about_Jobs

Short description

Provides information about how PowerShell background jobs run a command or expression in the background without interacting with the current session.

Long description

PowerShell concurrently runs commands and scripts through jobs. There are three jobs types provided by PowerShell to support concurrency.

• RemoteJob - Commands and scripts run on a remote session. For information, see about_Remote_Jobs.
• BackgroundJob - Commands and scripts run in a separate process on the local machine.
• PSTaskJob or ThreadJob - Commands and scripts run in a separate thread within the same process on the local machine. For more information, see about_Thread_Jobs.

Running scripts remotely, on a separate machine or in a separate process, provides great isolation. Any errors that occur in the remote job don't affect other running jobs or the parent session that started the job. However, the remoting layer adds overhead, including object serialization. All objects are serialized and deserialized as they're passed between the parent session and the remote (job) session. Serialization of large complex data objects can consume large amounts of compute and memory resources and transfer large amounts of data across the network.

Thread-based jobs aren't as robust as remote and background jobs, because they run in the same process on different threads. If one job has a critical error that crashes the process, then all other jobs in the process are terminated.

However, thread-based jobs require less overhead. They don't use the remoting layer or serialization. The result objects are returned as references to live objects in the current session. Without this overhead, thread-based jobs run faster and use fewer resources than the other job types.

Important

The parent session that created the job also monitors the job status and collects pipeline data. The job child process is terminated by the parent process once the job reaches a finished state. If the parent session is terminated, all running child jobs are terminated along with their child processes.

There are two ways work around this situation:

1. Use Invoke-Command to create jobs that run in disconnected sessions. For more information, see about_Remote_Jobs.
2. Use Start-Process to create a new process rather than a job. For more information, see Start-Process.

The job cmdlets

• Start-Job - Starts a background job on a local computer.
• Get-Job - Gets the background jobs that were started in the current session.
• Receive-Job - Gets the results of background jobs.
• Stop-Job - Stops a background job.
• Wait-Job - Suppresses the command prompt until one or all jobs are complete.
• Remove-Job - Deletes a background job.
• Invoke-Command - The AsJob parameter creates a background job on a remote computer. You can use Invoke-Command to run any job command remotely, including Start-Job.

How to start a job on the local computer

To start a background job on the local computer, use the Start-Job cmdlet.

To write a Start-Job command, enclose the command that the job runs in curly braces ({}). Use the ScriptBlock parameter to specify the command.

The following command starts a background job that runs a Get-Process command on the local computer.

Start-Job -ScriptBlock {Get-Process}

When you start a background job, the command prompt returns immediately, even if the job takes an extended time to complete. You can continue to work in the session without interruption while the job runs.

The Start-Job command returns an object that represents the job. The job object contains useful information about the job, but it doesn't contain the job results.

You can save the job object in a variable and then use it with the other Job cmdlets to manage the background job. The following command starts a job object and saves the resulting job object in the $job variable.

$job = Start-Job -ScriptBlock {Get-Process}

Getting job objects

The Get-Job cmdlet returns objects that represent the background jobs that were started in the current session. Without parameters, Get-Job returns all of the jobs that were started in the current session.

Get-Job

The job object contains the state of the job, which indicates whether the job has finished. A finished job has a state of Complete or Failed. A job might also be Blocked or Running.

Id  Name  PSJobTypeName State      HasMoreData  Location   Command
--  ----  ------------- -----      -----------  --------   -------
1   Job1  BackgroundJob Complete   True         localhost  Get-Process

You can save the job object in a variable and use it to represent the job in a later command. The following command gets the job with ID 1 and saves it in the $job variable.

$job = Get-Job -Id 1

Getting the results of a job

When you run a background job, the results don't appear immediately. To get the results of a background job, use the Receive-Job cmdlet.

The following example, the Receive-Job cmdlet gets the results of the job using job object in the $job variable.

Receive-Job -Job $job

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)    Id ProcessName
-------  ------    -----      ----- -----   ------    -- -----------
    103       4    11328       9692    56           1176 audiodg
    804      14    12228      14108   100   101.74  1740 CcmExec
    668       7     2672       6168   104    32.26   488 csrss
...

You can save the results of a job in a variable. The following command saves the results of the job in the $job variable to the $results variable.

$results = Receive-Job -Job $job

Getting and keeping partial job results

The Receive-Job cmdlet gets the results of a background job. If the job is complete, Receive-Job gets all job results. If the job is still running, Receive-Job gets the results that have been generated thus far. You can run Receive-Job commands again to get the remaining results.

