BASH Using varaibles as variable names

SwingingSimian · 08-29-2008, 08:33 AM

Hi all

Is it possible to use a variable as a variable name, I see many documented instances where people mistakely do something like:

$foo=bar

WHen they really want to do:

foo=bar

I've also seen many people explain that this will try and set the value of foo as a variable name. I actually want to do this, e.g. set the value of a var called 'test'.

test=
foo=test
$foo=value_of_test

This results in:

bash: test=value_of_test: command not found

Is there a way around this? I've tried all sort of eval'ing, back ticks and dereferencing but I can't seem to get anything to work.

Thanks in advance

N

colucix · 08-29-2008, 08:37 AM

You are looking for indirect reference. I let the Advanced Bash Scripting Guide to explain this. See http://www.tldp.org/LDP/abs/html/ivr.html

makyo · 08-29-2008, 08:44 AM

Hi.

You must have been close if you used eval:

Code:

#!/bin/bash -

# @(#) s1       Demonstrate indirect setting with bash, eval.

echo
echo "(Versions displayed with local utility \"version\")"
version >/dev/null 2>&1 && version "=o" $(_eat $0 $1)
set -o nounset

echo

test=
foo=test
$foo=value_of_test
echo " no   eval: foo is \"$foo\", test is \"$test\""

echo
eval $foo=value_of_test
echo " with eval: foo is \"$foo\", test is \"$test\""

exit 0

Producing:

Code:

% ./s1

(Versions displayed with local utility "version")
Linux 2.6.11-x1
GNU bash 2.05b.0

./s1: line 14: test=value_of_test: command not found
 no   eval: foo is "test", test is ""

 with eval: foo is "test", test is "value_of_test"

More examples of eval at http://www.tldp.org/LDP/abs/html/internal.html#EX43 ... cheers, makyo

gnashley · 08-29-2008, 01:28 PM

Is this what you were looking for?

Code:

#!/bin/bash
test=my_value
foo=$test
echo "foo="${!foo}

test=my_value_2
echo "foo="${!foo}

makyo · 08-29-2008, 01:46 PM

Hi.

Quote:

Originally Posted by gnashley

Is this what you were looking for?

Code:

#!/bin/bash
test=my_value
foo=$test
echo "foo="${!foo}

test=my_value_2
echo "foo="${!foo}

Probably intended to write:

Code:

#!/bin/bash -

test=my_value
# foo=$test
foo=test
echo "foo="${!foo}

test=my_value_2
echo "foo="${!foo}

Producing:

Code:

foo=my_value
foo=my_value_2

cheers, makyo

SwingingSimian · 09-02-2008, 02:56 AM

Quote:

Originally Posted by makyo

Hi.

...

test=
foo=test
$foo=value_of_test
echo " no eval: foo is \"$foo\", test is \"$test\""

echo
eval $foo=value_of_test
echo " with eval: foo is \"$foo\", test is \"$test\""

...
... cheers, makyo

Thanks Mayko, this is exactly what I want, setting the value of the var via the 'aliased' variable name, rather than inheriting it via the alias.

Thanks very much for all your comments.