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About ForEach

JuanPablo Jofre|Last Updated: 11/17/2016
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2 Contributors

about_ForEach

SHORT DESCRIPTION

Describes a language command you can use to traverse all the items in a collection of items.

LONG DESCRIPTION

The Foreach statement (also known as a Foreach loop) is a language construct for stepping through (iterating) a series of values in a collection of items.

The simplest and most typical type of collection to traverse is an array. Within a Foreach loop, it is common to run one or more commands against each item in an array.

Syntax The following shows the ForEach syntax:

foreach ($ in $){}

Simplified syntax Starting in Windows PowerShell 3.0, syntax with language keywords such as Where and ForEach was simplified. Comparison operators that work on the members of a collection are treated as parameters. You can use a method on the members of a collection without containing it in a script block or adding the automatic variable "$_.". Consider the following two examples:

dir cert:\ -Recurse | foreach GetKeyAlgorithm dir cert:\ -Recurse | foreach {$_.GetKeyAlgorithm()}

Although both commands work, the first returns results without using a script block or the $_. automatic variable. The method GetKeyAlgorithm is treated as a parameter of ForEach. The first command returns the same results, but without errors, because the simplified syntax does not attempt to return results for items for which the specified argument did not apply.

In this example, the Get-Process property Description is passed as a parameter argument of the ForEach statement. The results are the descriptions of active processes.

Get-Process | ForEach Description

The Foreach statement outside a command pipeline The part of the Foreach statement enclosed in parenthesis represents a variable and a collection to iterate. Windows PowerShell creates the variable ($) automatically when the Foreach loop runs. Prior to each iteration through the loop, the variable is set to a value in the collection. The block following a Foreach statement {} contains a set of commands to execute against each item in a collection.

Examples For example, the Foreach loop in the following example displays the values in the $letterArray array.

$letterArray = "a","b","c","d" foreach ($letter in $letterArray) { Write-Host $letter }

In this example, the $letterArray array is created and initialized with the string values "a", "b", "c", and "d". The first time the Foreach statement runs, it sets the $letter variable equal to the first item in $letterArray ("a"). Then, it uses the Write-Host cmdlet to display the letter a. The next time through the loop, $letter is set to "b", and so on. After the Foreach loop displays the letter d, Windows PowerShell exits the loop.

The entire Foreach statement must appear on a single line to run it as a command at the Windows PowerShell command prompt. The entire Foreach statement does not have to appear on a single line if you place the command in a .ps1 script file instead.

Foreach statements can also be used together with cmdlets that return a collection of items. In the following example, the Foreach statement steps through the list of items that is returned by the Get-ChildItem cmdlet.

foreach ($file in Get-ChildItem) { Write-Host $file }

You can refine the example by using an If statement to limit the results that are returned. In the following example, the Foreach statement performs the same looping operation as the previous example, but it adds an If statement to limit the results to files that are greater than 100 kilobytes (KB):

foreach ($file in Get-ChildItem) { if ($file.length -gt 100KB) { Write-Host $file } }

In this example, the Foreach loop uses a property of the $file variable to perform a comparison operation ($file.length -gt 100KB). The $file variable contains all the properties in the object that is returned by the Get-ChildItem cmdlet. Therefore, you can return more than just a file name. In the next example, Windows PowerShell returns the length and the last access time inside the statement list:

foreach ($file in Get-ChildItem) { if ($file.length -gt 100KB) { Write-Host $file Write-Host $file.length Write-Host $file.lastaccesstime } }

In this example, you are not limited to running a single command in a statement list.

You can also use a variable outside a Foreach loop and increment the variable inside the loop. The following example counts files over 100 KB in size:

$i = 0 foreach ($file in Get-ChildItem) { if ($file.length -gt 100KB) { Write-Host $file "file size:" ($file.length / 1024).ToString("F0") KB $i = $i + 1 } }

if ($i -ne 0) { Write-Host Write-Host $i " file(s) over 100 KB in the current directory."} else { Write-Host "No files greater than 100 KB in the current directory." }

In the preceding example, the $i variable is set to 0 outside the loop, and the variable is incremented inside the loop for each file that is found that is larger than 100 KB. When the loop exits, an If statement evaluates the value of $i to display a count of all the files over 100 KB. Or, it displays a message stating that no files over 100 KB were found.

The previous example also demonstrates how to format the file length results:

($file.length / 1024).ToString("F0")

The value is divided by 1,024 to show the results in kilobytes rather than bytes, and the resulting value is then formatted using the fixed-point format specifier to remove any decimal values from the result. The 0 makes the format specifier show no decimal places.

