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OK I don't really have a good title yet but I figure I can post works in progress and other tips I've come across or other interesting things.
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having fun with bash varaibles: indirect variable references

Posted 11-28-2009 at 01:38 PM by lumak
Updated 11-28-2009 at 02:59 PM by lumak

Well I recently started messing around with bash scripts as cgi scripts and was wondering how to pass variables in and what not.

Well it turns out, Apache will define the variable $QUERY_STRING which will contain everything after the first question mark. You can then pars this string to define variables. The general format is "name1=val1&name2=val2". It turns out you can actually declare variables in bash using 'declare' and parameter expansion.

Code:
VAR="hello"
VAL="how are you"
declare $VAR="$VAL"

echo $hello
# >> how are you
unfortunately, I couldn't find a very intuitive way to do the opposite. That is, get the value of a variable name if the name is given as text... But this seemd to do the job well enough.

Code:
eval echo '${'$VAR'}'
# >> how are you
In that example, $VAR is expanded to "hello" then eval will execute echo as if you had actually typed "echo ${hello}" on the console! if you remove the 'echo' it will be as if you typed "`$hello`" and in this case say "bash: how: command not found" which may or may not be your intention.

Anyways... I'm cleaning up a sample cgi script that utilizes this and will upload it when it's done.
Attached Files
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Comments

  1. Old Comment
    Not sure I understand everything you are trying to do but this may interest you:

    Code:
    $ GREETING=HELLO
    $ HELLO=howdy
    $ echo ${!GREETING}
    howdy
    Posted 11-29-2009 at 08:16 PM by lwasserm lwasserm is offline
  2. Old Comment
    How did I miss that one?!

    Yeah, that's somewhat of what I was looking for. Would have helped to read the bash manual more closely >.o

    Oddly enough, Linux in a nutshell 5th edition didn't explain the ${!name[@]} and neither explains just ${!name}.

    Either way, I completely forgot the usefulness of what I was trying to do with it... something along the lines of parsing out the $QUERY_STRING, storing them as bash variables, then later referencing them by a text string.

    Anyway, you would still have to do any text processing to "name" first then store it in another variable and reference it as ${!newvar}

    e.g.
    name="var1 var2"
    var1="foo"
    var2="bar"
    echo ${!${name% var2}}
    bash: ${!${name% var2}}: bad substitution

    var3=${name% var2}
    echo $var3
    >> var1

    echo ${!var3}
    >> foo



    BASH MANUAL
    Parameter Expansion
    ...

    ${!prefix*}
    ${!prefix@}
    Expands to the names of variables whose names begin with prefix, separated by the first character of
    the IFS special variable.

    ${!name[@]}
    ${!name[*]}
    If name is an array variable, expands to the list of array indices (keys) assigned in name. If name
    is not an array, expands to 0 if name is set and null otherwise. When @ is used and the expansion
    appears within double quotes, each key expands to a separate word.
    Posted 12-19-2009 at 03:26 AM by lumak lumak is offline
 

  



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