What is the difference between test, [ and [[ ?

[ ("test" command) and [[ ("new test" command) are used to evaluate expressions. [[ works only in Bash and Korn shell, and is more powerful; [ and test are available in POSIX shells. Here are some examples:

    if [ -z "$variable" ]
    then
        echo "variable is empty!"
    fi

    if [ ! -f "$filename" ]
    then
        echo "not a valid, existing file name: $filename"
    fi

and

    if [[ ! -e $file ]]
    then
        echo "directory entry does not exist: $file"
    fi

    if [[ $file0 -nt $file1 ]]
    then
        echo "file $file0 is newer than $file1"
    fi

To cut a long story short: test implements the old, portable syntax of the command. In almost all shells (the oldest Bourne shells are the exception), [ is a synonym for test (but requires a final argument of ]). Although all modern shells have built-in implementations of [, there usually still is an external executable of that name, e.g. /bin/[. POSIX defines a mandatory feature set for [, but almost every shell offers extensions to it. So, if you want portable code, you should be careful not to use any of those extensions.

[[ is a new improved version of it, and is a keyword, not a program. This makes it easier to use, as shown below. [[ is understood by KornShell and BASH (e.g. 2.03), but not by the older POSIX or BourneShell.

Although [ and [[ have much in common, and share many expression operators like "-f", "-s", "-n", "-z", there are some notable differences. Here is a comparison list:

(*) This is an extension to the POSIX standard; some shells may have it, and some may not.

(**) The -a and -o operators, and ( ... ) grouping, are defined in POSIX but only for strictly limited cases. Use of these operators is discouraged; you should use multiple [ commands instead:

• if [ "$a" = a ] && [ "$b" = b ]; then ...

• if { [ "$a" = a ] || [ "$b" = b ] ; } && [ "$c" = c ]; then ...

Special primitives that [[ is defined to have, but [ may be lacking (depending on the implementation):

But there are more subtle differences.

• No WordSplitting or glob expansion will be done for [[ (and therefore many arguments need not be quoted):

   file="file name"
   [[ -f $file ]] && echo "$file is a file"

  will work even though $file is not quoted and contains whitespace. With [ the variable needs to be quoted:

   file="file name"
   [ -f "$file" ] && echo "$file is a file"

  This makes [[ easier to use and less error-prone.

• Parentheses in [[ do not need to be escaped:

   [[ -f $file1 && ( -d $dir1 || -d $dir2) ]]
   [ -f "$file1" -a \( -d "$dir1" -o -d "$dir2" \) ]

• As of bash 4.1, string comparisons using < or > respect the current locale when done in [[, but not in [ or test. In fact, [ and test have never used locale collating order even though past man pages said they did. Bash versions prior to 4.1 do not use locale collating order for [[ either.

As a rule of thumb, [[ is used for strings and files. If you want to compare numbers, use an ArithmeticExpression, e.g.

# Bash
i=0
while ((i<10)); do ...

When should the new test command [[ be used, and when the old one [? If portability to the BourneShell is a concern, the old syntax should be used. If on the other hand the script requires BASH or KornShell, the new syntax is much more flexible.

See the Tests and Conditionals chapter in the BashGuide.

Theory

The theory behind all of this is that [ is a simple command, whereas [[ is a compound command. [ receives its arguments as any other command would, but most compound commands introduce a special parsing context which is performed before any other processing. Typically this step looks for special reserved words or control operators specific to each compound command which split it into parts or affect control-flow. The Bash test expression's logical and/or operators can short-circuit because they are special in this way (as are e.g. ;;, elif, and else). Contrast with ArithmeticExpression, where all expansions are performed left-to-right in the usual way, with the resulting string being subject to interpretation as arithmetic.

• The arithmetic compound command has no special operators. It has only one evaluation context - a single arithmetic expression. Arithmetic expressions have operators too, some of which affect control flow during the arithmetic evaluation step (which happens last).

      # Bash
      (( 1 + 1 == 2 ? 1 : $(echo "This doesn't do what you think..." >&2; echo 1) ))

• Test expressions on the other hand do have "operators" as part of their syntax, which lie on the other end of the spectrum (evaluated first).

      # Bash
      [[ '1 + 1' -eq 2 && $(echo "...but this probably does what you expect." >&2) ]]

• Old-style tests have no way of controlling evaluation because its arguments aren't special.

      # Bash
      [ $((1 + 1)) -eq 2 -o $(echo 'No short-circuit' >&2) ]

• Different error handling is made possible by searching for special compound command tokens before performing expansions. [[ can detect the presence of expansions that don't result in a word yet still throw an error if none are specified. Ordinary commands can't.

      # Bash 
      ( set -- $(echo 'Unquoted null expansions don't result in "null" parameters.' >&2); echo $# )
      [[ -z $(:) ]] && echo '-z was supplied an arg and evaluated empty.'
      [ -z ] && echo '-z wasn't supplied an arg, and no errors are reported. There's no possible way Bash could enforce specifying an argument here.'
      [[ -z ]] # This will cause an error that ordinary commands can't detect.

• For the very same reason, because ['s operators are just "arguments", unlike [[, you can specify operators as parameters to an ordinary test command. This might be seen as a limitation of [[, but the downsides outweigh the good almost always.

      # ksh93
  
      args=('0' '-gt' '1')
   
      (( $(print '0 > 1') )) # Valid command, Exit status is 1 as expected.
      [ "${args[@]}" ]       # Also exit 1. 
      [[ ${args[@]} ]]       # Valid command, but is misleading. Exit status 0. set -x reveals the resulting command is [[ -n '0 -gt 1 ]]

• Do keep in mind which operators belong to which shell constructs. Order of expansions can cause surprising results especially when mixing and nesting different evaluation contexts!

      # ksh93
      typeset -i x=0
      
      ( print "$(( ++x, ${ x+=1; print $x >&2;}1, x ))"      ) # Prints 1, 2
      ( print "$(( $((++x)), ${ x+=1; print $x >&2;}1, x ))" ) # Prints 2, 2 - because expansions are performed first.

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