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| [[Anchor(faq6)]] == How can I use associative arrays or variable variables? == Sometimes it's convenient to have associative arrays, arrays indexed by a string. Perl calls them "hashes". KornShell93 already supports this kind of array: {{{ # KornShell93 script - does not work with BASH typeset -A homedir # Declare KornShell93 associative array homedir[jim]=/home/jim homedir[silvia]=/home/silvia homedir[alex]=/home/alex for user in ${!homedir[@]} # Enumerate all indices (user names) do echo "Home directory of user $user is ${homedir[$user]}" done}}} BASH (including version 3.x) does not (yet) support them. However, we could simulate this kind of array by dynamically creating variables like in the following example: {{{ for user in jim silvia alex do eval homedir_$user=/home/$user done}}} This creates the variables {{{ homedir_jim=/home/jim homedir_silvia=/home/silvia homedir_alex=/home/alex}}} with the corresponding content. Note the use of the {{{eval}}} command, which interprets a command line not just one time like the shell usually does, but '''twice'''. In the first step, the shell uses the input {{{homedir_$user=/home/$user}}} to create a new line {{{homedir_jim=/home/jim}}}. In the second step, caused by {{{eval}}}, this variable assignment is executed, actually creating the variable. Print the variables using {{{ for user in jim silvia alex do varname=homedir_$user # e.g. "homedir_jim" eval varcontent='$'$varname # e.g. "/home/jim" echo "home directory of $user is $varcontent" done}}} The {{{eval}}} line needs some explanation. In a first step the command substitution is run: {{{ eval varcontent='$'$varname}}} becomes {{{ eval varcontent=$homedir_jim}}} In a second step the {{{eval}}} re-evaluates the line, and converts this to {{{ varcontent=/home/jim}}} Before starting to use dynamically created variables, think again of a simpler approach. If it still seems to be the best thing to do, have a look at the following disadvantages: 1. it's hard to read and to maintain 1. the variable names must match the regular expression ^[a-zA-Z_][a-zA-Z_0-9]* , i.e. a variable name cannot contain arbitrary characters but only letters, digits, and underscores. In the example above we e.g. could not have processed the home directory of a user named {{{hong-hu}}}, because a dash '-' can be no valid part of a user name. 1. Quoting is hard to get right. If a content (not variable name) string can contain whitespace characters, it's hard to quote it right to preserve it. Here is the summary. "{{{var}}}" is a constant prefix, "{{{$index}}}" contains index string, "{{{$content}}}" is the string to store. Note that quoting is absolutely essential here. A missing backslash \ or a wrong type of quote (e.g. apostrophes '...' instead of quotation marks "...") can (and probably will) cause the examples to fail: * Set variables {{{ eval "var$index=\"$content\"" # index must only contain characters from [a-zA-Z0-9_]}}} * Print variable content {{{ eval "echo \"var$index=\$$varname\""}}} * Check if a variable is empty {{{ if eval "[ -z "\$var$index\" ]" then echo "variable is empty: $var$index" fi}}} You've seen the examples. Now maybe you can go a step back and consider using AWK associative arrays, or a multi-line environment variable instead of dynamically created variables. |
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