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Windows PowerShell Job Tips and Tricks

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Timothy Warner, author of Sams Teach Yourself Windows PowerShell in 24 Hours, teaches you how to take your PowerShell local and remote job skills to the next level.
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The Windows PowerShell jobs engine allows us to run local and/or remote tasks in the background of an elevated console session. For instance, let's say we needed to find all .PDF files in a huge folder tree:

Get-ChildItem -Path E:\ -Recurse -Filter *.pdf

If your E:\ drive is anything like mine, the previous task is going to hang up your PowerShell console session for a long time. Annoying!

By contrast, if we put the previous Get-ChildItem call into a job, we can happily continue working in our session:

Start-Job -Name 'PDFSearch-EDrive' -ScriptBlock { Get-ChildItem -Path E:\ -Recurse -Filter *.pdf }

Id     Name            PSJobTypeName   State         HasMoreData     Location
--     ----            -------------   -----         -----------     --------
2      PDFSearch-ED... BackgroundJob   Running       True            localhost

We can use Get-Job to check job status:

Get-Job -Id 2

Id     Name            PSJobTypeName   State         HasMoreData     Location
--     ----            -------------   -----         -----------     --------
2      PDFSearch-CD... BackgroundJob   Completed     True            localhost

Finally, we can use Retrieve-Job with the -Keep switch to view the job results and keep them in memory for future reference:

Receive-Job -Id 2 -Keep | Select-Object -First 2

    Directory: E:\lab\db\commbutton

Mode             LastWriteTime      Length Name
----             -------------      ------ ----
-a----     4/23/2015   9:55 AM      27667CCENTv3.pdf
-a----     4/23/2015   9:55 AM      56913ITTv3.pdf

All of this information I've given you is well and good, but it should just be review. I want to take your PowerShell job skills to the next level by teaching you four "power user" tips and tricks:

  • Saving your job results by default
  • Leveraging the -AsJob switch parameter
  • Troubleshooting child jobs
  • Making use of community contributions

With no further ado, let's begin!

Saving Your Job Results by Default

If you forget to add the -Keep switch parameter to the Receive-Job cmdlet, your job results go up the proverbial chimney. Personally, that default behavior in Windows PowerShell ticks me off.

My friend and Windows PowerShell MVP Jeff Hicks taught me a neat trick to ensure that Receive-Job always runs with the -Keep switch, at least for the current session.

All we have to do is modify the default parameter values for our session. Try this:

$PSDefaultParameterValues = @{'Receive-Job:Keep'=$true}

Now create, run, and receive another PowerShell job. You should find that the job results persist in your current runspace thanks to our new default parameter value. We can view the contents of this automatic variable at any time:

$PSDefaultParameterValues

Name                           Value
----                           -----
Receive-Job:Keep               True

This default parameter change disappears when you close your current PowerShell session. Thus, you'll want to add that line to your PowerShell profile(s) as necessary if it's something you need for the future.

For some reason I'm inspired to remind you that you're free to export job results to, say, a text file or a delimited file. For instance, the following code creates a job that pulls event log data, and then stores it in a simple text file:

Start-Job -Name 'systemlog' -ScriptBlock { Get-EventLog -LogName System -Newest 50 }
Receive-Job -Id 2 -Keep | Out-File -Path '.\event.txt'

Ever the detective, PowerShell MVP Boe Prox taught me another way to retrieve job data if we forget the -Keep switch. For this example, I'll create a job showing processes that consume more than 100MB of RAM on my Windows 8.1 system:

Start-Job -Name 'highmem' -ScriptBlock { Get-Process | Where-Object { $_.WS -gt 100MB } }

If I run a Receive-Job and then a Get-Job again, the HasMoreData property is False as expected:

Receive-Job -Name highmem
Get-Job -Name highmem

Id     Name            PSJobTypeName   State         HasMoreData     Location
--     ----            -------------   -----         -----------     --------
4      highmem         BackgroundJob   Completed     False           localhost

Okay, here's where the .NET magic comes in. I'll store the highmem job as a variable, and then check out its ChildJobs.output property:

PS C:\> $highmem.ChildJobs.output | Select-Object -First 2

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----      ----- -----   ------     -- -----------
   1156      37   190532     167728   794 8,549.25   1120 chrome
    188      26   168340     157188   915   434.78   1672 chrome

Whoa—isn't that awesome?

