ForEach-Object

Syntax

ForEach-Object
            [-InputObject <PSObject>]
            [-Begin <ScriptBlock>]
            [-Process] <ScriptBlock[]>
            [-End <ScriptBlock>]
            [-RemainingScripts <ScriptBlock[]>]
            [-WhatIf]
            [-Confirm]
            [<CommonParameters>]

ForEach-Object
            [-InputObject <PSObject>]
            [-MemberName] <String>
            [-ArgumentList <Object[]>]
            [-WhatIf]
            [-Confirm]
            [<CommonParameters>]

ForEach-Object
            -Parallel <scriptblock>
            [-InputObject <psobject>]
            [-ThrottleLimit <int>]
            [-TimeoutSeconds <int>]
            [-AsJob]
            [-UseNewRunspace]
            [-WhatIf]
            [-Confirm]
            [<CommonParameters>]

Description

The ForEach-Object cmdlet performs an operation on each item in a collection of input objects. The input objects can be piped to the cmdlet or specified using the InputObject parameter.

Starting in Windows PowerShell 3.0, there are two different ways to construct a ForEach-Object command.

• Script block. You can use a script block to specify the operation. Within the script block, use the $_ variable to represent the current object. The script block is the value of the Process parameter. The script block can contain any PowerShell script.

  For example, the following command gets the value of the ProcessName property of each process on the computer.

  Get-Process | ForEach-Object {$_.ProcessName}

  ForEach-Object supports the begin, process, and end blocks as described in about_functions.

  Note

  The script blocks run in the caller's scope. Therefore, the blocks have access to variables in that scope and can create new variables that persist in that scope after the cmdlet completes.

• Operation statement. You can also write an operation statement, which is much more like natural language. You can use the operation statement to specify a property value or call a method. Operation statements were introduced in Windows PowerShell 3.0.

  For example, the following command also gets the value of the ProcessName property of each process on the computer.

  Get-Process | ForEach-Object ProcessName

• Parallel running script block. Beginning with PowerShell 7.0, a third parameter set is available that runs each script block in parallel. The ThrottleLimit parameter limits the number of parallel scripts running at a time. As before, use the $_ variable to represent the current input object in the script block. Use the $using: keyword to pass variable references to the running script.

  In PowerShell 7, a new runspace is created for each loop iteration to ensure maximum isolation. This can be a large performance and resource hit if the work you are doing is small compared to creating new runspaces or if there are a lot of iterations performing significant work. As of PowerShell 7.1, runspaces from a runspace pool are reused by default. The ThrottleLimit parameter sets the runspace pool size. The default runspace pool size is 5. You can still create a new runspace for each iteration using the UseNewRunspace switch.

  By default, the parallel scriptblocks use the current working directory of the caller that started the parallel tasks.

  For more information, see the NOTES section of this article.

Examples

Example 1: Divide integers in an array

This example takes an array of three integers and divides each one of them by 1024.

30000, 56798, 12432 | ForEach-Object -Process {$_/1024}

29.296875
55.466796875
12.140625

Example 2: Get the length of all the files in a directory

This example processes the files and directories in the PowerShell installation directory $PSHOME.

Get-ChildItem $PSHOME |
  ForEach-Object -Process {if (!$_.PSIsContainer) {$_.Name; $_.Length / 1024; " " }}

If the object isn't a directory, the script block gets the name of the file, divides the value of its Length property by 1024, and adds a space (" ") to separate it from the next entry. The cmdlet uses the PSISContainer property to determine whether an object is a directory.

Example 3: Operate on the most recent System events

This example writes the 1000 most recent events from the System event log to a text file. The current time is displayed before and after processing the events.

Get-EventLog -LogName System -Newest 1000 |
    ForEach-Object -Begin {Get-Date} -Process {
        Out-File -FilePath Events.txt -Append -InputObject $_.Message
    } -End {Get-Date}

Get-EventLog gets the 1000 most recent events from the System event log and pipes them to the ForEach-Object cmdlet. The Begin parameter displays the current date and time. Next, the Process parameter uses the Out-File cmdlet to create a text file that's named events.txt and stores the message property of each of the events in that file. Last, the End parameter is used to display the date and time after all the processing has completed.

Example 4: Change the value of a Registry key

This example changes the value of the RemotePath registry entry in all the subkeys under the HKCU:\Network key to uppercase text.

Get-ItemProperty -Path HKCU:\Network\* |
  ForEach-Object {
    Set-ItemProperty -Path $_.PSPath -Name RemotePath -Value $_.RemotePath.ToUpper()
  }

You can use this format to change the form or content of a registry entry value.

