Use string as bash variable name in alternative value expansion [duplicate] - bash

Question

This question already has answers here:
How can I look up a variable by name with #!/bin/sh (POSIX sh)?
(4 answers)
Closed 7 years ago.
How can I use the value of one variable as the name of another variable in an alternative value expansion (${var+alt}) in bash?
I would think that
#!/bin/bash
cat='dog'
varname='cat'
if [ -z ${`echo "${varname}"`+x} ]; then
echo 'is null'
fi
should be roughly equivalent to
#!/bin/bash
if [ -z ${dog+x} ]; then
echo 'is null'
fi
but when I try to do this, I get
${`echo "${cat}"`+x}: bad substitution
I guess part of the problem is that the subshell doing the command substitution doesn't know about $varname anymore? Do I need to export that variable?
My reason for doing this is that I learned from this answer how to check if a variable is null, and I'm trying to encapsulate that check in a function called is_null, like this:
function is_null {
if [ $# != 1 ]; then
echo "Error: is_null takes one argument"
exit
fi
# note: ${1+x} will be null if $1 is null, but "x" if $1 is not null
if [ -z ${`echo "${1}"`+x} ]; then
return 0
else
return 1
fi
}
if is_null 'some_flag'; then
echo 'Missing some_flag'
echo $usage
exit
fi

Answer 1

I'm not sure if I understand your problem.
If I got, what you need is eval command.
$ cat='dog'
$ varname='cat'
$ echo ${varname}
cat
$ eval echo \$${varname}
dog