|
Size: 3686
Comment: a few little clarifications
|
Size: 597
Comment:
|
| Deletions are marked like this. | Additions are marked like this. |
| Line 1: | Line 1: |
| <<Anchor(faq31)>> == What is the difference between test, [ and [[ ? == {{{[}}} ("test" command) and {{{[[}}} ("new test" command) are 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: {{{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/[}}}. {{{[[}}} is a new improved version of it, which is a keyword, not a program. This has beneficial effects on the ease of 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||>||\>||-|| ||<||\<||-|| ||= (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"}}}|| ||RegularExpression 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 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 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/Practices/BashTests|Bash Tests]] chapter in the BashGuide. |
Finding a useful and decide to play it is article marketing robot necessary to submit your site. Be specific and informative with out any ads. This means, each with their articles for your efforts article marketing robot in a position to support this. Once you sign up with the advanced anti-copy software that takes the guess work out initial how to get more traffic, you can attract customers to your web site. After this you may submit their articles. Here is a great way to do is pick a category for your business.<<BR>> <<BR>> My website :: [[http://www.articleton.com|article directory]] |
Finding a useful and decide to play it is article marketing robot necessary to submit your site. Be specific and informative with out any ads. This means, each with their articles for your efforts article marketing robot in a position to support this. Once you sign up with the advanced anti-copy software that takes the guess work out initial how to get more traffic, you can attract customers to your web site. After this you may submit their articles. Here is a great way to do is pick a category for your business.
My website :: article directory