Repeat Operations with Loops

1. Problem

You want to execute the same block of code more than once.

2. Solution

Use one of PowerShell’s looping statements (for, foreach, while, and do), or PowerShell’s Foreach-Object cmdlet to run a command or script block more than once. For a detailed description of these looping statements, see “Looping Statements” in . For example:

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        for loop
                 for($counter = 1; $counter -le 10; $counter++)
                 {
                    "Loop number $counter"
                 }
        foreach loop
                 foreach($file in dir)
                 {
                    "File length: " + $file.Length
                 }
        Foreach-Object cmdlet
                Get-ChildItem | Foreach-Object { "File length: " + $_.Length }
 
        while loop
                 $response = ""
                 while($response -ne "QUIT")
                 {
                    $response = Read-Host "Type something"
                 }
        do..while loop
                 $response = ""
                 do
                 {
                    $response = Read-Host "Type something"
                 } while($response -ne "QUIT")                                      



3. Discussion

Although any of the looping statements can be written to be functionally equivalent to any of the others, each lends itself to certain problems.

You usually use a for loop when you need to perform an operation an exact number of times. Because using it this way is so common, it is often called a counted for loop.

You usually use a foreach loop when you have a collection of objects and want to visit each item in that collection. If you do not yet have that entire collection in memory (as in the dir collection from the foreach example above), the Foreach-Object cmdlet is usually a more efficient alternative.

Unlike the foreach loop, the Foreach-Object cmdlet allows you to process each element in the collection as PowerShell generates it. This is an important distinction; asking PowerShell to collect the entire output of a large command (such as Get-Content hugefile.txt) in a foreach loop can easily drag down your system.

A handy shortcut to repeat an operation on the command line is:

        PS >1..10 | foreach { "Working" }
        Working
        Working
        Working
        Working
        Working
        Working
        Working 
        Working
        Working
        Working

The while and do..while loops are similar, in that they continue to execute the loop as long as its condition evaluates to true. A while loop checks for this before ever running your script block, while a do..while loop checks the condition after running your script block.

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July 22nd, 2008 | administration, power shell, server, windows