Test for a Bash variable being unset, using a function

Question

A simple Bash variable test goes:

${varName:?    \"${varName} is not defined\"}

I\'d like to re-use this, by putting it in a function. How

Answer 1

This function tests for variables that ARE CURRENTLY set. The variable may even be an array. Note that in bash: 0 == TRUE, 1 == FALSE.

function var.defined {
    eval '[[ ${!'$1'[@]} ]]'
}

# Typical Usage of var.defined {}

declare you="Your Name Here" ref='you';

read -p "What's your name: " you;

if var.defined you; then   # Simple demo using literal text

    echo "BASH recognizes $you";
    echo "BASH also knows a reference to $ref as ${!ref}, by indirection."; 

fi

unset you # have just been killed by a master :D

if ! var.defined $ref; then    # Standard demo using an expanded literal value

    echo "BASH doesn't know $ref any longer";

fi

read -s -N 1 -p "Press any key to continue...";
echo "";

So to be clear here, the function tests literal text. Everytime a command is called in bash, variables are GENERALLY 'swapped-out' or 'substituted' with the underlying value unless:

• $varRef ($) is escaped: \$varRef
• $varRef is single quoted '$varRef'