Difference between single and double square brackets in Bash

Single [] are posix shell compliant condition tests.

Double [[]] are an extension to the standard [] and are supported by bash and other shells (e.g. zsh, ksh). They support extra operations (as well as the standard posix operations). For example: || instead of -o and regex matching with =~. A fuller list of differences can be found in the bash manual section on conditional constructs.

Use [] whenever you want your script to be portable across shells. Use [[]] if you want conditional expressions not supported by [] and don't need to be portable.

Behavior differences

Tested in Bash 4.3.11:

• POSIX vs Bash extension:

  • [ is POSIX
  • [[ is a Bash extension inspired from Korn shell

• regular command vs magic

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

    ] is just the last argument of [.

  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

• <

  • [[ a < b ]]: lexicographical comparison
  • [ a \< b ]: Same as above. \ required or else does redirection like for any other command. Bash extension.
  • expr x"$x" \< x"$y" > /dev/null or [ "$(expr x"$x" \< x"$y")" = 1 ]: POSIX equivalents, see: How to test strings for lexicographic less than or equal in Bash?

• && and ||

  • [[ a = a && b = b ]]: true, logical and
  • [ a = a && b = b ]: syntax error, && parsed as an AND command separator cmd1 && cmd2
  • [ a = a ] && [ b = b ]: POSIX reliable equivalent
  • [ a = a -a b = b ]: almost equivalent, but deprecated by POSIX because it is insane and fails for some values of a or b like ! or ( which would be interpreted as logical operations

• (

  • [[ (a = a || a = b) && a = b ]]: false. Without ( ), would be true because [[ && ]] has greater precedence than [[ || ]]
  • [ ( a = a ) ]: syntax error, () is interpreted as a subshell
  • [ \( a = a -o a = b \) -a a = b ]: equivalent, but (), -a, and -o are deprecated by POSIX. Without \( \) would be true because -a has greater precedence than -o
  • { [ a = a ] || [ a = b ]; } && [ a = b ] non-deprecated POSIX equivalent. In this particular case however, we could have written just: [ a = a ] || [ a = b ] && [ a = b ] because the || and && shell operators have equal precedence unlike [[ || ]] and [[ && ]] and -o, -a and [

• 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?' ]]: false, loses magic with '' in Bash 3.2 and above and provided compatibility to bash 3.1 is not enabled (like with BASH_COMPAT=3.1)
  • [[ 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.

Thanks to Stéphane Chazelas for important corrections and additions.

Inside single brackets for condition test (i.e. [ ... ]), some operators such as single = is supported by all shells, whereas use of operator == is not supported by some of the older shells.

Inside double brackets for condition test (i.e. [[ ... ]]), there is no difference between using = or == in old or new shells.

Edit: I should also note that: In bash, always use double brackets [[ ... ]] if possible, because it is safer than single brackets. I'll illustrate why with the following example:

if [ $var == "hello" ]; then

if $var happens to be null / empty, then this is what the script sees:

if [ == "hello" ]; then

which will break your script. The solution is to either use double brackets, or always remember to put quotes around your variables ("$var"). Double brackets is better defensive coding practice.