Default values (${var:-default}) in Bash Scripting

Introduction

This helps you use a default value when a variable is empty or not set, so your script keeps working smoothly.

You want to use a default file name if the user does not provide one.

You want to set a default greeting if no name is given.

You want to avoid errors when a variable might be empty.

You want to make your script more flexible with optional inputs.

Syntax

Bash Scripting

${variable:-default_value}

If 'variable' is unset or null, 'default_value' is used instead.

This does not change the variable's value, it only provides a fallback.

Examples

Prints the value of 'name' if set; otherwise prints 'Guest'.

Bash Scripting

echo "${name:-Guest}"

Assigns 'file' to 'filename' if set; otherwise assigns 'default.txt'. Then prints it.

Bash Scripting

filename=${file:-default.txt}
echo "$filename"

Greets the user by name if set; otherwise uses 'friend'.

Bash Scripting

echo "Welcome, ${user:-friend}!"

Sample Program

This script shows how to use a default value 'blue' when the variable 'color' is not set.

Bash Scripting

#!/bin/bash

# Variable 'color' is not set

# Use default value 'blue' if 'color' is unset or null
chosen_color=${color:-blue}
echo "The chosen color is $chosen_color"

Important Notes

This syntax only uses the default value temporarily; it does not assign it to the variable.

If you want to assign the default value to the variable when it is empty, use ${var:=default} instead.

Summary

Use ${var:-default} to safely use a default when a variable is empty or missing.

It helps avoid errors and makes scripts more user-friendly.

It does not change the original variable's value.