391 lines
7.5 KiB
HTML
391 lines
7.5 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>Recursion: a script calling itself</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Advanced Bash-Scripting Guide"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="Miscellany"
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HREF="miscellany.html"><LINK
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TITLE="Tests and Comparisons: Alternatives"
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HREF="testsandcomparisons.html"><LINK
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REL="NEXT"
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TITLE="Colorizing Scripts"
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HREF="colorizing.html"></HEAD
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><TR
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><TH
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COLSPAN="3"
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>Advanced Bash-Scripting Guide: </TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 36. Miscellany</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="colorizing.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="RECURSIONSCT"
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></A
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>36.4. Recursion: a script calling itself</H1
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><P
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><A
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NAME="SCRIPTRECURSION"
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></A
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></P
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><P
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>Can a script <A
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HREF="localvar.html#RECURSIONREF"
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>recursively</A
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>
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call itself? Indeed.</P
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><DIV
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CLASS="EXAMPLE"
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><A
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NAME="RECURSE"
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></A
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><P
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><B
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>Example 36-10. A (useless) script that recursively calls itself</B
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></P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="PROGRAMLISTING"
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>#!/bin/bash
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# recurse.sh
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# Can a script recursively call itself?
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# Yes, but is this of any practical use?
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# (See the following.)
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RANGE=10
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MAXVAL=9
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i=$RANDOM
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let "i %= $RANGE" # Generate a random number between 0 and $RANGE - 1.
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if [ "$i" -lt "$MAXVAL" ]
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then
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echo "i = $i"
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./$0 # Script recursively spawns a new instance of itself.
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fi # Each child script does the same, until
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#+ a generated $i equals $MAXVAL.
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# Using a "while" loop instead of an "if/then" test causes problems.
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# Explain why.
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exit 0
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# Note:
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# ----
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# This script must have execute permission for it to work properly.
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# This is the case even if it is invoked by an "sh" command.
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# Explain why.</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="EXAMPLE"
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><A
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NAME="PBOOK"
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></A
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><P
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><B
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>Example 36-11. A (useful) script that recursively calls itself</B
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></P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="PROGRAMLISTING"
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>#!/bin/bash
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# pb.sh: phone book
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# Written by Rick Boivie, and used with permission.
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# Modifications by ABS Guide author.
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MINARGS=1 # Script needs at least one argument.
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DATAFILE=./phonebook
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# A data file in current working directory
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#+ named "phonebook" must exist.
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PROGNAME=$0
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E_NOARGS=70 # No arguments error.
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if [ $# -lt $MINARGS ]; then
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echo "Usage: "$PROGNAME" data-to-look-up"
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exit $E_NOARGS
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fi
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if [ $# -eq $MINARGS ]; then
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grep $1 "$DATAFILE"
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# 'grep' prints an error message if $DATAFILE not present.
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else
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( shift; "$PROGNAME" $* ) | grep $1
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# Script recursively calls itself.
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fi
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exit 0 # Script exits here.
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# Therefore, it's o.k. to put
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#+ non-hashmarked comments and data after this point.
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# ------------------------------------------------------------------------
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Sample "phonebook" datafile:
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John Doe 1555 Main St., Baltimore, MD 21228 (410) 222-3333
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Mary Moe 9899 Jones Blvd., Warren, NH 03787 (603) 898-3232
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Richard Roe 856 E. 7th St., New York, NY 10009 (212) 333-4567
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Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678
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Zoe Zenobia 4481 N. Baker St., San Francisco, SF 94338 (415) 501-1631
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# ------------------------------------------------------------------------
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$bash pb.sh Roe
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Richard Roe 856 E. 7th St., New York, NY 10009 (212) 333-4567
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Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678
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$bash pb.sh Roe Sam
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Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678
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# When more than one argument is passed to this script,
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#+ it prints *only* the line(s) containing all the arguments.</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="EXAMPLE"
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><A
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NAME="USRMNT"
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></A
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><P
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><B
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>Example 36-12. Another (useful) script that recursively calls itself</B
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></P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="PROGRAMLISTING"
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>#!/bin/bash
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# usrmnt.sh, written by Anthony Richardson
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# Used in ABS Guide with permission.
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# usage: usrmnt.sh
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# description: mount device, invoking user must be listed in the
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# MNTUSERS group in the /etc/sudoers file.
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# ----------------------------------------------------------
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# This is a usermount script that reruns itself using sudo.
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# A user with the proper permissions only has to type
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# usermount /dev/fd0 /mnt/floppy
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# instead of
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# sudo usermount /dev/fd0 /mnt/floppy
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# I use this same technique for all of my
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#+ sudo scripts, because I find it convenient.
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# ----------------------------------------------------------
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# If SUDO_COMMAND variable is not set we are not being run through
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#+ sudo, so rerun ourselves. Pass the user's real and group id . . .
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if [ -z "$SUDO_COMMAND" ]
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then
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mntusr=$(id -u) grpusr=$(id -g) sudo $0 $*
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exit 0
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fi
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# We will only get here if we are being run by sudo.
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/bin/mount $* -o uid=$mntusr,gid=$grpusr
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exit 0
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# Additional notes (from the author of this script):
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# -------------------------------------------------
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# 1) Linux allows the "users" option in the /etc/fstab
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# file so that any user can mount removable media.
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# But, on a server, I like to allow only a few
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# individuals access to removable media.
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# I find using sudo gives me more control.
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# 2) I also find sudo to be more convenient than
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# accomplishing this task through groups.
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# 3) This method gives anyone with proper permissions
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# root access to the mount command, so be careful
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# about who you allow access.
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# You can get finer control over which access can be mounted
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# by using this same technique in separate mntfloppy, mntcdrom,
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# and mntsamba scripts.</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="CAUTION"
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><P
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></P
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><TABLE
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CLASS="CAUTION"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/caution.gif"
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HSPACE="5"
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ALT="Caution"></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>Too many levels of recursion can exhaust the
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script's stack space, causing a segfault.</P
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></TD
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></TR
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></TABLE
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></DIV
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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ALIGN="left"
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VALIGN="top"
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HREF="testsandcomparisons.html"
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ACCESSKEY="P"
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>Prev</A
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VALIGN="top"
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HREF="index.html"
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HREF="colorizing.html"
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WIDTH="33%"
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VALIGN="top"
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>Tests and Comparisons: Alternatives</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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HREF="miscellany.html"
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>Up</A
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VALIGN="top"
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CLASS="QUOTE"
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>"Colorizing"</SPAN
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> Scripts</TD
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