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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. ${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. }}} |
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