usr/src/linux-1.2.10-includes/include/asm-sparc/ +usr/src/patch-1.2.11.gz +# + + + Unfortunately, tar can't notice when a file's + inode information has changed, for example, that its permission + bits have been changed, or when its name has been changed. + This can be worked around using find and + comparing current filesystem state with lists of files that have + been previously backed up. Scripts and programs for doing this + can be found on Linux ftp sites. + + + + +Restoring files with <command>tar</command> + + The () + option for tar extracts files: + + +# tar --extract --same-permissions +--verbose --file +/dev/fd0H1440 +usr/src/ +usr/src/linux +usr/src/linux-1.2.10-includes/ +usr/src/linux-1.2.10-includes/include/ +usr/src/linux-1.2.10-includes/include/linux/ +usr/src/linux-1.2.10-includes/include/linux/hdreg.h +usr/src/linux-1.2.10-includes/include/linux/kernel.h +... +# + + + You also extract only specific files or directories (which + includes all their files and subdirectories) by naming on the + command line: + + +# tar xpvf /dev/fd0H1440 +usr/src/linux-1.2.10-includes/include/linux/hdreg.h +usr/src/linux-1.2.10-includes/include/linux/hdreg.h +# + + + Use the () option, + if you just want to see what files are on a backup volume: + + +# tar --list --file +/dev/fd0H1440 +usr/src/ +usr/src/linux +usr/src/linux-1.2.10-includes/ +usr/src/linux-1.2.10-includes/include/ +usr/src/linux-1.2.10-includes/include/linux/ +usr/src/linux-1.2.10-includes/include/linux/hdreg.h +usr/src/linux-1.2.10-includes/include/linux/kernel.h +... +# + + + Note that tar always reads the backup volume + sequentially, so for large volumes it is rather slow. It is not + possible, however, to use random access database techniques when + using a tape drive or some other sequential medium. + + tar doesn't handle deleted files + properly. If you need to restore a filesystem from a full and + an incremental backup, and you have deleted a file between + the two backups, it will exist again after you have done the + restore. This can be a big problem, if the file has sensitive + data that should no longer be available. + + + + + + +Multilevel backups + + The simple backup method outlined in the previous section + is often quite adequate for personal use or small sites. For more + heavy duty use, multilevel backups are more appropriate. + + The simple method has two backup levels: full and + incremental backups. This can be generalised to any number of + levels. A full backup would be level 0, and the different levels + of incremental backups levels 1, 2, 3, etc. At each incremental + backup level you back up everything that has changed since the + previous backup at the same or a previous level. + + The purpose for doing this is that it allows a longer + backup history cheaply. In the example in + the previous section, the backup history went back to the previous + full backup. This could be extended by having more tapes, but + only a week per new tape, which might be too expensive. A longer + backup history is useful, since deleted or corrupted files are + often not noticed for a long time. Even a version of a file that + is not very up to date is better than no file at all. + + With multiple levels the backup history can be extended + more cheaply. For example, if we buy ten tapes, we could use + tapes 1 and 2 for monthly backups (first Friday each month), + tapes 3 to 6 for weekly backups (other Fridays; note that there + can be five Fridays in one month, so we need four more tapes), + and tapes 7 to 10 for daily backups (Monday to Thursday). + With only four more tapes, we've been able to extend the backup + history from two weeks (after all daily tapes have been used) + to two months. It is true that we can't restore every version + of each file during those two months, but what we can restore + is often good enough. + + shows which backup + level is used each day, and which backups can be restored from + at the end of the month. + +
+ A sample multilevel backup schedule. + +
