Advanced Bash−Scripting Guide

if [[ −e $file ]]
then
echo "Password file exists."
fi
Advanced Bash−Scripting Guide
Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example,
the &&, ||, operators work within a [[ ]] test, despite giving an error within a [ ] construct.
Following an if, neither the test command nor the test brackets ( [ ] or [[ ]] ) are strictly
necessary.
dir=/home/bozo
if cd "$dir" 2>/dev/null; then
echo "Now in $dir."
else
echo "Can't change to $dir."
fi
# "2>/dev/null" hides error message.
The "if COMMAND" construct returns the exit status of COMMAND.
Similarly, a condition within test brackets may stand alone without an if, when used in
combination with a list construct.
var1=20
var2=22
[ "$var1" −ne "$var2" ] && echo "$var1 is not equal to $var2"
home=/home/bozo
[ −d "$home" ] || echo "$home directory does not exist."
The (( )) construct expands and evaluates an arithmetic expression. If the expression evaluates as zero, it
returns an exit status of 1, or "false". A non−zero expression returns an exit status of 0, or "true". This is in
marked contrast to using the test and [ ] constructs previously discussed.
Example 7−3. Arithmetic Tests using (( ))
#!/bin/bash
# Arithmetic tests.
# The (( ... )) construct evaluates and tests numerical expressions.
# Exit status opposite from [ ... ] construct!
(( 0 ))
echo "Exit status of \"(( 0 ))\" is $?." # 1
(( 1 ))
echo "Exit status of \"(( 1 ))\" is $?." # 0
(( 5 > 4 )) # true
echo "Exit status of \"(( 5 > 4 ))\" is $?." # 0
(( 5 > 9 )) # false
echo "Exit status of \"(( 5 > 9 ))\" is $?." # 1
Chapter 7. Tests 45