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See the [http://wooledge.org:8000/BashGuide#Bash_Tests 'Bash Tests' chapter in the Bash Guide].

Anchor(faq31)

What is the difference between the old and new test commands ([ and [[)?

[ ("test" command) and [[ ("new test" command) are both used to evaluate expressions. Some examples:

    if [ -z "$variable" ]
    then
        echo "variable is empty!"
    fi

    if [ ! -f "$filename" ]
    then
        echo "not a valid, existing file name: $filename"
    fi

and

    if [[ -e $file ]]
    then
        echo "directory entry does not exist: $file"
    fi

    if [[ $file0 -nt $file1 ]]
    then
        echo "file $file0 is newer than $file1"
    fi

To cut a long story short: [ implements the old, portable syntax of the command. Although all modern shells have built-in implementations, there usually still is an external executable of that name, e.g. /bin/[. [[ is a new improved version of it, which is a keyword, not a program. This has beneficial effects on the ease of use, see below. [[ is understood by KornShell, ["BASH"] (e.g. 2.03), and KornShell93, but not by the older 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

>

\>

-

<

\<

-

== (or =)

=

-

!=

!=

-

expression grouping

&&

-a

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

||

-o

-

Pattern matching

== (or =)

(not available)

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

In-process regular expression matching

=~

(not available)

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

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

!

-

But there are more subtle differences.

  • No field splitting will be done for [[ (and therefore many arguments need not be quoted)

     file="file name"
     [[ -f $file ]] && echo "$file is a file"

    will work even though $file is not quoted and contains whitespace. With [ the variable needs to be quoted:

     file="file name"
     [ -f "$file" ] && echo "$file is a file"

    This makes [[ easier to use and less error-prone.

  • No file name generation will be done for [[. Therefore the following line tries to match the contents of the variable $path with the pattern /*

     [[ $path = /* ]] && echo "\$path starts with a forward slash /: $path"

    The next command most likely will result in an error, because /* is subject to file name generation:

     [ $path = /* ] && echo "this does not work"

    (If you need to do that using Bourne shells, use case instead.)

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

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

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

See the [http://wooledge.org:8000/BashGuide#Bash_Tests 'Bash Tests' chapter in the Bash Guide].

BashFAQ/031 (last edited 2022-05-09 13:49:40 by 27)