+ + Backup levels can also be used to keep filesystem + restoration time to a minimum. If you have many incremental + backups with monotonously growing level numbers, you need to + restore all of them if you need to rebuild the whole filesystem. + Instead you can use level numbers that aren't monotonous, and + keep down the number of backups to restore. + + To minimise the number of tapes needed to restore, you + could use a smaller level for each incremental tape. However, + then the time to make the backups increases (each backup copies + everything since the previous full backup). A better scheme is + suggested by the dump manual page and described + by the table XX (efficient-backup-levels). Use the following + succession of backup levels: 3, 2, 5, 4, 7, 6, 9, 8, 9, etc. + This keeps both the backup and restore times low. The most you + have to backup is two day's worth of work. The number of tapes + for a restore depends on how long you keep between full backups, + but it is less than in the simple schemes. + + +Efficient backup scheme using many backup levels + + +Tape Level Backup +(days) Restore +tapes + + +1 0 n/a +1 +2 3 1 1, +2 +3 2 2 1, +3 +4 5 1 1, 2, +4 +5 4 2 1, 2, +5 +6 7 1 1, 2, +5, 6 +7 6 2 1, 2, +5, 7 +8 9 1 1, 2, +5, 7, 8 +9 8 2 1, 2, +5, 7, 9 +10 9 1 1, 2, +5, 7, 9, 10 +11 9 1 1, 2, +5, 7, 9, 10, +11 +... 9 1 1, +2, 5, 7, 9, 10, 11, +... + + +
+ + A fancy scheme can reduce the amount of labour needed, but + it does mean there are more things to keep track of. You must + decide if it is worth it. + + dump has built-in support for backup + levels. For tar and cpio + it must be implemented with shell scripts. + + + + +What to back up + + You want to back up as much as possible. The major + exception is software that can be easily reinstalled, + + You get to decide what's easy. + Some people consider installing from dozens of floppies + easy. + + but even they may have configuration files that it is + important to back up, lest you need to do all the work to + configure them all over again. Another major exception is + the /proc filesystem; since that only + contains data that the kernel always generates automatically, + it is never a good idea to back it up. Especially the + /proc/kcore file is unnecessary, since it + is just an image of your current physical memory; it's pretty + large as well. + + Gray areas include the news spool, log files, and many + other things in /var. You must decide what + you consider important. + + The obvious things to back up are user files + (/home) and system configuration files + (/etc, but possibly other things scattered + all over the filesystem). + + + + +Compressed backups + + Backups take a lot of space, which can cost quite + a lot of money. To reduce the space needed, the backups + can be compressed. There are several ways of doing this. + Some programs have support for for compression built in; for + example, the () + option for GNU tar pipes the whole backup + through the gzip compression program, before + writing it to the backup medium. + + Unfortunately, compressed backups can cause trouble. + Due to the nature of how compression works, if a single bit is + wrong, all the rest of the compressed data will be unusable. + Some backup programs have some built in error correction, but no + method can handle a large number of errors. This means that if + the backup is compressed the way GNU tar does + it, with the whole output compressed as a unit, a single error + makes all the rest of the backup lost. Backups must be reliable, + and this method of compression is not a good idea. + + An alternative way is to compress each file separately. + This still means that the one file is lost, but all other files + are unharmed. The lost file would have been corrupted anyway, + so this situation is not much worse than not using compression + at all. The afio program (a variant of + cpio) can do this. + + + Compression takes some time, which may make the backup program + unable to write data fast enough for a tape drive. + + If a tape drive doesn't data fast enough, + it has to stop; this makes backups even slower, and can + be bad for the tape and the drive. + + This can be avoided by buffering the output (either internally, if + the backup program if smart enough, or by using another program), + but even that might not work well enough. This should only be + a problem on slow computers. + + + + + + +Keeping Time + +
Time is an illusion. Lunchtime double + so. (Douglas Adams.)
