Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
then
echo "(null)"
return
fi
#+ send error message
#+ (C−style void−pointer error message)
#+ and return from function.
echo "$@" | tr A−Z a−z
# Translate all passed arguments ($@).
return
# Use command substitution to set a variable to function output.
# For example:
# oldvar="A seT of miXed−caSe LEtTerS"
# newvar=`tolower "$oldvar"`
# echo "$newvar" # a set of mixed−case letters
#
# Exercise: Rewrite this function to change lowercase passed argument(s)
# to uppercase ... toupper() [easy].
}
• Use special−purpose comment headers to increase clarity and legibility in scripts.
## Caution.
rm −rf *.zzy
## The "−rf" options to "rm" are very dangerous,
##+ especially with wildcards.
#+ Line continuation.
# This is line 1
#+ of a multi−line comment,
#+ and this is the final line.
#* Note.
#o List item.
#> Another point of view.
while [ "$var1" != "end" ] #> while test "$var1" != "end"
• Using the $? exit status variable, a script may test if a parameter contains only digits, so it can be
treated as an integer.
#!/bin/bash
SUCCESS=0
E_BADINPUT=65
test "$1" −ne 0 −o "$1" −eq 0 2>/dev/null
# An integer is either equal to 0 or not equal to 0.
# 2>/dev/null suppresses error message.
if [ $? −ne "$SUCCESS" ]
then
echo "Usage: `basename $0` integer−input"
exit $E_BADINPUT
fi
let "sum = $1 + 25"
echo "Sum = $sum"
# Would give error if $1 not integer.
# Any variable, not just a command line parameter, can be tested this way.
exit 0
Chapter 34. Miscellany 349