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| <<Anchor(faq31)>> == 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/[}}}. [[http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html|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: ||'''Feature'''||'''new test''' {{{[[}}}||'''old test''' {{{[}}}||'''Example'''|| ||<rowspan=4>string comparison||>||\> [[#np|(*)]]||{{{[[ a > b ]] || echo "a does not come before b"}}}|| ||<||\< [[#np|(*)]]||{{{[[ az < za ]] && echo "az comes before za"}}}|| ||= (or ==)||=||{{{[[ a == a ]] && echo "a equals a"}}}|| ||!=||!=||{{{[[ a != b ]] && echo "a is not equal to b"}}}|| ||<rowspan=6>integer comparison||-gt||-gt||{{{[[ 5 -gt 10 ]] || echo "5 is not bigger than 10"}}}|| ||-lt||-lt||{{{[[ 8 -lt 9 ]] && echo "8 is less than 9"}}}|| ||-ge||-ge||{{{[[ 3 -ge 3 ]] && echo "3 is greater than or equal to 3"}}}|| ||-le||-le||{{{[[ 3 -le 8 ]] && echo "3 is less than or equal to 8"}}}|| ||-eq||-eq||{{{[[ 5 -eq 05 ]] && echo "5 equals 05"}}}|| ||-ne||-ne||{{{[[ 6 -ne 20 ]] && echo "6 is not equal to 20"}}}|| ||<rowspan=2>conditional evaluation||&&||-a [[#np2|(**)]]||{{{[[ -n $var && -f $var ]] && echo "$var is a file"}}}|| ||{{{||}}}||-o [[#np2|(**)]]||{{{[[ -b $var || -c $var ]] && echo "$var is a device"}}}|| ||expression grouping||{{{(...)}}}||{{{\( ... \)}}} [[#np2|(**)]]||{{{[[ $var = img* && ($var = *.png || $var = *.jpg) ]] &&}}}<<BR>>{{{echo "$var starts with img and ends with .jpg or .png"}}}|| ||Pattern matching||= (or ==)||(not available)||{{{[[ $name = a* ]] || echo "name does not start with an 'a': $name"}}}|| ||RegularExpression matching||=~||(not available)||{{{[[ $(date) =~ ^Fri\ ...\ 13 ]] && echo "It's Friday the 13th!"}}}|| <<Anchor(np)>> (*) This is an extension to the POSIX standard; some shells may have it, and some may not. <<Anchor(np2)>> (**) The `-a` and `-o` operators, and `( ... )` grouping, are defined in POSIX but only for strictly limited cases. Use of these operators is discouraged; you should use multiple `[` commands instead: * `if [ "$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): ||'''Description'''||'''Primitive'''||'''Example'''|| ||entry (file or directory) exists||-e||{{{[[ -e $config ]] && echo "config file exists: $config"}}}|| ||file is newer/older than other file||-nt / -ot||{{{[[ $file0 -nt $file1 ]] && echo "$file0 is newer than $file1"}}}|| ||two files are the same||-ef||{{{[[ $input -ef $output ]] && { echo "will not overwrite input file: $input"; exit 1; } }}}|| ||negation||!||{{{[[ ! -u $file ]] && echo "$file is not a setuid file"}}}|| 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 [[BashGuide/TestsAndConditionals|Tests and Conditionals]] chapter in the BashGuide. ---- CategoryShell |
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