+ + This chapter explains how a Linux system keeps time, + and what you need to do to avoid causing trouble. Usually, + you don't need to do anything about time, but it is good to + understand it. + + +Time zones + + Time measurement is based on mostly regular natural + phenomena, such as alternating light and dark periods caused + by the rotation of the planet. The total time taken by two + successive periods is constant, but the lengths of the light + and dark period vary. One simple constant is noon. + + Noon is the time of the day when the Sun is at its + highest position. Since the Earth is round, + + According to + recent research. + + noon happens at different times in different places. This leads + to the concept of local time. Humans + measure time in many units, most of which are tied to natural + phenomena like noon. As long as you stay in the same place, + it doesn't matter that local times differ. + + As soon as you need to communicate with distant places, + you'll notice the need for a common time. In modern times, + most of the places in the world communicate with most other + places in the world, so a global standard for measuring time + has been defined. This time is called universal + time (UT or UTC, formerly known as Greenwich Mean Time + or GMT, since it used to be local time in Greenwich, England). + When people with different local times need to communicate, + they can express times in universal time, so that there is no + confusion about when things should happen. + + Each local time is called a time zone. While geography + would allow all places that have noon at the same time have the + same time zone, politics makes it difficult. For various reasons, + many countries use daylight savings time, + that is, they move their clocks to have more natural light + while they work, and then move the clocks back during winter. + Other countries do not do this. Those that do, do not agree when + the clocks should be moved, and they change the rules from year + to year. This makes time zone conversions definitely non-trivial. + + + Time zones are best named by the location or by telling + the difference between local and universal time. In the US + and some other countries, the local time zones have a name and + a three letter abbreviation. The abbreviations are not unique, + however, and should not be used unless the country is also named. + It is better to talk about the local time in, say, Helsinki, + than about East European time, since not all countries in Eastern + Europe follow the same rules. + + Linux has a time zone package that knows about all + existing time zones, and that can easily be updated when the + rules change. All the system administrator needs to do is to + select the appropriate time zone. Also, each user can set his + own time zone; this is important since many people work with + computers in different countries over the Internet. When the + rules for daylight savings time change in your local time zone, + make sure you'll upgrade at least that part of your Linux system. + Other than setting the system time zone and upgrading the time + zone data files, there is little need to bother about time. + + + + + +The hardware and software clocks + + A personal computer has a battery driven hardware clock. + The battery ensures that the clock will work even if the rest of + the computer is without electricity. The hardware clock can be + set from the BIOS setup screen or from whatever operating system + is running. + + The Linux kernel keeps track of time independently from + the hardware clock. During the boot, Linux sets its own clock + to the same time as the hardware clock. After this, both clocks + run independently. Linux maintains its own clock because looking + at the hardware is slow and complicated. + + The kernel clock always shows universal time. This way, + the kernel does not need to know about time zones at all. The + simplicity results in higher reliability and makes it easier + to update the time zone information. Each process handles time + zone conversions itself (using standard tools that are part of + the time zone package). + + The hardware clock can be in local time or in universal + time. It is usually better to have it in universal time, + because then you don't need to change the hardware clock when + daylight savings time begins or ends (UTC does not have DST). + Unfortunately, some PC operating systems, including MS-DOS, + Windows, and OS/2, assume the hardware clock shows local time. + Linux can handle either, but if the hardware clock shows local + time, then it must be modified when daylight savings time begins + or ends (otherwise it wouldn't show local time). + + + + +Showing and setting time + + In the Debian system, the system time zone is determined + by the symbolic link /etc/localtime. + This link points at a time zone data file that describes + the local time zone. The time zone data files are stored in + /usr/lib/zoneinfo. Other Linux distributions + may do this differently. + + A user can change his private time zone by setting the + TZ environment variable. If it is unset, the system time zone + is assumed. The syntax of the TZ variable is described in the + tzset manual page. + + + The date command shows the current date and + time. + + Beware of the time command, +which does + not show the current time. + + For example: + + +$ date +Sun Jul 14 21:53:41 EET DST 1996 +$ + + + That time is Sunday, 14th of July, 1996, at about ten before + ten at the evening, in the time zone called ``EET DST'' + (which might be East