bash: check if a variable's value is the name of an existing variable

Question

I will be reading variables in from a config file. Say, for instance, that the file looks like this:

cfg1=hello
cfg2=there

My script will read that in, so it will have variables names $cfg1 and $cfg2. I now want to determine if the command line argument matches one of the defined variables. I.e., if I execute script.sh cfg1 I want my if statement to pass, but if I execute script.sh cfg3 it should fail.

I'm aware of the \$$varname syntax, but frankly I can't figure out how it works. I've tried:

if [[ \$$1 ]]

but that's always passes.

choroba · Accepted Answer · 2016-07-08 17:00:24Z

5

You can use parameter expansion:

#!/bin/bash

cfg1=hello
cfg2=''

for varname in cfg{1..3} ; do
    echo $varname
    if [[ ${!varname+1} ]] ; then
        echo ok
    fi
done

+1 replaces the value with 1 if the variable is defined.
! introduces variable indirection, i.e. the variable $varname is used as the variable's name.

edited Jul 8, 2016 at 17:00

answered Jul 8, 2016 at 16:54

choroba

245k27 gold badges221 silver badges304 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Community · Accepted Answer · 2017-05-23 12:16:16Z

3

Use the -v test in bash 4.2 or later:

if [[ -v $1 ]]; then
    echo "Variable $1 is defined"
else
    echo "Variable $1 is not defined"
fi

@choroba's answer should work in any earlier version one is likely to be using.

edited May 23, 2017 at 12:16

CommunityBot

11 silver badge

answered Jul 8, 2016 at 17:09

chepner

538k77 gold badges594 silver badges746 bronze badges

1 Comment

mklement0 Over a year ago

Good to know about -v. Just to avoid potential confusion: -v $1 works perfectly here, because the value of $1 is the name of the variable whose existence to test. By contrast, you can use neither -v $1 nor -v 1 to test if $1 itself was passed or not (examine $# for that).

Collectives™ on Stack Overflow

bash: check if a variable's value is the name of an existing variable

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related