By default, Receive-Job deletes the results from the cache where job results are stored. When you run Receive-Job again, you get only the new results that arrived after the first run.

The following commands show the results of Receive-Job commands run before the job is complete.

C:\PS> Receive-Job -Job $job

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----      ----- -----   ------     -- -----------
    103       4    11328       9692    56            1176 audiodg
    804      14    12228      14108   100   101.74   1740 CcmExec

C:\PS> Receive-Job -Job $job

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----      ----- -----   ------     -- -----------
    68       3     2632        664    29     0.36   1388 ccmsetup
   749      22    21468      19940   203   122.13   3644 communicator
   905       7     2980       2628    34   197.97    424 csrss
  1121      25    28408      32940   174   430.14   3048 explorer

Use the Keep parameter to prevent Receive-Job from deleting the job results that are returned. The following commands show the effect of using the Keep parameter on a job that's not yet complete.

C:\PS> Receive-Job -Job $job -Keep

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----      ----- -----   ------     -- -----------
    103       4    11328       9692    56            1176 audiodg
    804      14    12228      14108   100   101.74   1740 CcmExec

C:\PS> Receive-Job -Job $job -Keep

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----      ----- -----   ------     -- -----------
    103       4    11328       9692    56            1176 audiodg
    804      14    12228      14108   100   101.74   1740 CcmExec
     68       3     2632        664    29     0.36   1388 ccmsetup
    749      22    21468      19940   203   122.13   3644 communicator
    905       7     2980       2628    34   197.97    424 csrss
   1121      25    28408      32940   174   430.14   3048 explorer

Waiting for the results

If you run a command that takes a long time to complete, you can use the properties of the job object to determine when the job is complete. The following command uses the Get-Job object to get all the background jobs in the current session.

Get-Job

The results appear in a table. The status of the job appears in the State column.

Id Name  PSJobTypeName State    HasMoreData Location  Command
-- ----  ------------- -----    ----------- --------  -------
1  Job1  BackgroundJob Complete True        localhost Get-Process
2  Job2  BackgroundJob Running  True        localhost Get-EventLog -Log ...
3  Job3  BackgroundJob Complete True        localhost dir -Path C:\* -Re...

In this case, the State property reveals that Job 2 is still running. If you were to use the Receive-Job cmdlet to get the job results now, the results would be incomplete. You can use the Receive-Job cmdlet repeatedly to get all of the results. Use the State property to determine when the job is complete.

You can also use the Wait parameter of the Receive-Job cmdlet. When use use this parameter, the cmdlet doesn't return the command prompt until the job is completed and all results are available.

You can also use the Wait-Job cmdlet to wait for any or all of the results of the job. Wait-Job lets you wait for one or more specific job or for all jobs. The following command uses the Wait-Job cmdlet to wait for a job with ID 10.

Wait-Job -ID 10

As a result, the PowerShell prompt is suppressed until the job is completed.

You can also wait for a predetermined period of time. This command uses the Timeout parameter to limit the wait to 120 seconds. When the time expires, the command prompt returns, but the job continues to run in the background.

Wait-Job -ID 10 -Timeout 120

Stopping a job

To stop a background job, use the Stop-Job cmdlet. The following command starts a job to get every entry in the System event log. It saves the job object in the $job variable.

$job = Start-Job -ScriptBlock {Get-EventLog -Log System}

The following command stops the job. It uses a pipeline operator (|) to send the job in the $job variable to Stop-Job.

$job | Stop-Job

Deleting a job

To delete a background job, use the Remove-Job cmdlet. The following command deletes the job in the $job variable.

Remove-Job -Job $job

Investigating a failed job

Jobs can fail for many reasons. the job object contains a Reason property that contains information about the cause of the failure.

The following example starts a job without the required credentials.

$job = Start-Job -ScriptBlock {New-Item -Path HKLM:\Software\MyCompany}
Get-Job $job

Id Name  PSJobTypeName State  HasMoreData  Location  Command
-- ----  ------------- -----  -----------  --------  -------
1  Job1  BackgroundJob Failed False        localhost New-Item -Path HKLM:...

Inspect the Reason property to find the error that caused the job to fail.

$job.ChildJobs[0].JobStateInfo.Reason

In this case, the job failed because the remote computer required explicit credentials to run the command. The Reason property contains the following message:

Connecting to remote server failed with the following error message: "Access is denied".

See also

• about_Job_Details
• about_PSSessions
• about_Remote
• about_Remote_Jobs
• about_Thread_Jobs
• Invoke-Command
• Get-Job
• Receive-Job
• Remove-Job
• Start-Job
• Stop-Job
• Wait-Job