European Daylight Savings Time). + date can also show the universal time: + + +$ date -u +Sun Jul 14 18:53:42 UTC 1996 +Sun Jul 14 18:53:42 UTC 1996 +$ + + + date is also used to set the kernel's software + clock: + + +# date 07142157 +Sun Jul 14 21:57:00 EET DST 1996 +# date +Sun Jul 14 21:57:02 EET DST 1996 +# + + + See the date manual page for more details; + the syntax is a bit arcane. Only root can set the time. + While each user can have his own time zone, the clock is the + same for everyone. + + date only shows or sets the software + clock. The clock commands synchronises + the hardware and software clocks. It is used when the system + boots, to read the hardware clock and set the software clock. + If you need to set both clocks, you first set the software clock + with date, and then the hardware clock with + clock -w. + + The option to clock + tells it that the hardware clock is in universal time. + You must use the + option correctly. If you don't, your computer will be quite + confused about what the time is. + + The clocks should be changed with care. Many parts of a + Unix system require the clocks to work correctly. For example, + the cron daemon runs commands periodically. + If you change the clock, it can be confused of whether + it needs to run the commands or not. On one early Unix + system, someone set the clock twenty years into the future, + and cron wanted to run all the periodic + commands for twenty years all at once. Current versions of + cron can handle this correctly, but you should + still be careful. Big jumps or backward jumps are more dangerous + than smaller or forward ones. + + + + +When the clock is wrong + + The Linux software clock is not always accurate. It is + kept running by a periodic timer interrupt + generated by PC hardware. If the system has too many processes + running, it may take too long to service the timer interrupt, and + the software clock starts slipping behind. The hardware clock + runs independently and is usually more accurate. If you boot + your computer often (as is the case for most systems that aren't + servers), it will usually keep fairly accurate time. + + If you need to adjust the hardware clock, it is usually + simplest to reboot, go into the BIOS setup screen, and do it + from there. This avoids all trouble that changing system time + might cause. If doing it via BIOS is not an option, set the new + time with date and clock + (in that order), but be prepared to reboot, if some part of the + system starts acting funny. + + A networked computer (even if just over the modem) can + check its own clock automatically, by comparing it to some other + computer's time. If the other computer is known to keep very + accurate time, then both computers will keep accurate time. + This can be done by using the rdate and + netdate commands. Both check the time of a + remote computer (netdate can handle several + remote computers), and set the local computer's time to that. + By running one these commands regularly, your computer will keep + as accurate time as the remote computer. + + XXX say something intelligent about NTP + + + + + + +Finding Help + +
Help me if you can I'm feeling down. And I do +appreciate you being 'round. - The +Beatles
+ + +Newsgroups and Mailing Lists + +This guide cannot teach you everything about Linux. There +just isn't enough space. It is almost inevitable that at some point +you will find something you need to do, that isn't covered in +this (or any other) document at the LDP. + + + +One of the nicest things about Linux is the large number of forums +devoted to it. There are forums relating to almost all facets of +Linux ranging from newbie FAQs to in depth kernel development issues. + +To receive the most from them, there are a few things you can do. + + + +Finding The Right Forum + +The first thing to do is to find an appropriate forum. There are many +newsgroups and mailing lists devoted to Linux, so try to find and use +the +one which most closely matches your question. For example, there +isn't much point in you asking a question about sendmail in a forum +devoted to Linux kernel development. At best the people there will +think +you are stupid and you will get few responses, at worst you may +receive +lots of highly insulting replies (flames). A quick look +through the newsgroups available finds comp.mail.sendmail, which +looks like an appropriate place to ask a sendmail question. Your news +client probably has a list of the newsgroups available to you, but if +not then a full list of newsgroups is available at +http://groups.google.com/groups?group=*. + + + + +Before You Post + +Now that you have found your appropriate forum, you may think you are +ready +to post your question. Stop. You aren't ready yet. Have you already +looked for the answer yourself? There are a huge number of HOWTOs and +FAQs available, if any of them relate to the thing you are having a +problem with then read them first. Even if they +don't +contain the answer to your problem, what they will do is give you a +better understanding of the subject area, and that understanding will +allow you +to ask a more informed and sensible question. There are also archives +of newsgroups and mailing lists and it is entirely possible that your +question has been asked and answered previously. http://www.google.com or a similar +search engine should be something you try before +posting a question. + + + + +Writing Your Post +Okay, you have found your appropriate forum, you have read the +relevant HOWTOs and FAQs, you have searched the web, but you still +have +not found the answer you need. Now you can start writing your post. +It is always a good idea to make it clear that you already have read +up +on the subject by saying something like ``I have read the +Winmodem-HOWTO and +the PPP FAQ, but neither contained what I was looking for, +searching for `Winmodem Linux PPP Setup' on google didn't return +anything +of use either''. This shows you to be someone who is willing to make +an +effort rather than a lazy idiot who requires spoonfeeding. The former +is likely to receive help if anyone knows the answer, the latter +is likely to meet with either stony silence or outright +derision. + +Write in clear, grammatical and correctly spelt English. This +is +incredibly important. It marks you as a precise and considered +thinker. +There are no such words as ``u'' or ``b4.'' Try to make yourself look +like an educated and intelligent person rather than an idiot. It will +help. I promise. + +Similarly do not type in all capitals LIKE THIS. That is +considered shouting and looks very rude. + +Provide clear details stating what the problem is and what you +have +already tried to do to fix it. A question like ``My linux has stopped +working, what can I do?'' is totally useless. Where has it stopped +working? In what way has it stopped working? You need to be as +precise +as possible. There are limits however. Try not to include irrelevant +information either. If you are having problems with your mail client +it +is unlikely that a dump of your kernel boot log +(dmesg) would be of help. + +Don't ask for replies by private email. The point of most Linux +forums is that everybody can learn something from each other. Asking +for private replies simply removes value from the newsgroup or mailing +list. + + + +Formatting Your Post + Do not post in HTML. Many Linux users have mail clients which +can't easily read HTML email. Whilst with some effort, they +can read HTML email, they usually don't. If you +send them HTML +mail it often gets deleted unread. Send plain text +emails, they will reach a wider audience that way. + + + +Follow Up +After your problem has been solved, post a short followup +explaining what the problem was and how you solved it. People will +appreciate this as it not only gives a sense of closure about the +problem but +also helps the next time someone has a similar question. When they +look at the archives of the newsgroup or mailing list, they will see +you +had the same problem, the discussion that followed your question and +your final solution. + + + +More Information +This short guide is simply a paraphrase +and summary of the excellent (and more detailed) document ``How To +Ask +Questions The Smart Way'' by Eric S Raymond. +http://www.tuxedo.org/~esr/faqs/smart-questions.html. It is +recommend that you read it before you post anything. It will help +you formulate +your question to maximise your +chances of getting the answer you are looking for. + + + + +IRC +IRC (Internet Relay Chat) is not covered in the Eric Raymond +document, but IRC +can also be an excellent way of finding the answers you need. +However it +does require some practice in asking questions in the right way. +Most IRC +networks have busy #linux channels and if the answer to your question +is contained in the manpages, or in the HOWTOs then expect to be told +to +go read them. The rule about typing in clear and grammatical English +still applies. + +Most of what has been said about newsgroups and mailing lists +is still +relevant for IRC, with a the following additions + + +Colours +Do not use colours, bold, underline or strange (non ASCII) +characters. This breaks some older terminals and is just plain ugly +to +look at. If you arrive in a channel and start spewing colour or bold +then expect to be kicked out. + + +Be Polite +Remember you are not entitled to an answer. If you ask the +question in the right way then you will probably get one, but you have +no right to get one. The people in Linux IRC channels are all there +on +their own time, nobody is paying them, especially not you. + +Be polite. Treat others as you would like to be +treated. If you think people are not being polite to you then don't +start calling them names or getting annoyed, become even politer. +This makes +them look foolish rather than dragging you down to their level. + +Don't go slapping anyone with large trouts. Would you believe +this +has been done before once or twice? And that we it wasn't +funny the first time? + + +Type Properly, in English +Most #linux channels are English channels. Speak English whilst +in them. Most of the larger IRC networks also have #linux channel in +other languages, for example the French language channel might be +called #linuxfr, the Spanish one might be #linuxes or #linuxlatino. +If +you can't find the right channel then asking in the main #linux +channel +(preferably in English) should help you find the one you are looking +for. + +Do not type like a ``1337 H4X0R d00d!!!''