Leveraging the -AsJob Switch Parameter

Instead of formally defining background jobs with Get-Job, we can tack on the -AsJob parameter to some cmdlets to create ad hoc background jobs. Let's see which of the PowerShell core cmdlets support this switch:

PS C:\> Get-Command -ParameterName AsJob | Select-Object -Property Name

Name
----
Get-WmiObject
Invoke-Command
Invoke-WmiMethod
Remove-WmiObject
Restart-Computer
Set-WmiInstance
Stop-Computer
Test-Connection

Let's focus on Invoke-Command because it uses WS-Man remoting and is therefore more efficient than the old DCOM/RPC remoting used by most of those other commands.

Check this out:

Invoke-Command -ComputerName dc1,mem1,mem2 -ScriptBlock { Get-EventLog -LogName Security -Newest 2 } -AsJob

Id     Name            PSJobTypeName   State         HasMoreData     Location
--     ----            -------------   -----         -----------     --------
15     Job15           RemoteJob       Running       True            dc1,mem1,mem2

Notice that the PSJobTypeName is RemoteJob instead of the BackgroundJob type we've seen thus far.

Troubleshooting Child Jobs

Something many PowerShell administrative scripts don't (yet) understand is that jobs in PowerShell always consist of one parent job and one or more child jobs. For instance, consider the Invoke-Command -AsJob example we used previously:

Invoke-Command -ComputerName dc1,mem1,mem2 -ScriptBlock { Get-EventLog -LogName Security -Newest 2 } -AsJob

Id     Name            PSJobTypeName   State         HasMoreData     Location
--     ----            -------------   -----         -----------     --------
15     Job15           RemoteJob       Running       True            dc1,mem1,mem2

That ID of 15 refers to the parent job container, and I'm sure that PowerShell has a separate child job defined for each computer queried by our Get-EventLog script block. How do I know? By using the -IncludeChildJob switch of the Get-Job cmdlet:

PS C:\> Get-Job -IncludeChildJob

Id     Name            PSJobTypeName   State         HasMoreData     Location
--     ----            -------------   -----         -----------     --------
15     Job15           RemoteJob       Completed     True            dc1,mem1,mem2
16     Job16                           Completed     True            dc1
17     Job17                           Completed     True            mem1
18     Job18                           Completed     True            mem2

Understanding child jobs makes troubleshooting easier because a failure in a child job can cause the parent job to report a failed status. To that point, we can retrieve child job contents simply by using the child job ID:

Receive-Job -Id 18 -Keep

   Index Time          EntryType   Source                 InstanceID Message    PSCompute
                                                                                rName
   ----- ----          ---------   ------                 ---------- -------    ---------
    4104 Jan 13 10:09  SuccessA... Microsoft-Windows...         4624 An acco... mem2
    4103 Jan 13 10:09  SuccessA... Microsoft-Windows...         4672 Special... mem2

Making Use of Community Contributions

Please keep in mind that Windows PowerShell is largely a community-driven technology stack; for proof, notice how many PowerShell features are hosted by Microsoft in GitHub repositories.

Once again, Boe Prox offers us an alternative to the native PowerShell job engine in his PoshRSJob module. Let's use PowerShellGet to install it:

Find-Module -Name poshrsjob | Install-Module -Verbose

Boe wrote the PoshRSJob module in an attempt to improve background job performance in PowerShell. He also wanted to give users the ability to throttle their jobs' resource utilization. Here's the command set:

PS C:\> Get-Command -Module PoshRSJob | Select-Object -Property Name

Name
----
Get-RSJob
Receive-RSJob
Remove-RSJob
Start-RSJob
Stop-RSJob
Wait-RSJob

Let's say we want to write a job that checks the status of the Windows Update service on our three servers, but we want to make the job more resource-friendly on our system and the network. For instance, normally Invoke-Command operates on 25 computers simultaneously, but here we want to throttle that value back to two. Enter Start-RSJob!

Start-RSJob -Name 'wucheck' -Throttle 2 -ScriptBlock { Invoke-Command -ComputerName dc1, mem1, mem2 -ScriptBlock {Get-Service -Name wuauserv}}

Do you see the -Throttle switch? You should also notice peppier performance with the RSJob architecture as compared to the PowerShell core job engine.

One more thing—these RSJobs are managed separately from PowerShell core jobs. Therefore, don't forget to use Get-RSJob and Receive-RSJob in your experimentation.

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