Getting Loopy: An Introduction to PowerShell Loops

• For
• ForEach-Object
• Foreach
• While
• Do-While
• Do-Until

Some of these loops are traditional in how they function, while others such as ForEach-Object are unique to PowerShell and how the PowerShell object pipeline works. All of them have their place and are useful in different ways. With that being said, let’s jump into PowerShell loops!

For ($i = 1; $i -LE 5; $i++) {
		$i * 2 
		}

# Output 2, 4, 6, 8, 10

This simple loop operates on the $i variable and as you can see simply multiplies the value of the iterator by 2 each time the loop is run.

To counteract that, the foreach-statement in PowerShell allows front-loading all items of a collection of objects that should be iterated over instead of streaming those objects in like ForEach-Object does. This comes at the cost of higher memory usage but increased performance.

$Array = 1..5 

Foreach ($Number In $Array) {
		$Number
		} 
		
# Output 
1, 2, 3, 4, 5

A useful ability is that Foreach can be used with a cmdlet that returns a collection of objects. One of the most common examples of this is using Get-ChildItem to return a collection of files.

 

$i = 1 Foreach ($File in Get-ChildItem) {
"File {0}: {1}" -F $i, $File.FullName $i++
}
# Output
File 1: C:\Test\File1.txt
File 2: C:\Test\File2.txt

Another very useful feature is the ability to skip a number by using the special function of $foreach.MoveNext(). This allows you to skip the current loop and move to the next value. In this case I am sending that output to Out-Null as the default returned value is true which I do not want to display.

$Array = 1..20
Foreach ($Number In $Array) { 
		If ($Number % 2) {
		$Foreach.MoveNext() | Out-Null 
		} 
		$Number 
		} 
		
# Output
1, 3, 5, 7, 9, 11, 13, 15, 17, 19

• Ability to use Begin, Process, and End blocks
• Stream resulting objects to cmdlets further down the pipeline

$Array = @("Item4", "Item2", "Item1", "Item3") 
$Array | ForEach-Object {
		"{0}.{1}" -F $_, "txt" 
		} | Sort-Object 
		
# Output Item1.txt, Item2.txt, Item3.txt, Item4.txt

As mentioned above, the other useful feature is the ability to define a Begin and End block, along with the standard Process block.

$Array = @("Item4", "Item2", "Item1", "Item3")
$Array | ForEach-Object { $i = 0 } {
		"File {0}: {1}.{2}" -F $i, $_, "txt" 
		$i++ 
		} { Write-Host "Files Count: $i" } 
		
# Output 
File 0: Item4.txt 
File 1: Item2.txt 
File 2: Item1.txt 
File 3: Item3.txt 
Files Count: 4

While ($Value -NE 3) { 
		$Value++ 
		Write-Host "Value: $Value" 
		} 

#Output 
Value: 1 
Value: 2 
Value: 3

An example of an infinite loop is by simply having a condition that will always evaluate to true. You can do this by the following code.

While ($True) { 
		"This will run forever!" 
		}

#Output
This will run forever!

Do { 
		Write-Host "This will display no matter what" 
		} While ($False) 
		
# Output
This will display no matter what

Do { 
		Write-Host "This will display no matter what" 
		} Until ($True) 
		
# Output
This will display no matter what

A whole range of examples can be found in the ScriptRunner ActionPacks. These ready-to-use and free PowerShell scripts allow you to quickly and easily implement numerous use cases in the area of systems administration and IT operations management.