Each subkey in the Network key represents a mapped network drive that reconnects at sign on. The RemotePath entry contains the UNC path of the connected drive. For example, if you map the E: drive to \\Server\Share, an E subkey is created in HKCU:\Network with the RemotePath registry value set to \\Server\Share.

The command uses the Get-ItemProperty cmdlet to get all the subkeys of the Network key and the Set-ItemProperty cmdlet to change the value of the RemotePath registry entry in each key. In the Set-ItemProperty command, the path is the value of the PSPath property of the registry key. This is a property of the Microsoft .NET Framework object that represents the registry key, not a registry entry. The command uses the ToUpper() method of the RemotePath value, which is a string REG_SZ.

Because Set-ItemProperty is changing the property of each key, the ForEach-Object cmdlet is required to access the property.

Example 5: Use the $null automatic variable

This example shows the effect of piping the $null automatic variable to the ForEach-Object cmdlet.

1, 2, $null, 4 | ForEach-Object {"Hello"}

Hello
Hello
Hello
Hello

Because PowerShell treats $null as an explicit placeholder, the ForEach-Object cmdlet generates a value for $null as it does for other objects piped to it.

Example 6: Get property values

This example gets the value of the Path property of all installed PowerShell modules using the MemberName parameter of the ForEach-Object cmdlet.

Get-Module -ListAvailable | ForEach-Object -MemberName Path
Get-Module -ListAvailable | Foreach Path

The second command is equivalent to the first. It uses the Foreach alias of the ForEach-Object cmdlet and omits the name of the MemberName parameter, which is optional.

The ForEach-Object cmdlet is useful for getting property values, because it gets the value without changing the type, unlike the Format cmdlets or the Select-Object cmdlet, which change the property value type.

Example 7: Split module names into component names

This example shows three ways to split two dot-separated module names into their component names. The commands call the Split method of strings. The three commands use different syntax, but they are equivalent and interchangeable. The output is the same for all three cases.

"Microsoft.PowerShell.Core", "Microsoft.PowerShell.Host" |
    ForEach-Object {$_.Split(".")}
"Microsoft.PowerShell.Core", "Microsoft.PowerShell.Host" |
    ForEach-Object -MemberName Split -ArgumentList "."
"Microsoft.PowerShell.Core", "Microsoft.PowerShell.Host" |
    Foreach Split "."

Microsoft
PowerShell
Core
Microsoft
PowerShell
Host

The first command uses the traditional syntax, which includes a script block and the current object operator $_. It uses the dot syntax to specify the method and parentheses to enclose the delimiter argument.

The second command uses the MemberName parameter to specify the Split method and the ArgumentList parameter to identify the dot (.) as the split delimiter.

The third command uses the Foreach alias of the ForEach-Object cmdlet and omits the names of the MemberName and ArgumentList parameters, which are optional.

Example 8: Using ForEach-Object with two script blocks

In this example, we pass two script blocks positionally. All the script blocks bind to the Process parameter. However, they're treated as if they had been passed to the Begin and Process parameters.

1..2 | ForEach-Object { 'begin' } { 'process' }

begin
process
process

Example 9: Using ForEach-Object with more than two script blocks

In this example, we pass four script blocks positionally. All the script blocks bind to the Process parameter. However, they're treated as if they had been passed to the Begin, Process, and End parameters.

1..2 | ForEach-Object { 'begin' } { 'process A' }  { 'process B' } { 'end' }

begin
process A
process B
process A
process B
end

Example 10: Run multiple script blocks for each pipeline item

As shown in the previous example, multiple script blocks passed using the Process parameter get mapped to the Begin and End parameters. To avoid this mapping, you must provide explicit values for the Begin and End parameters.

1..2 | ForEach-Object -Begin $null -Process { 'one' }, { 'two' }, { 'three' } -End $null

one
two
three
one
two
three

Example 11: Run slow script in parallel batches

This example runs a script block that evaluates a string and sleeps for one second.

$Message = "Output:"

1..8 | ForEach-Object -Parallel {
    "$using:Message $_"
    Start-Sleep 1
} -ThrottleLimit 4

Output: 1
Output: 2
Output: 3
Output: 4
Output: 5
Output: 6
Output: 7
Output: 8

The ThrottleLimit parameter value is set to 4 so that the input is processed in batches of four. The $using: keyword is used to pass the $Message variable into each parallel script block.

