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 -         


        

Answer 1

Behavior differences

Some differences on Bash 4.3.11:

• POSIX vs Bash extension:

  • [ is POSIX
  • [[ is a Bash extension¹

• 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

• <

  • [[ a < b ]]: lexicographical comparison
  • [ a \< b ]: Same as above. \ required or else does redirection like for any other command. Bash extension.
  • expr a \< b > /dev/null: POSIX equivalent², 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 -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.}