. Even if other people +are. It looks silly and thereby makes you look silly. At best you +will only +look like an idiot, at worst you will be derided then kicked +out. + + +Port scanning +Never ever as anyone to port scan you, or +try +to ``hack'' you. This is inviolable. There is no way of knowing that +you are who you say you are, or that the IP that you are connected +from +belongs to you. Don't put people in the position where they have to +say +no to a request like this. + +Don't ever port scan anyone, even if they +ask you +to. You have no way to tell +that they are who they say they are or that the IP they are connected +from +is their own IP. In some jurisdictions port scanning may be illegal +and it +is certainly against the Terms of Service of most ISPs. +Most people log TCP connections, they will notice they are being +scanned. Most people will report you to your ISP +for this (it is trivial to find out who that is). + + +Keep it in the Channel +Don't /msg anyone unless they ask you to. It diminishes the +usefulness of the channel and some people just prefer that +you not do it. + + +Stay On Topic +Stay on topic. The channel is a ``Linux'' channel, not a ``What +Uncle Bob Got Up To Last Weekend'' channel. Even if you see other +people being off topic, this does not mean that you should be. They +are +probably channel regulars and different conventions apply to +them. + + +CTCPs +If you are thinking of mass CTCP + If you are not familiar with IRC, CTCP stands + for Client To Client Protocol. It is a method whereby you can + find out things about other peoples' clients. See the + documentation for your IRC client for more + details +pinging the channel or CTCP +version or CTCP anything, then think again. It is liable to get you +kicked out very quickly. + + +Hacking, Cracking, Phreaking, Warezing +Don't ask about exploits, unless you are looking for a further +way +to be unceremoniously kicked out. + +Don't be in hacker/cracker/phreaker/warezer channels whilst in a +#linux channel. For some reason the people in charge of #linux +channels +seem to hate people who like causing destruction to people's machines +or who like to steal software. Can't imagine why. + + +Round Up +Apologies if that seems like a lot of DON'Ts, and very few DOs. + The +DOs were already pretty much covered in the section on newsgroups and +mailing lists. + +Probably the best thing you can do is to go into a #linux +channel, +sit there and watch, getting the feel for a half hour before +you say anything. This can help you to recognise the correct tone you +should be using. + + + +Further Reading +There are excellent FAQs about how to get the most of IRC #linux +channels. Most #linux channels have an FAQ and/or set or channel +rules. +How to find this will usually be in the channel topic (which you can +see +at any time using the /topic command. Make sure +you +read the rules if there are any and follow them. One fairly generic +set +of rules and advice is the ``Undernet #linux FAQ'' which can be found +at +http://linuxfaq.quartz.net.nz +. + + + + + +GNU Free Documentation License + + + + + + + Version 1.1, March 2000 + +
+ Copyright (C) 2000 Free Software Foundation, Inc. +59 Temple Place, Suite 330, Boston, MA 02111-1307 USA +Everyone is permitted to copy and distribute verbatim copies +of this license document, but changing it is not allowed. +
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The Librarian of the Unseen University + had unilaterally decided to aid comprehension + by producing an Orang-utan/Human Dictionary. + He'd been working on it for three months. + It wasn't easy. He'd got as far as `Oook.' + (Terry Pratchett, ``Men At Arms'')
+ + This is a short list of word definitions for concepts + relating to Linux and system administration. + + + CMOS RAM + + CMOS stands for "Complementary Metal Oxide Semiconductor". + It is a complex technology, but put very simply it is a type + of transistor which maintains its state even if there is no + power flowing, so it provides a sort of static RAM. ie RAM + which does not lose what it was storing when the power is + switched off. + + + + + account + + A Unix system gives users accounts. It + gives them a username and a password with which to log on to the + account. A home directory in which to store files is usually + provided, and permissions to access hardware and software. These + things taken as a whole are an account. + + + + + application program + + Software that does something useful. The results of using an + application program is what the computer was bought for. + See also system program, operating system. + + + + + bad block + + A block (usually one sector on a disk) that cannot reliably hold + data. + + + + + bad sector + + Similar to bad block but more precise in + the case where a block and a sector may be of differing sizes. + + + + + boot sector + + Usually the first sector on any given partition. It contains + a very short program (on the order of a few hundred bytes) which + will load and start running the operating system proper. + + + + + booting + + Everything that happens between the time the computer is + switched on and it is ready to accept commands/input from + the user is known as booting. + + + + + bootstrap loader + + A very small program (usually residing in ROM) which reads + a fixed location on a disk (eg. the MBR) + and passes control over to it. The data residing on that fixed + location is, in general, slightly bigger and more sophisticated, + and it then takes responsibility for loading the actual operating + system and passing control to it. + + + + + cylinder + + The set of tracks on a multi-headed disk + that may be accessed without head movement. In other words the + tracks which are the same distance from the spindle about which + the disk platters rotate. Placing data + that is more likely to be accessed at the same time on the same + cylinder can reduce the access time significantly as moving the + read-write heads is slow compared to the speed with which the + disks rotate. + + + + + daemon + + A process lurking in the background, usually unnoticed, until + something triggers it