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Using Variables in Script Workflows

Published: June 24, 2013

Updated: June 24, 2013

Applies To: Windows PowerShell 3.0, Windows PowerShell 4.0, Windows Server 2012 R2



This topic explains the rules for using variables in Windows PowerShell Workflow.

All Windows PowerShell automatic variables are valid in workflows, except for the automatic variables in the following list. For a complete list of automatic variables, see about_Automatic_Variables.

  • $Args

  • $Error

  • $MyInvocation

  • $PID

  • $PSBoundParameters

  • $PsCmdlet

  • $PSCommandPath

  • $PSScriptRoot

  • $StackTrace

In addition, Windows PowerShell Workflow adds the Workflow Runtime Variables to every workflow. For more information, see the PSWorkflowRunTimeVariable Enumeration help topic in the MSDN Library.

All Windows PowerShell preference variables are valid in workflows. For a complete list of preference variables, see about_Preference_Variables.

Workflow preference variables are valid only in a workflow. Unlike Windows PowerShell preference variables that are set in a session and affect all commands that are run in the session, workflow preference variables are set in a workflow and affect only that workflow.

The following sections list and explain the workflow preference variables.

Establishes an identifier for activities in the current scope and all child scopes. The identifier appears in the progress records for activities in the affected scopes.

For example, in the following workflow, the Get-Data nested workflow includes the PSParentActivityID preference variable with a value of "GetData_Section". That ID will appear on all progress records of all activities run in the Get-Data nested workflow and in any nested workflows or functions in that workflow, such as the Get-SourceData workflow.

Workflow Test-Workflow
{
    Workflow Get-Data
    {
        $PSParentActivityId = "GetData_Section"
        ...

        Workflow Get-SourceData
        {
          ...
        }
    }

    Get-Data    
}

Adds a checkpoint to a group of activities. A value of $True takes a checkpoint after every activity that follows the variable definition. This value is equivalent to setting the value of the PSPersist activity parameter to $true for all affected activities. To turn off the behavior, set the value of the variable to $False.

In the following workflow, the $PSPersistPreference variable takes a checkpoint after Activity 2, 3, and 4.

Workflow Test-Workflow
{
    Activity1
    $PSPersistPreference = $true    
    Activity2
    Activity3
    Activity4
    $PSPersistPreference = $false    
    Activity5
}

This variable takes a Boolean value. Valid values are:

  1. $True: Adds a checkpoint after each activity, in addition to checkpoints that are specified in the workflow. It affects only activities that follow the variable definition.

  2. $False: Stops adding checkpoints. Checkpoints are taken when specified in the workflow

The workflow variable that controls whether activities are run in the process. If this variable is specified, all activities in the enclosing scope are run in the workflow process.

Windows PowerShell Workflow adds the workflow runtime variables to every workflow. The runtime variables include the workflow common parameters and their values, so you do not have to redefine those variables in your workflow. For more information about the Workflow Runtime Variables, see PSWorkflowRuntimeVariable Enumeration. For more information about the workflow common parameters, see about_WorkflowCommonParameters.

For example, the following workflow uses the $PSComputerName, $ParentJobName, and $JobName variables. The values of these variables are passed to the workflow and are available for use in the workflow.

Workflow Test-Workflow
{
    $PSComputerName
    $ParentJobName
    $JobName
}
    
PS C:\>Test-Workflow -PSComputerName Server01, Server12 -JobName TestWFJob -AsJob
Id     Name            PSJobTypeName   State         HasMoreData     Location
--     ----            -------------   -----         -----------     --------
83     TestWFJob       PSWorkflowJob   Completed     True            Server01,.


PS C:\> Receive-Job -Keep -ID 83
Server01
TestWFJob
Job84
Server12
TestWFJob
Job85

The workflow runtime variables include other very useful variables. For example, the following workflow uses a few of the runtime variables to get information about the values of session options and the version of Windows PowerShell running on the target computer.


Workflow Get-SessionOptions
{
    "Computer: $PSComputerName"
    "Parent Command Name: $ParentCommandName"
    $p = $PSVersionTable.PSVersion.Major
    "PSVersion: $p"
    "Session Options:"
    $PSSessionOption
}

PS C:\ps-test> Get-SessionOptions -PSComputerName Server01
Computer: Server01
Parent Command Name: Test-Workflow
PSVersion: 4
Session Options:
PSComputerName                    : localhost
PSSourceJobInstanceId             : b8ecbf21-a49e-4326-a0f1-4829fcdff4a8
MaximumConnectionRedirectionCount : 5
NoCompression                     : False
NoMachineProfile                  : False
ProxyAccessType                   : None
ProxyAuthentication               : Negotiate
ProxyCredential                   : 
SkipCACheck                       : False
SkipCNCheck                       : False
SkipRevocationCheck               : False
OperationTimeout                  : 00:03:00
NoEncryption                      : False
UseUTF16                          : False
IncludePortInSPN                  : False
OutputBufferingMode               : None
Culture                           : 
UICulture                         : 
MaximumReceivedDataSizePerCommand : 
MaximumReceivedObjectSize         : 209715200
ApplicationArguments              : 
OpenTimeout                       : 00:03:00
CancelTimeout                     : 00:01:00
IdleTimeout                       : -00:00:00.0010000

To change the values of workflow runtime variables within a workflow, use the Set-PSWorkflowData activity.

