PowerShell – Check If Variable Is Null

1. Introduction

In PowerShell, checking if a variable is null (or in PowerShell terms, $null) is a fundamental task in scripting, especially when dealing with the output of commands, function returns, or processing user input.

For instance, a variable storing data from database might be null if no data is present.

In this article, we will see different ways to check if variable is Null in PowerShell.

2. What is null in PowerShell?

null refers to variable that has no value assigned to it and is represented by $null in PowerShell. It is generally used to portray absence of a value or non-existent state.

In Powershell, $null is treated as an object with a value of null. It is singleton object, meaning there is only one instance of $null in PowerShell environment.

3. Using the Equality Operator -eq

The eq operator is the most straightforward and recommended method to check if a variable is null.

$myVariable = $null
if ($myVariable -eq $null) { 
    Write-Host "Variable is null."
}
else{
    Write-Host "Variable is not null."
}

Variable is null.

Here, the expression $myVariable -eq $null checks if myVariable is null. If true, it prints Variable is null;otherwise, it prints Variable is not null.

This method is fast and efficient, as it does basic comparison operation.

3.1 Handling Result from Database Call

If we are doing null check for result from database query, it might return [DBNULL] which is different than $null.

Let’s write a custom function to check if variable is $null or DBNULL.

function IsNullOrDbNull {
    param (
        [Parameter(Mandatory=$true)]
        $Value
    )
    return $Value -eq $null -or [System.DBNull]::Value.Equals($Value)
}

# Example usage:
$myVar = $null  # This could be $null or a result from a database query
if (IsNullOrDbNull -Value $myVar) {
    Write-Host "Variable is null or DB null"
} else {
    Write-Host "Variable has a value"
}

Explanation:

• The function IsNullOrDbNull takes a mandatory parameter $Value.
• It checks if $Value is null using -eq operator or if it is equal to DBNULL using [System.DBNull]::Value.Equals($Value).
• $myVar = $null is used to assign $null to $myVar. $myVar can have result from database query as well.
• IsNullOrDbNull returns true in case $Value is either $null or DBNULL.
• The expression if (IsNullOrDbNull -Value $myVar) calls function IsNullOrDbNull with Parameter $myVar and checks if it returns true.

4. Using Conditional Statement

Conditional statement can be used to check if variable is null, especially when working with multiple conditions.

$myVariable = $null
if ($myVariable) {
    Write-Host "Variable has a value"
} elseif ($myVariable -eq $null) {
    Write-Host "Variable is null"
}

Here, if block checks if variable has a truthy value, so it will go to elseif block even if value is 0, $false, an empty String, an empty array, and not just $null, and that’s the reason, we have put explicit check in elseif statement if variable is $null.

5. Using Null-Coalescing Operators [ Supported in PowerShell 7.0 versions]

?? is the null-coalescing operator. It returns the left-hand operand if it’s not $null, otherwise the right-hand operand. It is generally useful when we want to assign default value to variable when it is $null.

Let’s see with the help of example:

$myVar = $null
$result = $myVar ?? "default value"
Write-Host $result  # Outputs 'default value'

Let’s assign some value to $myVar and check the output.

$myVar = "Java2blog"
$result = $myVar ?? "default value"
Write-Host $result  # Outputs 'Java2blog'

6. Checking for null for String Variables

For string values, we can use two more methods i.e. IsNullorEmpty() and IsNullOrWhiteSpace() to check if variable is null.

Let’s go through each of them:

6.1 Using IsNullorEmpty()

IsNullorEmpty() checks if a string variable is either null or empty. It is efficient and more comprehensive for basic string checks.

Let’s see with the help of example:

[string]::IsNullOrEmpty($newVariable)  

 True

In the above code, IsNullOrEmpty() method is used to determine whether the string variable named $newVariable is null or not, and we got True, which means the specified variable is null. Now, execute the following code to assess whether the given variable is empty.

$newVariable=''
[string]::IsNullOrEmpty($newVariable)  

True

The above code fence also returned True, meaning $newVariable is empty; otherwise, we would get False. So, let’s assign a value to $newVariable as given below and observe the output.

$newVariable = 'Hello World'
[string]::IsNullOrEmpty($newVariable) 

False

This time, we got False because the $newVariable is not null or empty but contains a string type value, Hello World.

6.2 Using IsNullOrWhiteSpace() Method

we can use the IsNullOrWhiteSpace() method to check if the specified string variable is null, empty, or contains whitespace characters only.

[string]::IsNullOrWhiteSpace($Variable)

True

This approach is similar to the previous one, where we used the IsNullorEmpty() method, except for one difference, it not only tells that the specified variable is null or empty but informs if the given variable contains whitespace characters only.

The IsNullOrWhiteSpace() method returns True if the given variable is null, empty, or contains a whitespace character; otherwise, False. Here, null means the $Variable is not yet assigned any value, empty means set with an empty string, and the third scenario holds whitespace characters only.

Let’s see another example to understand all of these situations.

[string]::IsNullOrWhiteSpace($Variable)

$Variable=''
[string]::IsNullOrWhiteSpace($Variable)

$Variable='        '
[string]::IsNullOrWhiteSpace($Variable)

$Variable='How are you?'
[string]::IsNullOrWhiteSpace($Variable)

True
True
True
False

First, we got three True values because $Variable was not initialized at the beginning, then assigned with an empty string and third, held whitespace characters only. Lastly, we received only one False because How are you? was assigned to $Variable.

Note: This method only applies in PowerShell versions 3.0 and above.

7. Handling $null in Collections

When dealing with collections like arrays or lists, checking for $null items requires iterating through the collection.

$myArray = @(1, $null, 3)
$nullItem = $myArray -contains $null

if ($nullItem) {
    Write-Host "Array contains null value"
} else {
    Write-Host "Array does not contain null value"
}

Array contains null value

-Contains operator checks if array contains element.

Above code checks if array contains any $null value. In our example, $myArray contains $null value, hence output is Array contains null value.

8. Conclusion

In this article, we have discussed ways to check if variable is null in PowerShell.

The equality operator (-eq) provides a straightforward check, while conditional statements offer more flexibility for complex scenarios.

The null-coalescing operator (??) is a modern and efficient way to handle $null values, especially when assigning default values.

The IsNullorEmpty() and IsNullOrWhiteSpace() methods are particularly useful for string variables, offering a more comprehensive check.