Example 12: Retrieve log entries in parallel

This example retrieves 50,000 log entries from 5 system logs on a local Windows machine.

$logNames = 'Security', 'Application', 'System', 'Windows PowerShell',
    'Microsoft-Windows-Store/Operational'

$logEntries = $logNames | ForEach-Object -Parallel {
    Get-WinEvent -LogName $_ -MaxEvents 10000
} -ThrottleLimit 5

$logEntries.Count

50000

The Parallel parameter specifies the script block that's run in parallel for each input log name. The ThrottleLimit parameter ensures that all five script blocks run at the same time.

Example 13: Run in parallel as a job

This example creates a job that runs a script block in parallel, two at a time.

PS> $job = 1..10 | ForEach-Object -Parallel {
    "Output: $_"
    Start-Sleep 1
} -ThrottleLimit 2 -AsJob

PS> $job

Id     Name            PSJobTypeName   State         HasMoreData     Location      Command
--     ----            -------------   -----         -----------     --------      -------
23     Job23           PSTaskJob       Running       True            PowerShell    …

PS> $job.ChildJobs

Id     Name            PSJobTypeName   State         HasMoreData     Location      Command
--     ----            -------------   -----         -----------     --------      -------
24     Job24           PSTaskChildJob  Completed     True            PowerShell    …
25     Job25           PSTaskChildJob  Completed     True            PowerShell    …
26     Job26           PSTaskChildJob  Running       True            PowerShell    …
27     Job27           PSTaskChildJob  Running       True            PowerShell    …
28     Job28           PSTaskChildJob  NotStarted    False           PowerShell    …
29     Job29           PSTaskChildJob  NotStarted    False           PowerShell    …
30     Job30           PSTaskChildJob  NotStarted    False           PowerShell    …
31     Job31           PSTaskChildJob  NotStarted    False           PowerShell    …
32     Job32           PSTaskChildJob  NotStarted    False           PowerShell    …
33     Job33           PSTaskChildJob  NotStarted    False           PowerShell    …

The ThrottleLimit parameter limits the number of parallel script blocks running at a time. The AsJob parameter causes the ForEach-Object cmdlet to return a job object instead of streaming output to the console. The $job variable receives the job object that collects output data and monitors running state. The $job.ChildJobs property contains the child jobs that run the parallel script blocks.

Example 14: Using thread safe variable references

This example invokes script blocks in parallel to collect uniquely named Process objects.

$threadSafeDictionary = [System.Collections.Concurrent.ConcurrentDictionary[string,object]]::new()
Get-Process | ForEach-Object -Parallel {
    $dict = $using:threadSafeDictionary
    $dict.TryAdd($_.ProcessName, $_)
}

$threadSafeDictionary["pwsh"]

NPM(K)    PM(M)      WS(M)     CPU(s)      Id  SI ProcessName
 ------    -----      -----     ------      --  -- -----------
     82    82.87     130.85      15.55    2808   2 pwsh

A single instance of a ConcurrentDictionary object is passed to each script block to collect the objects. Since the ConcurrentDictionary is thread safe, it's safe to be modified by each parallel script. A non-thread-safe object, such as System.Collections.Generic.Dictionary, would not be safe to use here.

Example 15: Writing errors with parallel execution

This example writes to the error stream in parallel, where the order of written errors is random.

1..3 | ForEach-Object -Parallel {
    Write-Error "Error: $_"
}

Write-Error: Error: 1
Write-Error: Error: 3
Write-Error: Error: 2

Example 16: Terminating errors in parallel execution

This example demonstrates a terminating error in one parallel running scriptblock.

1..5 | ForEach-Object -Parallel {
    if ($_ -eq 3)
    {
        throw "Terminating Error: $_"
    }

    Write-Output "Output: $_"
}

Exception: Terminating Error: 3
Output: 1
Output: 4
Output: 2
Output: 5

Output: 3 is never written because the parallel scriptblock for that iteration was terminated.

Example 17: Passing variables in nested parallel script ScriptBlockSet

You can create a variable outside a Foreach-Object -Parallel scoped scriptblock and use it inside the scriptblock with the $using keyword.

$test1 = 'TestA'
1..2 | Foreach-Object -Parallel {
    $using:test1
}

TestA
TestA

# You CANNOT create a variable inside a scoped scriptblock
# to be used in a nested foreach parallel scriptblock.
$test1 = 'TestA'
1..2 | Foreach-Object -Parallel {
    $using:test1
    $test2 = 'TestB'
    1..2 | Foreach-Object -Parallel {
        $using:test2
    }
}

Line |
   2 |  1..2 | Foreach-Object -Parallel {
     |         ~~~~~~~~~~~~~~~~~~~~~~~~~~
     | The value of the using variable '$using:test2' can't be retrieved because it has
     | not been set in the local session.

