Default values (${var:-default}) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the expression ${var:-default} do in bash scripting?

It checks if the variable var is set and not empty. If var is unset or empty, it uses the default value instead.

intermediate

How does ${var:-default} differ from ${var-default} in bash?

${var:-default} uses the default if var is unset or empty, while ${var-default} uses the default only if var is unset (empty is allowed).

beginner

Write a bash command using ${var:-default} to print the value of name or 'Guest' if name is empty or unset.

echo "Hello, ${name:-Guest}!"

intermediate

What happens if var is set to an empty string and you use ${var:-default}?

The expression treats var as empty and returns default because :- checks for unset or empty.

intermediate

Can ${var:-default} change the value of var?

No, it only returns the default value if var is unset or empty but does not assign it to var. To assign, use ${var:=default}.

What will echo ${user:-anonymous} print if user is unset?

A anonymous

B empty line

C user

D error

Since user is unset, ${user:-anonymous} returns the default 'anonymous'.

If var="" (empty string), what does echo ${var:-default} output?

A default

B empty line

C var

D error

:- treats empty as unset, so it outputs the default value.

Which expression assigns the default value to var if it is unset or empty?

A ${var:=default}

B ${var:-default}

C ${var-default}

D ${var=default}

${var:=default} assigns the default to var if unset or empty.

What does ${var-default} do differently from ${var:-default}?

A Uses default only if var is unset, not if empty

B Uses default if var is empty only

C Always uses default

D Assigns default to var

${var-default} uses default only if var is unset, but allows empty values.

What will echo ${var:-"No value"} print if var="Hello"?

A Hello

B No value

C empty line

D error

Since var is set and not empty, it prints its value 'Hello'.

Explain how the bash expression ${var:-default} works and when it is useful.

Think about how to avoid errors when variables might be empty.

Describe the difference between ${var:-default} and ${var:=default} in bash scripting.

One changes the variable, the other just returns a value.