= Test expressions =
A ''test'' is a predicate which can perform various logical operations on strings, patterns, filenames, and arithmetic. Tests are an important part of the shell language. Almost every practical shell script includes at least a few test expressions. There are two types of tests: the [[https://web.archive.org/web/20230407163534/https://wiki.bash-hackers.org/commands/classictest|test simple command]] (`[` or `test`), and the [[https://web.archive.org/web/20230326223230/https://wiki.bash-hackers.org/syntax/ccmd/conditional_expression|test expression compound command]] (`[[ ... ]]`).

The `[` command is the most common form of test, and is nearly identical to the `test` command. `[` and `test` are specified by [[Posix|POSIX]] and are the portable types of test that should be used in scripts requiring POSIX conformance.

The `[[` (test expresssion) compound command is a more powerful type of test introduced by AT&T ksh88, and later adopted by all other kshes (ksh93, mksh, pdksh, etc), Bash, and Zsh. While `[[` is relatively portable and consistent where implemented, it is not specified by POSIX. Minimal shells like [[Bashism|Dash]] don't support it.

When writing shell scripts, we recommend consistently using either `[[` when targeting Bash and other Ksh-like shells that don't require POSIX conformance, or `[`, only when POSIX conformance is required, in scripts that use only POSIX features exclusively.

This example shows typical test expression usage:

{{{#!highlight bash
#Bash/Ksh

unset -v suffix fileCnt
[[ $1 == -p ]] && suffix=png

if [[ -d ${HOME}/images ]]; then
    cd "$HOME"/images
else
    echo 'Image directory not found. Exiting.' >&2
    exit 1
fi

for file in *.gif; do
    if [[ -f $file ]]; then
        if mv -- "$file" "${file%gif}${suffix:-jpg}"; then
            ((fileCnt++))
        else
            printf 'Failed to move: %s\n' "$file" >&2
        fi
    fi
done

printf 'Finished moving %d files.\n' "$fileCnt" >&2
}}}

== Links ==

 * [[BashGuide/TestsAndConditionals|Tests and Conditionals]] in the BashGuide.
 * [[BashFAQ/031]] - ''What is the difference between `test`, `[`, and `[[` ?''
 * [[https://web.archive.org/web/20230407163534/https://wiki.bash-hackers.org/commands/classictest|The classic test command]] and [[https://web.archive.org/web/20230326223230/https://wiki.bash-hackers.org/syntax/ccmd/conditional_expression|test expresssion]] on [[https://web.archive.org/web/20230406205817/https://wiki.bash-hackers.org/|Bash Hackers]].
 * [[BashSheet#Tests|Test expression cheat sheet]]
 * [[https://tiswww.case.edu/php/chet/bash/bashref.html#SEC83|Bash Conditional Expressions]] in the Bash infopages.
 * [[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html|SUSv4 test utility]].