What is the difference between test, [ and [[ ?

[ ("test" command) and [[ ("new test" command) are used to evaluate expressions. [[ works only in Bash, Zsh and the Korn shell, and is more powerful; [ and test are available in POSIX shells. Here are some examples:

#POSIX
[ "$variable" ] || echo 'variable is unset or empty!' >&2
[ -f "$filename" ] || printf 'File does not exist or is not a regular file: %s\n' "$filename" >&2

if [[ ! -e $file ]]; then
    echo "File doesn't exist or is in an unaccessible directory or is a symlink to a file that doesn't exist: $file" >&2
fi

if [[ $file -nt ${file[1]} ]]; then
    printf 'file %s is newer than %s\n' "${file[@]}"
fi

To cut a long story short: test implements the old, portable syntax of the command. In almost all shells (the oldest Bourne shells are the exception), [ is a synonym for test (but requires a final argument of ]). Although all modern shells have built-in implementations of [, there usually still is an external executable of that name, e.g. /bin/[. POSIX defines a mandatory feature set for [, but almost every shell offers extensions to it. So, if you want portable code, you should be careful not to use any of those extensions.

[[ is a new improved version of it, and is a keyword, not a program. This makes it easier to use, as shown below. [[ is understood by KornShell, Zsh and BASH (e.g. 2.03), but not by other POSIX shell implementations (like posh, yash or dash) or the BourneShell .

Although [ and [[ have much in common, and share many expression operators like "-f", "-s", "-n", "-z", there are some notable differences. Here is a comparison list:

Feature

new test [[

old test [

Example

string comparison

>

\> (*)

[[ a > b ]] || echo "a does not come before b"

<

\< (*)

[[ az < za ]] && echo "az comes before za"

= (or ==)

=

[[ a = a ]] && echo "a equals a"

!=

!=

[[ a != b ]] && echo "a is not equal to b"

integer comparison

-gt

-gt

[[ 5 -gt 10 ]] || echo "5 is not bigger than 10"

-lt

-lt

[[ 8 -lt 9 ]] && echo "8 is less than 9"

-ge

-ge

[[ 3 -ge 3 ]] && echo "3 is greater than or equal to 3"

-le

-le

[[ 3 -le 8 ]] && echo "3 is less than or equal to 8"

-eq

-eq

[[ 5 -eq 05 ]] && echo "5 equals 05"

-ne

-ne

[[ 6 -ne 20 ]] && echo "6 is not equal to 20"

conditional evaluation

&&

-a (**)

[[ -n $var && -f $var ]] && echo "$var is a file"

||

-o (**)

[[ -b $var || -c $var ]] && echo "$var is a device"

expression grouping

(...)

\( ... \) (**)

[[ $var = img* && ($var = *.png || $var = *.jpg) ]] &&
echo "$var starts with img and ends with .jpg or .png"

Pattern matching

= (or ==)

(not available)

[[ $name = a* ]] || echo "name does not start with an 'a': $name"

RegularExpression matching

=~

(not available)

[[ $(date) =~ ^Fri\ ...\ 13 ]] && echo "It's Friday the 13th!"

(*) This is an extension to the POSIX standard; some shells may have it, and some may not.

(**) The -a and -o operators, and ( ... ) grouping, are defined by POSIX but only for strictly limited cases, and are marked as deprecated. Use of these operators is discouraged; you should use multiple [ commands instead:

Special primitives that [[ is defined to have, but [ may be lacking (depending on the implementation):

Description

Primitive

Example

entry (file or directory) exists

-e

[[ -e $config ]] && echo "config file exists: $config"

file is newer/older than other file

-nt / -ot

[[ $file0 -nt $file1 ]] && echo "$file0 is newer than $file1"

two files are the same

-ef

[[ $input -ef $output ]] && { echo "will not overwrite input file: $input"; exit 1; } 

negation

!

[[ ! -u $file ]] && echo "$file is not a setuid file"

But there are more subtle differences.

As a rule of thumb, [[ is used for strings and files. If you want to compare numbers, use an ArithmeticExpression, e.g.

# Bash
i=0
while (( i < 10 )); do ...

When should the new test command [[ be used, and when the old one [? If portability to POSIX or the BourneShell is a concern, the old syntax should be used. If on the other hand the script requires BASH, Zsh or KornShell, the new syntax is much more flexible.

See the Tests and Conditionals chapter in the BashGuide.

Theory

The theory behind all of this is that [ is a simple command, whereas [[ is a compound command. [ receives its arguments as any other command would, but most compound commands introduce a special parsing context which is performed before any other processing. Typically this step looks for special reserved words or control operators specific to each compound command which split it into parts or affect control-flow. The Bash test expression's logical and/or operators can short-circuit because they are special in this way (as are e.g. ;;, elif, and else). Contrast with ArithmeticExpression, where all expansions are performed left-to-right in the usual way, with the resulting string being subject to interpretation as arithmetic.


CategoryShell

BashFAQ/031 (last edited 2014-08-04 12:00:59 by GreyCat)