Shell Test Blocks

[ is defined by the POSIX standard and thus it’s supported by all POSIX shells. On the other hand, although [[ is more powerful it only works with bash, zsh and ksh.

Initially [ was implemented as a command (/usr/bin/[). Because of that, the expression needs to be surrounded by spaces: [ expr ] and [[ expr ]].

Differences

Feature	`[ ... ]` old style	`[[ ... ]]` new style	Comments
Variable expansion	`"$v"`	`$v`	New style will not split the variable if it contains spaces.
String comparison	`=`	`=` or `==`
	`!=`	`!=`
	`\>` and `\<`	`>` and `<`
Integer comparison	`-ge`, `-le`, `-eq`	`-gt`, `-lt`, `-eq`	No differences.
	`-gt`, `-lt`, `-ne`	`-gt`, `-lt`, `-eq`
Logical operators	Deprecated by POSIX	`&&` and \|\|	See below.
Grouping	Deprecated by POSIX	`(` and `)`	See below.
Glob matching	Not available	`[[ $v == *.txt ]]`
Regex matching	Not available	`[[ $v =~ .*\.txt ]]`

POSIX deprecates -a, -o, $ and $. So instead of using:

if [ -f file1 -a \( "$var1" -eq 0 -i "$var2" -eq 0 \) ]; then
	echo true
fi

It’s necessary to use:

if [ -f file1 ] && { [ "$var1" -eq 0 ] || [ "$var2" -eq 0 ]; }; then
	echo true
fi

While that could be written in the following way with [[:

if [[ -f file 1 && ( $var1 -eq 0 || $var2 -eq 0 ) ]]; then
	echo true
fi

Besides the differences above, [ might also lack the following operators depending on the implementation: -e, -nt (newer than), -ot (older than), -ef (exact same file) and ! (not).

Arithmetic Expansion

(( ... )) and $(( ... )) can be used for integer comparison and arithmetic operations.

Examples:

# POSIX sh
i=$((j + 3))
lvcreate -L "$((24 * 1024))" -n lv99 vg99
q=$((29 / 6)) r=$((29 % 6))
if test "$((a%4))" = 0; then echo "true"; fi

# Bash
((a=$a+7))         # Add 7 to a
((a = a + 7))      # Add 7 to a.  Identical to the previous command.
((a += 7))         # Add 7 to a.  Identical to the previous command.

if ((a > 5)); then echo "a is more than 5"; fi

true; echo "$?"       # Writes 0, because a successful command returns 0.
((10 > 6)); echo "$?" # Also 0.  An arithmetic command returns 0 for true.
echo "$((10 > 6))"    # Writes 1.  An arithmetic expression evaluates to 1 for true.

for ((i=0, j=0; i<100; i++, j+=5)); do echo "$i"; done

Unlike [ and [[, expressions do not need to be surrounded by spaces with $((expr)) and ((expr)).

Shell Test Blocks

Differences

Arithmetic Expansion

References