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| [[Anchor(faq73)]] == How can I use parameter expansion? How can I get substrings? How can I get a file without its extension, or get just a file's extension? == Parameter Expansion is a separate section of the [http://tiswww.case.edu/php/chet/bash/bash.html#lbBA bash manpage] (also {{{man bash -P 'less -p "^ Parameter Expansion"'}}} or see [http://tiswww.tis.case.edu/~chet/bash/bashref.html#SEC30 the reference] or [http://bash-hackers.org/wiki/doku.php?id=syntax:pe the bash hackers article] about it). It can be hard to understand parameter expansion without actually using it. (DO NOT think about parameter expansion like a regex. It is different and distinct.) Here's an example of how to use parameter expansion with something akin to a hostname (dot-separated components): {{{ parameter result ----------- ------------------------------ ${NAME} polish.ostrich.racing.champion ${NAME#*.} ostrich.racing.champion ${NAME##*.} champion ${NAME%%.*} polish ${NAME%.*} polish.ostrich.racing }}} And, here's an example of the parameter expansions for a typical filename. {{{ parameter result ----------- -------------------------------------------------------- ${FILE} /usr/share/java-1.4.2-sun/demo/applets/Clock/Clock.class ${FILE#*/} usr/share/java-1.4.2-sun/demo/applets/Clock/Clock.class ${FILE##*/} Clock.class ${FILE%%/*} ${FILE%/*} /usr/share/java-1.4.2-sun/demo/applets/Clock }}} You cannot nest parameter expansions. If you need to perform two separate expansions, use a temporary variable to hold the result of the first expansion. You may find it helpful to associate that, on your keyboard, the "#" is to the left of the "$" symbol and the "%" symbol is to its right; this corresponds with their acting upon the left (beginning) and right (end) parts of the parameter. Here are a few more examples (but ''please'' see the real documentation for a list of all the features!). I include these mostly so people won't break the wiki again, trying to add new questions that answer this stuff. {{{ ${string:2:1} # The third character of string (0, 1, 2 = third) ${string:1} # The string starting from the second character # Note: this is equivalent to ${string#?} ${string%?} # The string with its last character removed. ${string: -1} # The last character of string ${string:(-1)} # The last character of string, alternate syntax # Note: string:-1 means something entirely different; see below. ${file%.mp3} # The filename without the .mp3 extension # Very useful in loops of the form: for file in *.mp3; do ... ${file%.*} # The filename without its extension (assuming there was # only one extension in the first place...). ${file%%.*} # The filename without all of its extensions ${file##*.} # The extension only. }}} === Parameter Expansion on Arrays === [:BashFAQ#faq5:BASH arrays] are remarkably flexible, since they are so well integrated with the other shell expansions. Any expansion you can carry out on a scalar can equally be applied to the whole list in an array. Remember that quoting an array expansion using @, eg {{{"$@"}}} or {{{"${cmd[@]}"}}} results in the members being treated as individual words, irrespective of their content. So for example, {{{arr=("${list[@]}" foo)}}} correctly handles all parameters in the list; we use this to modify an array in place. First the expansions: {{{ $ a=(alpha beta gamma) # our base array $ echo "${a[@]#a}" # chop 'a' from the beginning of every member lpha beta gamma $ echo "${a[@]%a}" # from the end alph bet gamm $ echo "${a[@]//a/f}" # substitution flphf betf gfmmf }}} The following expansions (substitute at beginning or end) are very useful for the next part: {{{ $ echo "${a[@]/#a/f}" # substitute a for f at start flpha beta gamma $ echo "${a[@]/%a/f}" # at end alphf betf gammf }}} We use these to prefix or suffix every member of the list: {{{ $ echo "${a[@]/#/a}" # append a to beginning aalpha abeta agamma # (thanks to floyd-n-milan for this) $ echo "${a[@]/%/a}" # append a to end alphaa betaa gammaa }}} This works by substituting an empty string at beginning or end with the value we wish to append. So finally, a quick example of how you might use this in a script, say to add a user-defined prefix to every target: {{{ $ PFX=inc_ $ a=("${a[@]/#/$PFX}") $ echo "${a[@]}" inc_alpha inc_beta inc_gamma }}} This is very useful, as you might imagine, since it saves looping over every member of the array. === Portability === The original Bourne shell (7th edition Unix) only supports a very limited set of parameter expansion options: {{{ ${var-word} # if var is defined, use var; otherwise, "word" ${var+word} # if var is defined, use "word"; otherwise, nothing ${var=word} # if var is defined, use var; otherwise, use "word" AND... # also assign "word" to var ${var?error} # if var is defined, use var; otherwise print "error" and exit }}} These are the only completely portable options available. POSIX (as well as Korn and Bash) shells offer those, plus a slight variant: {{{ ${var:-word} # if var is defined AND NOT EMPTY, use var; otherwise, "word" etc. }}} POSIX, Korn (all versions) and Bash also support the `${var#word}`, `${var%word}`, `${var##word}` and `${var%%word}` expansions. ksh88 does not support `${var/replace/with}` or `${var//replace/all}`, but ksh93 and Bash do. ksh88 does not support fancy expansion with arrays (e.g., `${a[@]%.gif}`) but ksh93 and Bash do. |
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