Diff for "BashFAQ/031"

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

The open square bracket [ command (aka test command) and the [[ ... ]] test construct are used to evaluate expressions. [[ ... ]] works only in the Korn shell (where it originates), Bash, Zsh, and recent versions of Yash and busybox sh (if enabled at compilation time, and still very limited there especially in the hush-based variant), and is more powerful; [ and test are POSIX utilities (generally builtin). POSIX doesn't specify the [[ ... ]] construct (which has a specific syntax with significant variations between implementations) though allows shells to treat [[ as a keyword. Here are some examples:

#POSIX
[ "$variable" ] || echo 'variable is unset or empty!' >&2
[ -f "$filename" ] || printf 'File does not exist or is not a regular file: %s\n' "$filename" >&2

if [[ ! -e $file ]]; then
    echo "File doesn't exist or is in an inaccessible directory or is a symlink to a file that doesn't exist." >&2
fi

if [[ $file0 -nt $file1 ]]; then
    printf '%s\n' "file $file0 is newer than $file1 (or $file1 is no accessible)"
    # (behaviour varies between shells if $file1 is not accessible)
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 it is a keyword rather than a program. This makes it easier to use, as shown below.

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

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

(**) The -a and -o operators, and ( ... ) grouping, are defined by POSIX but only for strictly limited cases, and are marked as deprecated. 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 regular 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 regular 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/conformance to POSIX or the BourneShell is a concern, the old syntax should be used. If on the other hand the script requires BASH, Zsh, or KornShell, the new syntax is usually more flexible, but not necessarily backwards compatible.

For reasons explained in the theory section below, any problem with an operator used with [[ is an unhandleable parse-time error that will cause bash to terminate, even if the command is never evaluated.

# Example of improper [[ usage.
# Notice that isSet is never even called.

 $ bash-3.2 <<\EOF
if ((BASH_VERSINFO[0] > 4 || (BASH_VERSINFO[0] == 4 && BASH_VERSINFO[1] >= 2))); then
  isSet() { [[ -v $1 ]]; }
else
  isSet() { [[ ${1+_} ]]; }
fi
EOF
bash-3.2: line 2: conditional binary operator expected
bash-3.2: line 2: syntax error near `$1'
bash-3.2: line 2: `  isSet() { [[ -v $1 ]]; }'

If backwards-compatibility were desired then [ -v should have been used instead. The only other alternatives would be to use an alias to conditionally expand during the function definition, or eval to defer parsing until the command is actually reached at runtime.

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 do not 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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