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

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?

A typical situation where you cannot use [[ is in an autotools configure.ac script, there brackets has a special and different meaning, so you will have to use test instead of [ or [[ -- Note that test and [ are the same program.

If you are into following Google's style guide:

Test, [ and [[

[[ ... ]] reduces errors as no path name expansion or word splitting takes place between [[ and ]], and [[ ... ]] allows for regular expression matching where [ ... ] does not.

# This ensures the string on the left is made up of characters in the
# alnum character class followed by the string name.
# Note that the RHS should not be quoted here.
# For the gory details, see
# E14 at https://tiswww.case.edu/php/chet/bash/FAQ
if [[ "filename" =~ ^[[:alnum:]]+name ]]; then
  echo "Match"
fi

# This matches the exact pattern "f*" (Does not match in this case)
if [[ "filename" == "f*" ]]; then
  echo "Match"
fi

# This gives a "too many arguments" error as f* is expanded to the
# contents of the current directory
if [ "filename" == f* ]; then
  echo "Match"
fi

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: 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.

[[ ]] 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).

[[ ]] double brackets are unsuported under certain version of SunOS and totally unsuported inside function declarations by : GNU bash, version 2.02.0(1)-release (sparc-sun-solaris2.6)

In a nutshell, [[ is better because it doesn't fork another process. No brackets or a single bracket is slower than a double bracket because it forks another process.

In a question tagged 'bash' that explicitly has "in Bash" in the title, I'm a little surprised by all of the replies saying you should avoid [[...]] because it only works in bash!

It's true that portability is the primary objection: if you want to write a shell script which works in Bourne-compatible shells even if they aren't bash, you should avoid [[...]]. (And if you want to test your shell scripts in a more strictly POSIX shell, I recommend dash; though it is an incomplete POSIX implementation since it lacks the internationalization support required by the standard, it also lacks support for the many non-POSIX constructs found in bash, ksh, zsh, etc.)

The other objection I see is at least applicable within the assumption of bash: that [[...]] has its own special rules which you have to learn, while [...] acts like just another command. That is again true (and Mr. Santilli brought the receipts showing all the differences), but it's rather subjective whether the differences are good or bad. I personally find it freeing that the double-bracket construct lets me use (...) for grouping, && and || for Boolean logic, < and > for comparison, and unquoted parameter expansions. It's like its own little closed-off world where expressions work more like they do in traditional, non-command-shell programming languages.

A point I haven't seen raised is that this behavior of [[...]] is entirely consistent with that of the arithmetic expansion construct $((...)), which is specified by POSIX, and also allows unquoted parentheses and Boolean and inequality operators (which here perform numeric instead of lexical comparisons). Essentially, any time you see the doubled bracket characters you get the same quote-shielding effect.

(Bash and its modern relatives also use ((...)) – without the leading $ – as either a C-style for loop header or an environment for performing arithmetic operations; neither syntax is part of POSIX.)

So there are some good reasons to prefer [[...]]; there are also reasons to avoid it, which may or may not be applicable in your environment. As to your coworker, "our style guide says so" is a valid justification, as far as it goes, but I'd also seek out backstory from someone who understands why the style guide recommends what it does.

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.