The Foreach Statement Inside a Command Pipeline When Foreach appears in a command pipeline, Windows PowerShell uses the foreach alias, which calls the ForEach-Object command. When you use the foreach alias in a command pipeline, you do not include the ($ in $) syntax as you do with the Foreach statement. This is because the prior command in the pipeline provides this information. The syntax of the foreach alias when used in a command pipeline is as follows:

| foreach {}

For example, the Foreach loop in the following command displays processes whose working set (memory usage) is greater than 20 megabytes (MB).

The Get-Process command gets all of the processes on the computer. The Foreach alias performs the commands in the script block on each process in sequence.

The IF statement selects processes with a working set (WS) greater than 20 megabytes. The Write-Host cmdlet writes the name of the process followed by a colon. It divides the working set value, which is stored in bytes by 1 megabyte to get the working set value in megabytes. Then it converts the result from a double to a string. It displays the value as a fixed point number with zero decimals (F0), a space separator (" "), and then "MB".

Write-Host "Processes with working sets greater than 20 MB." Get-Process | foreach { if ($_.WS -gt 20MB) { Write-Host $.name ": " ($.WS/1MB).ToString("F0") MB -Separator ""} }

The foreach alias also supports beginning command blocks, middle command blocks, and end command blocks. The beginning and end command blocks run once, and the middle command block runs every time the Foreach loop steps through a collection or array.

The syntax of the foreach alias when used in a command pipeline with a beginning, middle, and ending set of command blocks is as follows:

| foreach {}{}{}

The following example demonstrates the use of the beginning, middle, and end command blocks.

Get-ChildItem | foreach { $fileCount = $directoryCount = 0}{ if ($_.PsIsContainer) {$directoryCount++} else {$fileCount++}}{ "$directoryCount directories and $fileCount files"}

The beginning block creates and initializes two variables to 0:

{$fileCount = $directoryCount = 0}

The middle block evaluates whether each item returned by Get-ChildItem is a directory or a file:

{if ($_.PsIsContainer) {$directoryCount++} else {$fileCount++}}

If the item that is returned is a directory, the $directoryCount variable is incremented by 1. If the item is not a directory, the $fileCount variable is incremented by 1. The ending block runs after the middle block completes its looping operation and then returns the results of the operation:

{"$directoryCount directories and $fileCount files"}

By using the beginning, middle, and ending command block structure and the pipeline operator, you can rewrite the earlier example to find any files that are greater than 100 KB, as follows:

Get-ChildItem | foreach{ $i = 0}{ if ($_.length -gt 100KB) { Write-Host $.name "file size:" ($.length / 1024).ToString("F0") KB $i++ } }{ if ($i -ne 0) { Write-Host Write-Host "$i file(s) over 100 KB in the current directory." } else { Write-Host "No files greater than 100 KB in the current directory."} }

The following example, a function which returns the functions that are used in scripts and script modules, demonstrates how to use the MoveNext method (which works similarly to "skip X" on a For loop) and the Current property of the $foreach variable inside of a foreach script block, even if there are unusually- or inconsistently-spaced function definitions that span multiple lines to declare the function name. The example also works if there are comments in the functions used in a script or script module.

function Get-FunctionPosition { [CmdletBinding()] [OutputType('FunctionPosition')] param( [Parameter(Position=0, Mandatory, ValueFromPipeline, ValueFromPipelineByPropertyName)] [ValidateNotNullOrEmpty()] [Alias('PSPath')] [System.String[]] $Path

)

process { try { $filesToProcess = if ($_ -is [System.IO.FileSystemInfo]) {

$_

} else { Get-Item -Path $Path } foreach ($item in $filesToProcess) { if ($item.PSIsContainer -or $item.Extension -notin @('.ps1','.psm1')) { continue } $tokens = $errors = $null $ast = [System.Management.Automation.Language.Parser]::ParseFile($item.FullName,([REF]$tokens),([REF]$errors)) if ($errors) { Write-Warning "File '$($item.FullName)' has $($errors.Count) parser errors." } :tokenLoop foreach ($token in $tokens) { if ($token.Kind -ne 'Function') { continue } $position = $token.Extent.StartLineNumber do { if (-not $foreach.MoveNext()) { break tokenLoop } $token = $foreach.Current } until ($token.Kind -in @('Generic','Identifier')) $functionPosition = [pscustomobject]@{ Name = $token.Text LineNumber = $position Path = $item.FullName } Add-Member -InputObject $functionPosition -TypeName FunctionPosition -PassThru } } } catch { throw } } }

SEE ALSO

about_Automatic_Variables about_If Foreach-Object

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