Is double square brackets [[ ]] preferable over single square brackets [ ] in Bash?


Question

A co-worker claimed recently in a code review that the [[ ]] construct is to be preferred over [ ] in constructs like

if [ "`id -nu`" = "$someuser" ] ; then 
     echo "I love you madly, $someuser"
fi

He couldn't provide a rationale. Is there one?

1
479
8/1/2018 5:13:11 AM

Accepted Answer

[[ has fewer surprises and is generally safer to use. But it is not portable - POSIX doesn't specify what it does and only some shells support it (beside bash, I heard ksh supports it too). For example, you can do

[[ -e $b ]]

to test whether a file exists. But with [, you have to quote $b, because it splits the argument and expands things like "a*" (where [[ takes it literally). That has also to do with how [ can be an external program and receives its argument just normally like every other program (although it can also be a builtin, but then it still has not this special handling).

[[ also has some other nice features, like regular expression matching with =~ along with operators like they are known in C-like languages. Here is a good page about it: What is the difference between test, [ and [[ ? and Bash Tests

537
6/4/2017 2:59:49 AM

Behavior differences

Some differences on Bash 4.3.11:

  • POSIX vs Bash extension:

  • regular command vs magic

    • [ is just a regular command with a weird name.

      ] is just an argument of [ that prevents further arguments from being used.

      Ubuntu 16.04 actually has an executable for it at /usr/bin/[ provided by coreutils, but the bash built-in version takes precedence.

      Nothing is altered in the way that Bash parses the command.

      In particular, < is redirection, && and || concatenate multiple commands, ( ) generates subshells unless escaped by \, and word expansion happens as usual.

    • [[ X ]] is a single construct that makes X be parsed magically. <, &&, || and () are treated specially, and word splitting rules are different.

      There are also further differences like = and =~.

      In Bashese: [ is a built-in command, and [[ is a keyword: https://askubuntu.com/questions/445749/whats-the-difference-between-shell-builtin-and-shell-keyword

  • <

  • && and ||

    • [[ a = a && b = b ]]: true, logical and
    • [ a = a && b = b ]: syntax error, && parsed as an AND command separator cmd1 && cmd2
    • [ a = a -a b = b ]: equivalent, but deprecated by POSIX³
    • [ a = a ] && [ b = b ]: POSIX and reliable equivalent
  • (

    • [[ (a = a || a = b) && a = b ]]: false
    • [ ( a = a ) ]: syntax error, () is interpreted as a subshell
    • [ \( a = a -o a = b \) -a a = b ]: equivalent, but () is deprecated by POSIX
    • { [ a = a ] || [ a = b ]; } && [ a = b ] POSIX equivalent⁵
  • word splitting and filename generation upon expansions (split+glob)

    • x='a b'; [[ $x = 'a b' ]]: true, quotes not needed
    • x='a b'; [ $x = 'a b' ]: syntax error, expands to [ a b = 'a b' ]
    • x='*'; [ $x = 'a b' ]: syntax error if there's more than one file in the current directory.
    • x='a b'; [ "$x" = 'a b' ]: POSIX equivalent
  • =

    • [[ ab = a? ]]: true, because it does pattern matching (* ? [ are magic). Does not glob expand to files in current directory.
    • [ ab = a? ]: a? glob expands. So may be true or false depending on the files in the current directory.
    • [ ab = a\? ]: false, not glob expansion
    • = and == are the same in both [ and [[, but == is a Bash extension.
    • case ab in (a?) echo match; esac: POSIX equivalent
    • [[ ab =~ 'ab?' ]]: false⁴, loses magic with ''
    • [[ ab? =~ 'ab?' ]]: true
  • =~

    • [[ ab =~ ab? ]]: true, POSIX extended regular expression match, ? does not glob expand
    • [ a =~ a ]: syntax error. No bash equivalent.
    • printf 'ab\n' | grep -Eq 'ab?': POSIX equivalent (single line data only)
    • awk 'BEGIN{exit !(ARGV[1] ~ ARGV[2])}' ab 'ab?': POSIX equivalent.

Recommendation: always use [].

There are POSIX equivalents for every [[ ]] construct I've seen.

If you use [[ ]] you:

  • lose portability
  • force the reader to learn the intricacies of another bash extension. [ is just a regular command with a weird name, no special semantics are involved.

¹ Inspired from the equivalent [[...]] construct in the Korn shell

² but fails for some values of a or b (like + or index) and does numeric comparison if a and b look like decimal integers. expr "x$a" '<' "x$b" works around both.

³ and also fails for some values of a or b like ! or (.

⁴ in bash 3.2 and above and provided compatibility to bash 3.1 is not enabled (like with BASH_COMPAT=3.1)

⁵ though the grouping (here with the {...;} command group instead of (...) which would run an unnecessary subshell) is not necessary as the || and && shell operators (as opposed to the || and && [[...]] operators or the -o/-a [ operators) have equal precedence. So [ a = a ] || [ a = b ] && [ a = b ] would be equivalent.


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