Default values (${var:-default}) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this script using default values?

Consider the following Bash script snippet. What will it print?

Bash Scripting

unset NAME
echo "Hello, ${NAME:-Guest}!"

A Hello, ${NAME:-Guest}!

B Hello, !

C Hello, Guest!

D Hello, NAME!

💡 Hint

If a variable is not set, the default value after :- is used.

Since NAME is unset, ${NAME:-Guest} uses the default 'Guest'. So the output is 'Hello, Guest!'.

💻 Command Output

intermediate

Output when variable is empty string with default value

What will this script print?

Bash Scripting

NAME=""
echo "Welcome, ${NAME:-User}!"

A Welcome, ${NAME:-User}!

B Welcome, User!

C Welcome, !

D Welcome, ""!

💡 Hint

Default value is used only if variable is unset or null (empty).

The variable NAME is set but empty (null). The :- operator uses the default 'User' when the variable is unset or null. So the output is 'Welcome, User!'.

💻 Command Output

advanced

Difference between ${var:-default} and ${var-default}

Given this script, what is the output?

Bash Scripting

NAME=""
echo "A: ${NAME:-Guest}"
echo "B: ${NAME-Guest}"

A

A: Guest
B: 

B

A: 
B: Guest

C

A: Guest
B: Guest

D

A: 
B: 

💡 Hint

:- uses default if variable is unset or null; - uses default only if unset.

NAME is set but empty. ${NAME:-Guest} treats empty as null and uses default 'Guest'. ${NAME-Guest} treats empty as set and uses empty string. So output is A: Guest and B: (empty).

🔧 Debug

advanced

Why does this script not print the default value?

This script is intended to print 'Hello, Guest!' if NAME is unset or empty. Why does it print 'Hello, !' instead? CODE: NAME="" echo "Hello, ${NAME-default}!"

A Because the default value must be in quotes to work.

B Because the syntax ${NAME-default} is invalid and causes empty output.

C Because the variable NAME is not exported to the environment.

D Because ${NAME-default} uses default only if NAME is unset, not if empty.

💡 Hint

Check difference between - and :- in parameter expansion.

The operator - uses the default only if the variable is unset. Since NAME is set (empty string), default is not used, so output is empty.

🚀 Application

expert

Write a script snippet that prints a default value only if variable is unset or empty

Which option correctly prints the value of VAR if set and not empty, or 'default_value' otherwise?

A echo "${VAR:-default_value}"

B echo "${VAR:+default_value}"

C echo "${VAR:=default_value}"

D echo "${VAR-default_value}"

💡 Hint

Use the operator that substitutes default if variable is unset or empty.

The :- operator substitutes the default if VAR is unset or empty. - substitutes default only if unset. := assigns default to VAR if unset or empty. + substitutes default only if VAR is set and not empty.