into action. For example, the +update + daemon wakes up every thirty seconds or so to flush the buffer + cache, and the sendmail daemon awakes whenever +someone sends + mail. + + + + + daylight savings time + + A time of the year during which clocks are set forward one hour. + Widely used around the world in summer so that evenings have more + daylight than they would otherwise. + + + + + disk controller + + A hardware circuit which translates instructions about disk access + from the operating system to the physical disk. This provides a + layer of abstraction that means that an operating system does not + need to know how to talk to the many different types of disks, but + only needs to know about the (comparatively low) number of types of + disk controller. Common disk controller types are IDE and SCSI. + + + + + file system + + The methods and data structures that an operating + system uses to keep track of files on a disk or partition; + the way the files are organised on the disk. Also used about + a partition or disk that is used to store the files + or the type of the filesystem. + + + + + emergency boot floppy + + A floppy disk which can be used to boot the system even + if the hard disk has suffered damage on its filesystem. + Most linux distributions offer to make one of these during + installation, this is highly recommended. If your Linux + distribution does not offer this facility then read the + Boot floppy HOWTO, available at the LDP (**Find URL to cite**). + + + + + filesystem + + A term which is used for two purposes and which can have two + subtly different meanings. It is either the collection of + files and directories on a drive (whether hard drive, floppy, + Cd-ROM, etc). Or it is the markers put onto the disk media + which the OS uses to decide where to write files to (inodes, + blocks, superblocks etc). The actual meaning can almost + always be inferred from context. + + + + + formatting + + Strictly, formatting is organising and marking the surface of + a disk into tracks, sectors + , and cylinders. It is also + sometimes (incorrectly) a term used to signify the action of + writing a filesystem to a disk (especially + in the MS Windows/MS DOS world). + + + + + fragmented + + When a file is not written to a disk in contiguous + blocks. If there is not enough free space to write + a full file to a disk in one continuous stream of + blocks then the file gets split up between two or + more parts of the disk surface. This is known as + fragmenting and can make the time for loading a + file longer as the disk has to seek for the rest of the file. + + + + + full backup + + Taking a copy of the whole filesystem to a backup media + (eg tape, floppy, or CD). + + + + + geometry + + How many cylinders, sectors per cylinder and heads a disk + drive has. + + + + + high level formatting + + An incorrect term for writing a filesystem to a disk. Often + used in the MS Windows and MS DOS world. + + + + + incremental backups + + A backup of what has changed in a filesystem since the last + full backup. Incremental + backups if used sensibly as part of a backup regime, + can save a lot of time and effort in maintaining a backup of data. + + + + + inode + + A data structure holding information about files in a Unix + file system. There is an inode for each file and a file is + uniquely identified by the file system on which it resides + and its inode number on that system. Each inode contains + the following information: the device where the inode resides, + locking information, mode and type of file, the number of links + to the file, the owner's user and group ids, the number of bytes + in the file, access and modification times, the time the inode + itself was last modified and the addresses of the file's + blocks on disk. A Unix directory is an association between + file leafnames and inode numbers. A file's inode number + can be found using the "-i" switch to ls. + + + + + kernel + + Part of an operating system that implements the interaction with + hardware and the sharing of resources. See also system program. + + + + + local time + + The official time in a local region (adjusted for location around + the Earth); established by law or custom. + + + + + logical partition + + A partition inside an extended partition, + which is ``logical'' in that it does not exist in reality, + but only inside the logical structure of the software. + + + + + low level formatting + + Synonymous with formatting and used in + the MS DOS world so differentiate from creating a filesystem + which is also known as formatting sometimes. + + + + + mail transfer agent + + (MTA) The program responsible for delivering e-mail messages. + Upon receiving a message from a mail user agent + or another MTA it stores it temporarily locally + and analyses the recipients and either delivers it (local + addressee) or forwards it to another MTA. In either case + it may edit and/or add to the message headers. A widely used + MTA for Unix is sendmail. + + + + + mail user agent + + (MUA) The program that allows the user to compose and read + electronic mail messages. The MUA provides the interface + between the user and the mail transfer agent + . Outgoing mail is eventually handed over to an + MTA for delivery while the incoming messages are picked up + from where the MTA left it (although MUAs running on + single-user machines may pick up mail using POP). + Examples of MUAs are pine, elm and mutt. + + + + + master boot record + + (MBR) The first logical sector on a disk, this is (usually) + where the BIOS looks to load a