updated

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex15.sh

@@ -1,19 +1,34 @@
 #!/bin/bash
 
-#When is a variable "naked", i.e., lacking the '$' in front?
+echo
+
+# When is a variable "naked", i.e., lacking the '$' in front?
 
 # Assignment
 a=879
-echo $a
+echo "The value of \"a\" is $a"
 
 # Assignment using 'let'
 let a=16+5
-echo $a
+echo "The value of \"a\" is now $a"
+
+echo
 
 # In a 'for' loop (really, a type of disguised assignment)
+echo -n "The values of \"a\" in the loop are "
 for a in 7 8 9 11
 do
-  echo $a
+  echo -n "$a "
 done
 
+echo
+echo
+
+# In a 'read' statement
+echo -n "Enter \"a\" "
+read a
+echo "The value of \"a\" is now $a"
+
+echo
+
 exit 0

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex21.sh

@@ -1,9 +1,94 @@
 #!/bin/bash
 
-# Prints different random integer
-# at each invocation.
+# $RANDOM returns a different random integer at each invocation.
+# Nominal range: 0 - 32767 (signed integer).
+
+MAXCOUNT=10
+count=1
+
+echo
+echo "$MAXCOUNT random numbers:"
+echo "-----------------"
+while [ $count -le $MAXCOUNT ]      # Generate 10 ($MAXCOUNT) random integers.
+do
+  number=$RANDOM
+  echo $number
+  let "count += 1"  # Increment count.
+done
+echo "-----------------"
+
+# If you need a random int within a certain range, then use the 'modulo' operator.
+
+RANGE=500
+
+echo
+
+number=$RANDOM
+let "number %= $RANGE"
+echo "Random number less than $RANGE  -->  $number"
+
+echo
+
+# If you need a random int greater than a lower bound,
+# then set up a test to discard all numbers below that.
+
+FLOOR=200
+
+number=0   #initialize
+while [ $number -le $FLOOR ]
+do
+  number=$RANDOM
+done
+echo "Random number greater than $FLOOR -->  $number"
+echo
+
+
+# May combine above two techniques to retrieve random number between two limits.
+number=0   #initialize
+while [ $number -le $FLOOR ]
+do
+  number=$RANDOM
+  let "number %= $RANGE"
+done
+echo "Random number between $FLOOR and $RANGE -->  $number"
+echo
+
+
+# May generate binary choice, that is, "true" or "false" value.
+BINARY=2
+number=$RANDOM
+let "number %= $BINARY"
+if [ $number -eq 1 ]
+then
+  echo "TRUE"
+else
+  echo "FALSE"
+fi  
+
+echo
+
+
+# May generate toss of the dice.
+SPOTS=7
+DICE=2
+die1=0
+die2=0
+
+# Tosses each die separately, and so gives correct odds.
+
+  while [ $die1 -eq 0 ]   #Can't have a zero come up.
+  do
+    let "die1 = $RANDOM % $SPOTS"
+  done  
+
+  while [ $die2 -eq 0 ]
+  do
+    let "die2 = $RANDOM % $SPOTS"
+  done  
+
+let "throw = $die1 + $die2"
+echo "Throw of the dice = $throw"
+echo
 
-a=$RANDOM
-echo $a
 
 exit 0

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex36.sh

@@ -4,9 +4,18 @@ echo -n "Enter the value of variable 'var1': "
 # -n option to echo suppresses newline
 
 read var1
-# Note no '$' in front of var1,
-# since it is being set.
+# Note no '$' in front of var1, since it is being set.
 
 echo "var1 = $var1"
 
+
+# Note that a single 'read' statement can set multiple variables.
+
+echo
+
+echo -n "Enter the values of variables 'var2' and 'var3' (separated by a space or tab): "
+read var2 var3
+echo "var2 = $var2      var3 = $var3"
+# If you input only one value, the other variable(s) will remain unset (null).
+
 exit 0

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex39.sh

@@ -12,12 +12,13 @@ updatedb /usr &
 # Must be run as root.
 
 wait
-# Don't run the rest of the script
-# until 'updatedb' finished.
-# In this case, you want the the database updated
-# before looking up the file name.
+# Don't run the rest of the script until 'updatedb' finished.
+# You want the the database updated before looking up the file name.
 
