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| [[Anchor(faq31)]] == What is the difference between the old and new test commands ([ and [[)? == {{{[}}} ("test" command) and {{{[[}}} ("new test" command) are both used to evaluate expressions. 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: {{{[}}} implements the old, portable syntax of the command. Although all modern shells have built-in implementations, there usually still is an external executable of that name, e.g. {{{/bin/[}}}. {{{[[}}} is a new improved version of it, which is a keyword, not a program. This has beneficial effects on the ease of use, see below. {{{[[}}} is understood by KornShell, ["BASH"] (e.g. 2.03), KornShell93, ["POSIX"] shell, but not by the older 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||>||(not available)||-|| ||<||(not available)||-|| ||== (or =)||=||-|| ||!=||!=||-|| ||<rowspan="2">expression grouping||&&||-a||{{{[[ -n $var && -f $var ]] && echo "$var is a file"}}}|| ||{{{||}}}||-o||-|| ||Pattern matching||== (or =)||(not available)||{{{[[ $name = a* ]] || echo "name does not start with an 'a': $name"}}}|| ||In-process regular expression matching||=~||(not available)||{{{[[ $(date) =~ ^Fri\ ...\ 13 ]] && echo "It's Friday the 13th!"}}}|| 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||!||-|| But there are more subtle differences. * No field splitting will be done for {{{[[}}} (and therefore many arguments need not to 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. * No file name generation will be done for {{{[[}}}. Therefore the following line tries to match the contents of the variable $path with the pattern {{{/*}}} {{{ [[ $path = /* ]] && echo "\$path starts with a forward slash /: $path"}}} The next command most likely will result in an error, because {{{/*}}} is subject to file name generation: {{{ [ $path = /* ] && echo "this does not work"}}} {{{[[}}} is strictly used for strings and files. If you want to compare numbers, use ArithmethicExpression ((''expression'')), e.g. {{{ i=0 while ((i<10)) do echo $i ((i=$i+1)) done}}} 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 could be preferable. |
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