Background jobs

Another new feature of PowerShell V2 is background jobs.  These allow you to run commands asynchronously.  In other words you can start a long running PowerShell job and immediately get control back.  This allows you to keep working while the jobs is running.  PowerShell supplies a number of new cmdlets to control these jobs. 

Name
—-
Get-PSJob
Receive-PSJob
Remove-PSJob
Start-PSJob
Stop-PSJob
Wait-PSJob

Start-PSJob is used to create a new job.  For these examples we will use get-service and get-process as examples.

PS> Start-PSJob -Command {get-service | sort status}

SessionId       Name     State           HasMoreData     Command
———          —-        —–           ———–            ——-
1                               Running         True                 get-service | sort status

PS> Start-PSJob -Command {get-process | sort CPU}

SessionId       Name            State           HasMoreData      Command
———            —-            —–           ———–              ——-
3                                      Running         True                  get-process | sort CPU

You can view the status of jobs

PS> Get-PSJob

SessionId       Name            State           HasMoreData     Command
———       —-            —–                  ———–           ——-
1                               Completed           True                   get-service | sort status
3                               Completed           True                   get-process | sort CPU

if you are going to have a lot of jobs running it will be easier to keep track of them by assigning names or using variables to hold the job objects.  As I have done either in this case I will need to use Receive-PSJob and Get-PSJob together to see the results

PS> Receive-PSJob  -Job ( Get-PSJob -SessionId 1)

Status   Name               DisplayName
——   —-               ———–
Stopped  MSIServer          Windows Installer
Stopped  MSDTC              Distributed Transaction Coordinator
Stopped  mnmsrvc            NetMeeting Remote Desktop Sharing
Stopped  NetTcpPortSharing  Net.Tcp Port Sharing Service
Stopped  NetDDEdsdm         Network DDE DSDM
Stopped  NetDDE             Network DDE
Stopped  ImapiService       IMAPI CD-Burning COM Service

etc

Even though the data has been received the job still shows in the list if Get-PSJob is used.  To remove the job you use

PS> Remove-PSJob  -Job ( Get-PSJob -SessionId 1)
PS> Get-PSJob

SessionId       Name            State           HasMoreData     Command
———       —-                 —–           ———–            ——-
3                                    Completed       True            get-process | sort CPU

Wait-PSJob suppresses the command prompt until the job has finished and Stop-PSJob cause the job to cease.

If we create a simple script file – test3.ps1 containing

Get-Process | Select-Object Name

we can run it as

$job = Start-PSJob -Command "C:scriptstest3.ps1"

and get the results by

PS> Receive-PSJob $job

Name                          PSIPHostEntry                 RunspaceID                    Pipeline
—-                          ————-                 ———-                    ——–
alg                           xpn                           70bcf494-6b26-41ab-b1fb-a1… C:scriptstest3.ps1
csrss                         xpn                           70bcf494-6b26-41ab-b1fb-a1… C:scriptstest3.ps1
ctfmon                        xpn                           70bcf494-6b26-41ab-b1fb-a1… C:scriptstest3.ps1
explorer                      xpn                           70bcf494-6b26-41ab-b1fb-a1… C:scriptstest3.ps1
hh                            xpn                           70bcf494-6b26-41ab-b1fb-a1… C:scriptstest3.ps1
Idle                          xpn                           70bcf494-6b26-41ab-b1fb-a1… C:scriptstest3.ps1
lsass                         xpn                           70bcf494-6b26-41ab-b1fb-a1… C:scriptstest3.ps1
powershell                    xpn                           70bcf494-6b26-41ab-b1fb-a1… C:scriptstest3.ps1
powershell                    xpn                           70bcf494-6b26-41ab-b1fb-a1… C:scriptstest3.ps1
PowerShell Assistant          xpn                           70bcf494-6b26-41ab-b1fb-a1… C:scriptstest3.ps1

etc

It is possible to restrict what is viewed on return

PS> Receive-PSJob $job | select name

Name
—-
alg
csrss
ctfmon
explorer
hh
Idle
lsass
powershell
powershell
PowerShell Assistant
etc

It is also possible to run jobs against one or more remote machines

$job = Start-PSJob -ComputerName dc02 -Command {get-process | sort CPU}

The output is retrieved by Receive-PSJob as previously

In many respects this is a better option for running PowerShell against remote machines as the job can be left running in the background while other tasks are performed – it even gives time for a quick game of PowerShell Space Invaders!!

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