 locate $1
 
+# Lacking the wait command, in the worse case scenario,
+# the script would exit while 'updatedb' was still running,
+# leaving it as an orphan process.
 
 exit 0

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex40.sh

@@ -16,7 +16,10 @@ else
     IMAGE_DIRECTORY=$1
 fi
     
-ls -lR $IMAGE_DIRECTORY > $IMAGE_DIRECTORY/contents
+ls -lRF $IMAGE_DIRECTORY > $IMAGE_DIRECTORY/contents
+# The "l" option gives a "long" file listing.
+# The "R" option makes the listing recursive.
+# The "F" option marks the file types (directories suffixed by a /).
 echo "Creating table of contents."
 
 mkisofs -r -o cdimage.iso $IMAGE_DIRECTORY

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex45.sh

@@ -60,6 +60,8 @@ fi
 echo
 echo
 
+
+
 # String Operators
 
 echo String Operators
@@ -67,15 +69,19 @@ echo
 
 a=1234zipper43231
 echo The string being operated upon is $a.
+
 # index: position of substring
 b=`expr index $a 23`
 echo Numerical position of first 23 in $a is $b.
+
 # substr: print substring, starting position & length specified
 b=`expr substr $a 2 6`
 echo Substring of $a, starting at position 2 and 6 chars long is $b.
+
 # length: length of string
 b=`expr length $a`
 echo Length of $a is $b.
+
 # 'match' operations similarly to 'grep'
 b=`expr match $a [0-9]*`
 echo Number of digits at the beginning of $a is $b.

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex5.sh

@@ -17,4 +17,7 @@ exit 143
 # To verify this, type $? after script terminates.
 
 # By convention, an 'exit 0' shows success,
-# while a non-zero exit value indicates an error.
+# while a non-zero exit value indicates an error or anomalous condition.
+
+# It is also appropriate for the script to use the exit status
+# to communicate with other processes, as when in a pipe with other scripts.

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex55.sh

@@ -5,6 +5,8 @@
 # --> This is part of the 'rc' script package
 # --> by Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>
 
+# --> This particular script seems to be Red Hat specific
+# --> (may not be present in other distributions).
 
 # Bring down all unneeded services that are still running (there shouldn't 
 # be any, so this is just a sanity check)

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex6.sh

@@ -1,9 +1,43 @@
 #!/bin/bash
 
-: ${HOSTNAME?} {USER?} {MAIL?}
-  echo $HOSTNAME
-  echo $USER
-  echo $MAIL
-  echo Critical env. variables set.
+# Let's check some of the system's environmental variables.
+# If, for example, $USER, the name of the person
+# at the console, is not set, the machine will not
+# recognize you.
+
+: ${HOSTNAME?} ${USER?} ${HOME} ${MAIL?}
+  echo
+  echo "Name of the machine is $HOSTNAME."
+  echo "You are $USER."
+  echo "Your home directory is $HOME."
+  echo "Your mail INBOX is located in $MAIL."
+  echo
+  echo "If you are reading this message,"
+  echo "critical environmental variables have been set."
+  echo
+  echo
+
+# The ':' operator seems fairly error tolerant.
+# This script works even if the '$' omitted in front of
+# {HOSTNAME}, {USER?}, {HOME?}, and {MAIL?}. Why?
+
+# ------------------------------------------------------
+
+# The ${variablename?} construction can also check
+# for variables set within the script.
+
+ThisVariable=Value-of-ThisVariable
+# Note, by the way, that string variables may be set
+# to characters disallowed in their names.
+: ${ThisVariable?}
+echo "Value of ThisVariable is $ThisVariable".
+echo
+echo
+
+# If ZZXy23AB has not been set...
+: ${ZZXy23AB?}
+# This will give you an error message and terminate.
+
+echo "You will not see this message."
 
 exit 0 

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex63.sh

@@ -1,10 +1,13 @@
 #!/bin/bash
 
+#               factorial
+#               ---------
+
+
 # Does bash permit recursion?
 # Well, yes, but...
 # You gotta have rocks in your head to try it.
 
