Check if a Variable is Empty or not in Bash

Programming in Rust has taught me that checking if the variable is empty or not is always a good thing to do, before using the value (or absense of) inside.

To check if a variable is empty, use the -z conditional operator. Following is an example demonstrating this:

if [[ -z "${MY_VAR}" ]]; then
    echo "Empty variable, do not use it"
else
    echo "Varible is not empty, safe to use it."
fi

You can use the -n test operator to test if a variable is not empty.

To summarize:

Test condition	Description
-z	To check if var is empty
-n	To check if var is not empty

Let's see it in details with examples.

Checking if a variable is empty

Bash has a few conditional expressions, of which you can use one to check if a variable is empty or not. That is the -z operator.

Using the -z operator followed by a variable will result in true if the variable is empty. The condition will evaluate to false if the variable is not empty.

Let's take a look:

#!/usr/bin/env bash

EMPTY_VAR=''
NOT_EMPTY='1000101'

if [[ -z "${EMPTY_VAR}" ]]; then
    echo "Variable 'EMPTY_VAR' is empty"
fi

if [[ ! -z "${NOT_EMPTY}" ]]; then
    echo "Variable 'NOT_EMPTY' is populated"
fi

Here, I have two variables, one of which is empty and the other is populated. I am checking if the empty variable is empty and the non-empty variable is not empty :D

Upon running this script, I get the following output:

Variable 'EMPTY_VAR' is empty
Variable 'NOT_EMPTY' is populated

Checking if a variable is NOT empty

"What do you mean? Didn't you JUST cover it with ! -z? I don't like redundant people!"

I get what you mean, but hear me out. What if I told you that there is a shorter way to check if a variable is not empty than what I demonstrated above. A ! -z already exists: -n.

The -n option, accompanied with a varible, returns true if the variable is not empty.

Let's take a look at the same script, but with just one modification.

#!/usr/bin/env bash

EMPTY_VAR=''
NOT_EMPTY='1000101'

if [[ -z "${EMPTY_VAR}" ]]; then
    echo "Variable 'EMPTY_VAR' is empty"
fi

if [[ -n "${NOT_EMPTY}" ]]; then
    echo "Variable 'NOT_EMPTY' is populated"
fi

The only modification that I have made here is I replaced ! -z with -n. I expect the output to the be the same since the logic--pedantically speaking--is the same.

Here is what I see upon running the modified script:

Variable 'EMPTY_VAR' is empty
Variable 'NOT_EMPTY' is populated

New to Bash? Start here

If you are new to bash scripting, we have a good starting point for you in this Bash tutorial course that covers the essential concepts in the nine chapters.