Checking if a variable defined by another variable exists in Bash

Question

I'm looking at this but I'm trying to adapt it to my case where I don't hardcode the variable name. In fact, I'm combining logic from a previous question of mine.

Something like this:

shopt -s extglob
export var=abc_def__123__456
echo ${var/%__*}
# does this variable (abc_def) exist?
echo This is not valid syntax: [ -z ${${var/%__*}+x} ] && echo ${var/%__*} does not exist
echo Effectively checking this: [ -z ${abc_def+x} ] && echo abc_def does not exist
shopt -u extglob

EDIT (answer from @ikkachu):

shopt -s extglob
export var=abc_def__123__456
# does this variable (abc_def) exist?
if [[ ! -v ${var/%__*} ]]; then echo ${var/%__*} does not exist; fi
# does this variable (abc_def) exist? (with temp variable)
export temp=${var/%__*}
echo ${temp}
if [ ! -z ${temp+x} ]; then echo \'temp\' exists; fi # checks temp variable only
if [ -z ${!temp+x} ]; then echo ${temp} does not exist; fi
if [ -z ${abc_def+x} ]; then echo abc_def does not exist; fi
shopt -u extglob

Additional information (from Bash manual):

When not performing substring expansion, using the form described below (e.g., ‘:-’), Bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both parameter’s existence and that its value is not null; if the colon is omitted, the operator tests only for existence.

Answer 1

In Bash in particular, you can use [[ -v var ]]:

$ foo=1; unset bar;
$ varname=foo
$ if [[ -v $varname ]]; then echo set; else echo not set; fi
set
$ varname=bar
$ if [[ -v $varname ]]; then echo set; else echo not set; fi
not set

Works with e.g. [[ -v ${var/%__*} ]] too.

Or you can use "alternate value" expansion with a nameref (but you need that temporary variable):

$ declare -n ref=foo
$ if [[ ${ref+set} = set ]]; then echo set; else echo not set; fi
set
$ declare -n ref=bar
$ if [[ ${ref+set} = set ]]; then echo set; else echo not set; fi
not set

Or the same with the ${!var} indirect expansion, i.e. ${!var+set} etc.