-# Name this script "factorial".
 
 MAX_ARG=5
 WRONG_ARGS=1
@@ -21,22 +24,20 @@ if [ $1 -gt $MAX_ARG ]
 then
   echo "Out of range (5 is maximum)."
   # Let's get real now...
-  # If you want greater range, rewrite this
-  # in a real programming language.
+  # If you want greater range than this, rewrite it in a real programming language.
   exit $RANGE_ERR
 fi  
 
 fact ()
 {
   local number=$1
-  # number must be declared as local
-  # otherwise this doesn't work.
+  # Variable "number" must be declared as local otherwise this doesn't work.
   if [ $number -eq 0 ]
   then
     factorial=1
   else
     let "decrnum = number - 1"
-    fact $decrnum
+    fact $decrnum  # Recursive function call.
     let "factorial = $number * $?"
   fi
 

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex67.sh

@@ -11,15 +11,25 @@ read -a colors
 
 echo
 
-element_count=${#colors[@]}
-# Special syntax to extract number of elements in array.
+  element_count=${#colors[@]} # Special syntax to extract number of elements in array.
+# element_count=${#colors[*]} works also.
 index=0
 
+# List all the elements in the array.
 while [ $index -lt $element_count ]
 do
   echo ${colors[$index]}
   let "index = $index + 1"
 done
+# Each array element listed on a separate line.
+# If this is not desired, use  echo -n "${colors[$index]} "
+
+echo
+
+# Again, list all the elements in the array, but using a more elegant method.
+  echo ${colors[@]}
+# echo ${colors[*]} works also.
+
 
 echo
 

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex7.sh

@@ -29,8 +29,12 @@ echo "$path_name, with first 11 chars stripped off, length 5 = $t"
 echo
 
 t=${path_name/bozo/clown}
-echo "$path_name with bozo replaced = $t"
+echo "$path_name with \"bozo\" replaced  by \"clown\" = $t"
+t=${path_name/today/}
+echo "$path_name with \"today\" deleted = $t"
 t=${path_name//o/O}
 echo "$path_name with all o's capitalized = $t"
+t=${path_name//o/}
+echo "$path_name with all o's deleted = $t"
 
 exit 0

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex76.sh

@@ -1,13 +1,12 @@
 #!/bin/bash
 
 trap 'echo Variable Listing --- a = $a  b = $b' EXIT
-# EXIT is the name of the signal generated
-# upon exit from a script.
+# EXIT is the name of the signal generated upon exit from a script.
 
 a=39
 
 b=36
 
 exit 0
-# Note that commenting out the 'exit' command
-# does not make a difference.
+# Note that commenting out the 'exit' command makes no difference,
+# since the script exits anyhow after running out of commands.

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex78.sh

@@ -12,6 +12,7 @@ echo "a = $a"
 
 # Indirect reference.
 echo "Now a = ${!a}"
+# The ${!variable} notation is greatly superior to the old "eval var1=\$$var2"
 
 echo
 

LDP/howto/docbook/Adv-Bash-Scr-HOWTO/ex9.sh

@@ -1,21 +1,40 @@
 #!/bin/bash
 
+# Variables: assignment and substitution
+
 a=37.5
 hello=$a
-# No space permitted on either side of = sign.
+# No space permitted on either side of = sign when initializing variables.
 
 echo hello
+# Not a reference.
 
 echo $hello
-echo ${hello} #Identical as above.
+echo ${hello} #Identical to above.
 
 echo "$hello"
 echo "${hello}"
 
 echo '$hello'
-# Variable referencing disabled by single quotes.
+# Variable referencing disabled by single quotes,
+# because $ interpreted literally.
 
-# Notice the effect of different
-# types of quoting.
+# Notice the effect of different types of quoting.
+
+# ------------------------------------------------
+
+echo; echo
+
+numbers="one two three"
+other_numbers="1 2 3"
+# If whitespace within variables, then quotes necessary.
+echo "numbers = $numbers"
+echo "other_numbers = $other_numbers"
+echo
+
+echo "uninitialized variable = $uninitialized_variable"
+# Uninitialized variable has null value (no value at all).
+
+echo
 
 exit 0