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

[[ 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

Behavior differences

Some differences on 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: What's 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.

[[ ]] has more features - I suggest you take a look at the Advanced Bash Scripting Guide for more info, specifically the extended test command section in Chapter 7. Tests.

Incidentally, as the guide notes, [[ ]] was introduced in ksh88 (the 1988 version of the Korn shell).

From Which comparator, test, bracket, or double bracket, is fastest? (http://bashcurescancer.com)

The double bracket is a “compound command” where as test and the single bracket are shell built-ins (and in actuality are the same command). Thus, the single bracket and double bracket execute different code.

The test and single bracket are the most portable as they exist as separate and external commands. However, if your using any remotely modern version of BASH, the double bracket is supported.