How to tell if a string is not defined in a Bash shell script

Question

If I want to check for the null string I would do

[ -z $mystr ]

but what if I want to check whether the variable has been defined at all? O

Answer 1

Here is what I think is a much clearer way to check if a variable is defined:

var_defined() {
    local var_name=$1
    set | grep "^${var_name}=" 1>/dev/null
    return $?
}

Use it as follows:

if var_defined foo; then
    echo "foo is defined"
else
    echo "foo is not defined"
fi