Default values for input in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using default values for input in bash scripts?

Default values provide a fallback when the user does not enter any input, ensuring the script can continue without errors.

beginner

How do you assign a default value to a variable if the user input is empty in bash?

Use the syntax: variable=${input:-default_value}. This assigns default_value if input is empty or unset.

intermediate

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

${var:-default} returns default if var is unset or empty but does not change var. ${var:=default} assigns default to var if it is unset or empty.

beginner

Write a simple bash script snippet that asks for a username and uses 'guest' as the default if no input is given.

read -p "Enter username: " user user=${user:-guest} echo "Hello, $user!"

beginner

Why is it helpful to use default values in scripts that require user input?

It makes scripts more user-friendly and robust by avoiding errors or empty variables when users skip input. It also speeds up usage by providing sensible defaults.

In bash, which syntax assigns a default value to a variable only if it is unset or empty, without changing the variable itself?

A var=default

B ${var:=default}

C read -p "Enter value: " var

D ${var:-default}

The syntax ${var:-default} returns the default if var is unset or empty but does not assign it to var.

What will the following bash code output if the user presses Enter without typing anything? read -p "Name: " name name=${name:-John} echo $name

A John

B Name

C Empty line

D Error

Since the user input is empty, the default 'John' is assigned and printed.

Which bash parameter expansion assigns the default value to the variable if it is empty or unset?

A ${var-default}

B ${var:-default}

C ${var:=default}

D ${var=default}

The := operator assigns the default value to var if it is unset or empty.

Why might you want to use default values for input in a bash script?

A To avoid errors and provide fallback values

B To make the script slower

C To ignore user input completely

D To make the script crash if input is missing

Default values help avoid errors and make scripts more user-friendly by providing fallback values.

What does this bash code do? read -p "Enter age: " age age=${age:-18} echo "Age is $age"

A Always prints 'Age is 18'

B Prints the entered age or 18 if empty

C Prints nothing

D Causes an error if no input

If the user enters nothing, age defaults to 18; otherwise, it prints the entered value.

Explain how to use default values for user input in a bash script and why it is useful.

Think about how to assign a value if the user presses Enter without typing.

Write a bash script snippet that asks for a filename and uses 'default.txt' if the user provides no input.

Use ${variable:-default} syntax after reading input.