Home » What is the difference between the Bash operators [[ vs [ vs ( vs ((?

What is the difference between the Bash operators [[ vs [ vs ( vs ((?

Solutons:


In Bourne-like shells, an if statement typically looks like

if
   command-list1
then
   command-list2
else
   command-list3
fi

The then clause is executed if the exit code of the command-list1 list of commands is zero. If the exit code is nonzero, then the else clause is executed. command-list1 can be
simple or complex. It can, for example, be a sequence of one or more pipelines separated by one of the operators ;, &, &&, || or newline. The if conditions shown below are just special cases of command-list1:

  1. if [ condition ]

    [ is another name for the traditional test command. [ / test is a standard POSIX utility. All POSIX shells have it builtin (though that’s not required by POSIX²). The test command sets an exit code and the if statement acts accordingly. Typical tests are whether a file exists or one number is equal to another.

  2. if [[ condition ]]

    This is a new upgraded variation on test¹ from ksh that bash, zsh, yash, busybox sh also support. This [[ ... ]] construct also sets an exit code and the if statement acts accordingly. Among its extended features, it can test whether a string matches a wildcard pattern (not in busybox sh).

  3. if ((condition))

    Another ksh extension that bash and zsh also support. This performs arithmetic. As the result of the arithmetic, an exit code is set and the if statement acts accordingly. It returns an exit code of zero (true) if the result of the arithmetic calculation is nonzero. Like [[...]], this form is not POSIX and therefore not portable.

  4. if (command)

    This runs command in a subshell. When command completes, it sets an exit code and the if statement acts accordingly.

    A typical reason for using a subshell like this is to limit side-effects of command if command required variable assignments or other changes to the shell’s environment. Such changes do not remain after the subshell completes.

  5. if command

    command is executed and the if statement acts according to its exit code.


¹ though not really a command but a special shell construct with its own separate syntax from that of normal command, and varying significantly between shell implementations

² POSIX does require that there be a standalone test and [ utilities on the system however, though in the case of [, several Linux distributions have been known to be missing it.

  • (…) parentheses indicate a subshell. What’s inside them isn’t an expression like in many other languages. It’s a list of commands (just like outside parentheses). These commands are executed in a separate subprocess, so any redirection, assignment, etc. performed inside the parentheses has no effect outside the parentheses.
    • With a leading dollar sign, $(…) is a command substitution: there is a command inside the parentheses, and the output from the command is used as part of the command line (after extra expansions unless the substitution is between double quotes, but that’s another story).
  • { … } braces are like parentheses in that they group commands, but they only influence parsing, not grouping. The program x=2; { x=4; }; echo $x prints 4, whereas x=2; (x=4); echo $x prints 2. (Also braces being keywords need to be delimited and found in command position (hence the space after { and the ; before }) whereas parentheses don’t. That’s just a syntax quirk.)
    • With a leading dollar sign, ${VAR} is a parameter expansion, expanding to the value of a variable, with possible extra transformations. The ksh93 shell also supports ${ cmd;} as form of command substitution that doesn’t spawn a subshell.
  • ((…)) double parentheses surround an arithmetic instruction, that is, a computation on integers, with a syntax resembling other programming languages. This syntax is mostly used for assignments and in conditionals. This only exists in ksh/bash/zsh, not in plain sh.
    • The same syntax is used in arithmetic expressions $((…)), which expand to the integer value of the expression.
  • [ … ] single brackets surround conditional expressions. Conditional expressions are mostly built on operators such as -n "$variable" to test if a variable is empty and -e "$file" to test if a file exists. Note that you need a space around each operator (e.g. [ "$x" = "$y" ], not [ "$x"="$y" ]), and a space or a character like ; both inside and outside the brackets (e.g. [ -n "$foo" ], not [-n "$foo"]).
  • [[ … ]] double brackets are an alternate form of conditional expressions in ksh/bash/zsh with a few additional features, for example you can write [[ -L $file && -f $file ]] to test if a file is a symbolic link to a regular file whereas single brackets require [ -L "$file" ] && [ -f "$file" ]. See Why does parameter expansion with spaces without quotes works inside double brackets [[ but not single brackets [? for more on this topic.

In the shell, every command is a conditional command: every command has a return status which is either 0 indicating success or an integer between 1 and 255 (and potentially more in some shells) indicating failure. The [ … ] command (or [[ … ]] syntax form) is a particular command which can also be spelled test … and succeeds when a file exists, or when a string is non-empty, or when a number is smaller than another, etc. The ((…)) syntax form succeeds when a number is nonzero. Here are a few examples of conditionals in a shell script:

  • Test if myfile contains the string hello:

    if grep -q hello myfile; then …
    
  • If mydir is a directory, change to it and do stuff:

    if cd mydir; then
      echo "Creating mydir/myfile"
      echo 'some content' >myfile
    else
      echo >&2 "Fatal error. This script requires mydir to exist."
    fi
    
  • Test if there is a file called myfile in the current directory:

    if [ -e myfile ]; then …
    
  • The same, but also including dangling symbolic links:

    if [ -e myfile ] || [ -L myfile ]; then …
    
  • Test if the value of x (which is assumed to be numeric) is at least 2, portably:

    if [ "$x" -ge 2 ]; then …
    
  • Test if the value of x (which is assumed to be numeric) is at least 2, in bash/ksh/zsh:

    if ((x >= 2)); then …
    

[ vs [[

This answer will cover the [ vs [[ subset of the question.

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: https://stackoverflow.com/questions/21294867/how-to-test-strings-for-lexicographic-less-than-or-equal-in-bash/52707989#52707989
  • && 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 'abn' | 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.

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