set -u for undefined variable errors in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does set -u do in a Bash script?

set -u makes the script stop and show an error if you try to use a variable that has not been set or defined. This helps catch mistakes early.

beginner

Why is using set -u helpful when writing scripts?

It prevents bugs caused by typos or missing variables by stopping the script immediately when an undefined variable is used. This makes scripts safer and easier to debug.

intermediate

How can you safely use a variable that might be undefined when set -u is active?

You can provide a default value using ${VAR:-default}. This way, if VAR is not set, the script uses default instead of stopping.

beginner

What happens if you run this script with set -u enabled?<br>

#!/bin/bash
set -u
echo "Hello $NAME"

The script will stop with an error because NAME is not defined. set -u catches this and prevents the script from continuing.

beginner

How do you enable set -u in a Bash script?

Add the line set -u near the top of your script, usually after the #!/bin/bash line.

What does set -u do in a Bash script?

A Prints all variables used in the script

B Allows undefined variables without errors

C Stops the script if an undefined variable is used

D Disables all error messages

set -u makes the script stop with an error if you use a variable that has not been set.

How can you avoid errors with set -u when a variable might be missing?

A Use <code>${VAR:-default}</code> to provide a default value

B Ignore the error and continue

C Use <code>unset VAR</code> before using it

D Disable <code>set -u</code> temporarily

Using ${VAR:-default} gives a default value if VAR is not set, avoiding errors.

Where should you place set -u in your Bash script?

A At the very end of the script

B At the top, after the <code>#!/bin/bash</code> line

C Anywhere in the middle

D Only inside functions

Placing set -u near the top ensures the whole script checks for undefined variables.

What error will you get if you use an undefined variable with set -u?

A bash: variable_name: unbound variable

B bash: variable_name: command not found

C bash: syntax error near unexpected token

D No error, script continues

The error message says the variable is unbound (undefined) when set -u is active.

If you want to allow undefined variables without errors, what should you do?

A Use <code>set -e</code> instead

B Always define all variables

C Use <code>set -x</code>

D Do not use <code>set -u</code>

Not using set -u means the script will not stop on undefined variables.

Explain what set -u does in a Bash script and why it is useful.

Think about how scripts behave when variables are missing.

Describe how to safely use variables that might not be set when set -u is enabled.

Look at variable expansion syntax.