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Sometimes, we need to check if a variable contains a value. This is important when working on projects that contain user input to be run.

In Bash scripting, we can check in many ways whether a variable contains a value. This article will show you how we can check the value of a variable.

Furthermore, we will see necessary examples and explanations to make the topic easier to understand.

Check if a variable has a value using if...else in Bash

The first method we will use to check a variable is if...elsethe statement. We can also use this method to check if a variable contains a value.

To do this, you can follow the sample code shown below:

VAR=
if [ x"${VAR}" == "x" ]; then
     echo "No value found for the variable"
  else
     echo "Value found for the variable"
  fi

In the above example, with the line if [ x "${VAR}" == "x" ]; , we have checked if the variable contains a value. When you execute the code, you will get the following output:

No value found for the variable

Use double brackets in Bash to check if a variable has a value

We can also [[]]perform the same task using double square brackets. This section will show how we can check the value of a variable using double square brackets using the sample code shared below.

VAR=
[[ x"${VAR}" == "x" ]] && echo "No value found for the variable" || echo "Value found for the variable"

In the above example, through the line [[ x"${VAR}" == "x" ]], we have checked whether the variable contains a value or not. Now, when you execute this code, you will get the output as follows:

No value found for the variable

Check if a variable has a value using an argument expression in Bash

Another way to check if a variable contains a value is to use an argument expression.

This section will use the built-in parameter expressions to check variables. To do this, you can follow the example below:

VAR=
[[ ${VAR:-"unset"} == "unset" ]] && echo "No value found for the variable" || echo "Value found for the variable"

In the above example, with the line [[ ${VAR:-"unset"} == "unset" ]], we checked if the variable contains a value.

Now, when you execute the above code, you will get output like this:

No value found for the variable

Check if a variable has a value using -z conditional expression in Bash

In Bash scripting, there is a -zconditional expression which we can use to check if a variable contains a value. If the string length is 0, it returns true.

To check if a variable contains a value through this method, you can follow the example code below:

var=
[[ -z "${var}" ]] && echo "No value was found for the variable !!!" || echo "Value found for the variable..."

In the example above, with [[ -z "${VAR}" ]]the line, we checked if the variable contains a value.

When you execute the above code, you will get the following output:

No value was found for the variable !!!

Check if a variable has a value using -n conditional expression in Bash

Additionally, Bash has a built-in -n conditional expressions that we can use to check variables.

If the string length is non-zero value, then the -n conditional expression returns true. We can also use this method to check if a variable contains a value.

You can follow the sample code shared below:

VAR=
[[ ! -n "${VAR}" ]] && echo "No value found for the variable" || echo "Value found for the variable"

In the above example, with [[ ! -n "${VAR}" ]], we check if the variable contains a value.

Now, when you execute this code, you will get output like this:

No value found for the variable

You can choose any of the above methods to check your variables based on your needs and complexity.

请注意, all the codes used in this article are written in Bash. It can only be run in the Linux Shell environment.