How to Assign Default Value in Bash

This article will introduce the approaches used for providing a default value to the variables in bash scripts.

Provide a Default Value to a Variable in a Bash Script

The basic approach we follow while providing a default value in a bash script is below.

variable={$variable:-value}

But we can use a better short-hand form of this by using colon at the beginning.

: ${variable:=value}

The colon at the beginning ignores the arguments.

Use ${variable-value} or ${variable:-value}

echo ${greet-hello}
echo ${greet:-hello}
greet=
echo ${greet-hello}
echo ${greet:-hello}

Output:

hello
hello

hello

The key difference between using ${greet-hello} and ${greet:-hello} is that ${greet-hello} will use the default value hello if the variable greet was never set to a value. On the other hand, ${greet:-hello} will use default value either if the variable was never set a value or was set to null i.e. greet= .

Use ${variable:-value} or ${variable:=value}

echo ${greet:-Hello}
echo ${greet:-Namaste}
echo ${greet:=Bonjour}
echo ${greet:=Halo}

Output:

Hello
Namaste
Bonjour
Bonjour

Using :- will substitute the variable with the default value, whereas := will assign the default value to the variable.

In the given example,

• ${greet:-Namaste} prints out Namaste as ${greet:-Hello} has substituted the greet variable with default value as it was not set.
• ${greet:=Bonjour} will set the value of greet to Bonjour as its value was never set.
• ${greet:=Halo} will not use the default value Halo as the variable greet was set value previously i.e. Bonjour.