small program that will boot + the computer. + + + + + network file system + + (NFS) A protocol developed by Sun Microsystems, and defined in + RFC 1094 (FIND URL), which allows a computer to access files + over a network as if they were on its local disks. + + + + + operating system + + Software that shares a computer system's resources (processor, + memory, disk space, network bandwidth, and so on) between + users and the application programs they run. Controls access + to the system to provide security. See also kernel, system program, + application program. + + + + + partition + + A logical section of a disk. Each partition normally has its + own file system. Unix tends to treat partitions as though + they were separate physical entities. + + + + + password file + + A file that holds usernames and information about their accounts + like their password. On Unix systems this file is usually + /etc/passwd. On most modern Linux systems + the /etc/passwd file does not actually hold + password data. That tends to be held in a different file + /etc/shadow for security reasons. See manual pages + passwd(5) and shadow(5) for more information. + + + + + platters + + A physical disk inside a hard drive. Usually a hard drive is + made up of multiple physical disks stacked up on top of each + other. One individual disk is known as a platter + . + + + + + power on self test + + (POST) A series of diagnostic tests which are run when a computer + is powered on. Typically this might include testing the memory, + testing that the hardware configuration is the same as the last + saved configuration, checking that any floppy drives, or hard + drives which are known about by the BIOS are installed and working. + + + + + print queue + + A file (or set of files) which the print daemon + uses so that applications which wish to use the + printer do not have to wait until the print job they have sent + is finished before they can continue. It also allows multiple + users to share a printer. + + + + + read-write head + + A tiny electromagnetic coil and metal pole used to write and read + magnetic patterns on a disk. These coils move laterally against + the rotary motion on the platters. + + + + + root filesystem + + The parent of all the other filesystems mounted in a Unix filesystem + tree. Mounted as / it might have other filesystems mounted on it + (/usr for example). If the root filesystem cannot be mounted then +the + kernel will panic and the system will not be + able to continue booting + + + + + run level + + Linux has up to 10 runlevels (0-9) available (of which usually only + the first 7 are defined). Each runlevel may start a different set + of services, giving multiple different configurations in the same + system. Runlevel 0 is defined as ``system halt'', runlevel 1 is + defined as ``single user mode'', and runlevel + 6 is defined as ``reboot system''. The remaining runlevels can, + theoretically, be defined by the system administrator in any way. + However most distributions provide some other predefined runlevels. + For example, runlevel 2 might be defined as ``multi-user console'', + and runlevel 5 as ``multi-user X-Window system''. These definitions + vary considerably from distribution to distribution, so please check + the documentation for your own distribution. + + + + + sectors + + The minimum track length that can be +allocated + to store data. This is usually (but not always) 512 bytes. + + + + + shadow passwords + + Because the password file on Unix systems +often + needs to be world readable it usually does not actually contain the + encrypted passwords for users' accounts. Instead a shadow file is + employed (which is not world readable) which holds the encrypted + passwords for users' accounts. + + + + + single user mode + + Usually runlevel 1. A runlevel where logins are not allowed except + by the root account. Used either for system repairs (if the + filesystem is partially damaged it may still be possible to boot into + runlevel 1 and repair it), or for moving filesystems around between + partitions. These are just two examples. Any task that requires a + system where only one person can write to a disk at a time is a + candidate for requiring runlevel 1. + + + + + spool + + To send a file (or other data) to a queue. Generally used in + conjunction with printers, but might also be used for other + things (mail for example). The term is reported to be an acronym + for ``Simultaneous Peripheral Operation On-Line'', but according + to the Jargon File + it may have been a backronym (something made up later + for effect). + + + + + system call + + The services provided by the kernel to application programs, + and the way in which they are invoked. See section 2 of the + manual pages. + + + + + swap space + + Space on a disk in which the system can write portions of memory + to. Usually this is a dedicated partition, but it may also be + a swapfile. + + + + + system program + + Programs that implement high level functionality of an operating + system, i.e., things that aren't directly dependent on the + hardware. May sometimes require special privileges to run + (e.g., for delivering electronic mail), but often just commonly + thought of as part of the system (e.g., a compiler). See also + application program, kernel, operating system. + + + + + track + + The part of a disk platter which passes + under one read-write head while the head + is stationary but the disk is spinning. Each track is divided + into sectors, and a vertical collection of + tracks is a cylinder + + + + + + + + + + + +