Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
echo "uninitialized_var = $uninitialized_var"
# uninitialized_var =
Mixing up = and −eq in a test. Remember, = is for comparing literal variables and −eq for integers.
if [ "$a" = 273 ]
if [ "$a" −eq 273 ]
# Is $a an integer or string?
# If $a is an integer.
# Sometimes you can mix up −eq and = without adverse consequences.
# However...
a=273.0
# Not an integer.
if [ "$a" = 273 ]
then
echo "Comparison works."
else
echo "Comparison does not work."
fi # Comparison does not work.
# Same with a=" 273" and a="0273".
# Likewise, problems trying to use "−eq" with non−integer values.
if [ "$a" −eq 273.0 ]
then
echo "a = $a'
fi # Aborts with an error message.
# test.sh: [: 273.0: integer expression expected
Mixing up integer and string comparison operators.
#!/bin/bash
# bad−op.sh
number=1
while [ "$number" < 5 ] # Wrong! Should be while [ "number" −lt 5 ]
do
echo −n "$number "
let "number += 1"
done
# Attempt to run this bombs with the error message:
# bad−op.sh: 5: No such file or directory
Sometimes variables within "test" brackets ([ ]) need to be quoted (double quotes). Failure to do so may cause
unexpected behavior. See Example 7−6, Example 16−4, and Example 9−6.
Commands issued from a script may fail to execute because the script owner lacks execute permission for
them. If a user cannot invoke a command from the command line, then putting it into a script will likewise
fail. Try changing the attributes of the command in question, perhaps even setting the suid bit (as root, of
course).
Attempting to use − as a redirection operator (which it is not) will usually result in an unpleasant surprise.
Chapter 32. Gotchas 328