Flow control

  • 4 minutes

Flow control refers to how your code runs in your console or script. It describes the flow the code follows and how you control that flow. There are various constructs available to help you control the flow. The code can run all the statements, or only some of them. It can also repeat certain statements until it meets a certain condition.

Let's examine these flow-control constructs to see what they can do:

  • Sanitize input. If you use parameters in a script, you need to ensure your parameters hold reasonable values so your script works as intended. Writing code to manage this process is called sanitizing input.

  • Control execution flow. The previous technique ensures you get reasonable and correct input data. This technique is more about deciding how to run code. The values set can determine which group of statements runs.

  • Iterate over data. Sometimes your data takes the form of an array, which is a data structure that contains many items. For such data, you might need to examine each item and perform an operation for each one. Many constructs in PowerShell can help you with that process.

    Note

    Iterating over arrays is outside the scope of this module. There are many constructs to handle flow control in PowerShell. We can't name them all, but we'll talk about some important ones that you're likely to encounter in scripts that you read or write.

Manage input and execution flow by using If, ElseIf, and Else

You can use an If construct to determine if an expression is True or False. Depending on that determination, you might run the statement defined by the If construct. The syntax for If looks like this:

If (<expression that evaluates to True or False>) 
{
  # Statement that runs only if the preceding expression is $True.
}

Operators

PowerShell has two built-in parameters to determine if an expression is True or False:

  • $True indicates that an expression is True.
  • $False indicates that an expression is False.

You can use operators to determine if an expression is True or False. There are a few operators. The basic idea is usually to determine if something on the left side of the operator matches something on the right side, given the operator's condition. An operator can express conditions like whether something is equal to something else, larger than something else, or matches a regular expression.

Here's an example of using an operator. The -le operator determines if the value on the left side of the operator is less than or equal to the value on the right side:

$Value = 3
If ($Value -le 0) 
{
  Write-Host "Is negative"
}

This code won't display anything because the expression evaluates to False. The value 3 is clearly positive.

Else

The If construct runs statements only if they evaluate to True. What if you want to handle cases where they evaluate to False? That's when you use the Else construct. If expresses "if this specific case is true, run this statement." Else doesn't take an expression. It captures all cases where the If clause evaluates to False. When If and Else are combined, the code runs the statements in one of the two constructs. Let's modify the previous code to include an Else construct:

$Value = 3
If ($Value -le 0) 
{
  Write-Host "Is negative"
} Else {
  Write-Host "Is Positive"
}

Because we put the Else next to the ending brace for the If, we created a joined construct that works as one. If you run this code in the console, you'll see that Is Positive prints. That's because If evaluates to False, but Else evaluates to True. So Else prints its statement.

Note

You can use Else only if there's an If construct defined immediately above it.

ElseIf

If and Else work great to cover all the paths code can take. ElseIf is another construct that can be helpful. ElseIf is meant to be used with If. It says "the expression in this construct will be evaluated if the preceding If statement evaluates to False." Like If, ElseIf can take an expression, so it helps to think of ElseIf as a secondary If.

Here's an example that uses ElseIf:

# _FullyTax.ps1_
# Possible values: 'Minor', 'Adult', 'Senior Citizen'
$Status = 'Minor'
If ($Status -eq 'Minor') 
{
  Write-Host $False
} ElseIf ($Status -eq 'Adult') {
  Write-Host $True
} Else {
  Write-Host $False
}

It's possible to write this code in a more compact way, but this way does show the use of ElseIf. It shows how If is evaluated first, then ElseIf, and then Else.

Note

As with Else, you can't use ElseIf if you don't define an If above it.