Advanced Bash−Scripting Guide

Example 7−5. arithmetic and string comparisons
Advanced Bash−Scripting Guide
#!/bin/bash
a=4
b=5
# Here "a" and "b" can be treated either as integers or strings.
# There is some blurring between the arithmetic and string comparisons,
#+ since Bash variables are not strongly typed.
# Bash permits integer operations and comparisons on variables
#+ whose value consists of all−integer characters.
# Caution advised.
echo
if [ "$a" −ne "$b" ]
then
echo "$a is not equal to $b"
echo "(arithmetic comparison)"
fi
echo
if [ "$a" != "$b" ]
then
echo "$a is not equal to $b."
echo "(string comparison)"
# "4" != "5"
# ASCII 52 != ASCII 53
fi
# In this particular instance, both "−ne" and "!=" work.
echo
exit 0
Example 7−6. testing whether a string is null
#!/bin/bash
# str−test.sh: Testing null strings and unquoted strings,
# but not strings and sealing wax, not to mention cabbages and kings...
# Using if [ ... ]
# If a string has not been initialized, it has no defined value.
# This state is called "null" (not the same as zero).
if [ −n $string1 ] # $string1 has not been declared or initialized.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Wrong result.
# Shows $string1 as not null, although it was not initialized.
Chapter 7. Tests 51