The nested scriptblock can't access the $test2 variable and an error is thrown.

Example 18: Creating multiple jobs that run scripts in parallel

The ThrottleLimit parameter limits the number of parallel scripts running during each instance of ForEach-Object -Parallel. It doesn't limit the number of jobs that can be created when using the AsJob parameter. Since jobs themselves run concurrently, it's possible to create multiple parallel jobs, each running up to the throttle limit number of concurrent scriptblocks.

$jobs = for ($i=0; $i -lt 10; $i++) {
    1..10 | ForEach-Object -Parallel {
        ./RunMyScript.ps1
    } -AsJob -ThrottleLimit 5
}

$jobs | Receive-Job -Wait

This example creates 10 running jobs. Each job runs no more that 5 scripts concurrently. The total number of instances running concurrently is limited to 50 (10 jobs times the ThrottleLimit of 5).

Parameters

-ArgumentList

-AsJob

-Begin

-Confirm

-End

-InputObject

-MemberName

-Parallel

-Process

-RemainingScripts

-ThrottleLimit

-TimeoutSeconds

-UseNewRunspace

-WhatIf

Inputs

PSObject

You can pipe any object to this cmdlet.

Outputs

PSObject

This cmdlet returns objects that are determined by the input.

Notes

PowerShell includes the following aliases for ForEach-Object:

• All Platforms:
  • %
  • foreach

The ForEach-Object cmdlet works much like the Foreach statement, except that you can't pipe input to a Foreach statement. For more information about the Foreach statement, see about_Foreach.

Starting in PowerShell 4.0, Where and ForEach methods were added for use with collections. You can read more about these new methods here about_arrays

Using ForEach-Object -Parallel:

• ForEach-Object -Parallel runs each script block in a new runspace. The new runspaces create significantly more overhead than running ForEach-Object with sequential processing. It's important to use Parallel where the overhead of running in parallel is small compared to work the script block performs. For example:

  • Compute intensive scripts on multi-core machines
  • Scripts that spend time waiting for results or doing file operations

  Using the Parallel parameter can cause scripts to run much slower than normal. Especially if the parallel scripts are trivial. Experiment with Parallel to discover where it can be beneficial.

• When running in parallel, objects decorated with ScriptProperties or ScriptMethods can't be guaranteed to function correctly if they're run in a different runspace than the scripts were originally attached to them.

  Scriptblock invocation always attempts to run in its home runspace, regardless of where it's actually invoked. However, ForEach-Object -Parallel creates temporary runspaces that get deleted after use, so there's no runspace for the scripts to execute in anymore.

  This behavior can work as long as the home runspace still exists. However, you may not get the desired result if the script is dependent on external variables that are only present in the caller's runspace and not the home runspace.

• Non-terminating errors are written to the cmdlet error stream as they occur in parallel running scriptblocks. Because parallel scriptblock execution order is non-deterministic, the order in which errors appear in the error stream is random. Likewise, messages written to other data streams, like warning, verbose, or information are written to those data streams in an indeterminate order.

  Terminating errors, such as exceptions, terminate the individual parallel instance of the scriptblocks in which they occur. A terminating error in one scriptblocks may not cause the termination of the Foreach-Object cmdlet. The other scriptblocks, running in parallel, continue to run unless they also encounter a terminating error. The terminating error is written to the error data stream as an ErrorRecord with a FullyQualifiedErrorId of PSTaskException. Terminating errors can be converted to non-terminating errors using PowerShell try/catch or trap blocks.

• PipelineVariable common parameter variables are not supported in parallel scenarios even with the $using: keyword.

  Important

  The ForEach-Object -Parallel parameter set runs script blocks in parallel on separate process threads. The $using: keyword allows passing variable references from the cmdlet invocation thread to each running script block thread. Since the script blocks run in different threads, the object variables passed by reference must be used safely. Generally it's safe to read from referenced objects that don't change. If you need to modify the object state then you must use thread safe objects, such as .NET System.Collection.Concurrent types (See Example 14).

Related Links

• Compare-Object
• Where-Object
• Group-Object
• Measure-Object
• New-Object
• Select-Object
• Sort-Object
• Tee-Object