For example, the following workflow includes Set-PSWorkflowData commands that increment the value of the PSConnectionRetryCount workflow common parameter and reverse the value of the Verbose common parameter from within the workflow.

workflow Test-Workflow
{
    "Before: PSConnectionRetryCount is $PSConnectionRetryCount."
     Set-PSWorkflowData -PSConnectionRetryCount ($PSConnectionRetryCount + 1)
    "After: PSConnectionRetryCount is $PSConnectionRetryCount."
  
    "Before: Verbose is $verbose."
    if ($Verbose) {Set-PSWorkflowData -OtherVariableName Verbose -Value $False}
    else {Set-PSWorkflowData -OtherVariableName Verbose -Value $True}
    "After: Verbose is $Verbose."
}
PS C:\> Test-Workflow -PSConnectionRetryCount 4 -Verbose
Before: PSConnectionRetryCount is 4.
After: PSConnectionRetryCount is 5.
Before: Verbose is True.
After: Verbose is False.

This section explains the general rules for using variables in workflows. The sections that follow explain the rules that are specific to activities, loops, and other control statements.

  • You cannot store the results of sub-expressions in a variable, because sub-expressions cannot be persisted.

    #Incorrect: 
    $a = $(Get-Process | foreach {$_.Name) 
    
    #Correct:
    $a = (Get-Process | foreach {$_.Name)
    
  • You cannot assign a variable value to another variable in the same statement.

    #Incorrect: 
    $a = $b = 23
     
    #Correct:
    $b = 23
    $a = $b
    
  • You cannot assign a value to an environment variable.

    #Incorrect: 
    $env:Temp = "C:\Tmp"
    
    #Correct:
    InlineScript {[System.Environment]::SetEnvironmentVariable("C:\Tmp")}
    
    
  • You cannot use variable references to change property values.

    #Incorrect: 
    $o = New-PSSessionOption
    $o.SkipCACheck = $true
    
    #Correct:
    $o = New-PSSessionOption -SkipCACheck
    
    

Variables that you create in the top-level scope of a Windows PowerShell workflow are called workflow variables. Like the variables in a script or function, the commands and expressions in the top-level scope of a workflow can view and change the value of variables that other commands and expressions create, as shown in the following example.

Workflow Test-Workflow
{
$a = 23
$a++
$a
}

PS C:\>Test-Workflow
24

However, nested scopes, such as loops, inline scripts, and Parallel and Sequence statements, have slightly different rules.

noteNote
Each command in a workflow runs in its own session ("runspace"). Therefore, without intervention, the command would not be able to view or change the variables created by other commands. However, Windows PowerShell Workflow adds all workflow variables to all of the sessions in which top-level commands run. This feature allows the commands to share variables and view and change their values.

The following table shows the variable rules for different types of nested statements in workflows.

 

Rules for Nested Statements in Workflows

View workflow variable values

Change workflow variable values

Internal variables are visible in workflow scope

Loops/Control Statements

Yes

Yes

Yes

Parallel / Sequence

Yes

No. Use $Workflow.

No, return the variable.

InlineScript

No. Use $Using.

No. Return the variable. $Workflow is invalid.

No, return the variable.

Loop statements, such as those that use the For, While, and Do keywords, and control statements, such as those that use the If and ForEach keywords, share workflow variables. These statements can view and change workflow variables and the top-level commands and expressions in a workflow can view and change the variables that are created in the statements.

The following example shows that the ForEach statement can access the $a workflow variable and the top-level commands in the workflow can access the $i variable that is defined in the ForEach statement.

Workflow Test-Workflow
{
$a = 1,2,3

foreach ($number in $a) 
       {
         "A = $number"
         $i++
       }
$i
}

PS C:\>Test-Workflow
A = 1
A = 2
A = 3
3

The statements in Parallel and Sequence script blocks can view the values of workflow variables. However, by default, the statements in Parallel and Sequence script blocks cannot change the values of workflow variables and cannot create variables that have the same name as workflow variables.

To change the value of a workflow variable in a Parallel or Sequence block, use the $Workflow scope modifier. When you do, the value that is assigned in the Parallel or Sequence block is available in the workflow scope.

The following example shows how to use the $Workflow scope modifier to change the value of a workflow variable and to make the new variable value available in workflow scope.

Workflow Test-Workflow
{
    #Workflow (top-level) variable
    $w = 4

    Parallel
    {
        Sequence
        {
            # You can get the values of workflow variables.
            "InSequenceW = $w"

            # To change the value of a workflow variable, 
            # use the $Workflow scope modifier.
            $Workflow:w = 5
            "InSequenceWNew = $w"           
        }
    }
    # The change is visible in workflow scope. 
    "WorkflowW = $w"    
}

PS C:\>Test-Workflow
InSequenceW = 4
InSequenceWNew = 5
WorkflowW = 5

Variables that are defined in a child scope, such as a Parallel or Sequence block, are not visible in any parent scope. To make them visible, return the value to the parent scope, as shown in the following example.

Workflow Test-Workflow
{       
    Parallel
    {
        Sequence
        {
            #Variables defined in parallel/sequence ...
            $s = 3            
        }        
    }
    # ... are not available in workflow scope.    
    "S = $s"    


    $TFromSequence = Parallel
    {
        Sequence
        {
            #To export variable values from 
            # parallel/sequence, return the value.
            $t = 3
            $t   
        }        
    }   
    "T = $TFromSequence"
}

PS C:\>Test-Workflow
S = 
T = 3

The InlineScript activity runs commands and expressions in a standard Windows PowerShell session and returns the results to the workflow.

By default, the variables that are defined in a workflow are not visible to the commands in an InlineScript script block. To make workflow variables visible in InlineScript, use the $Using scope modifier. The $Using scope modifier is required only once for each variable in the InlineScript script block, as shown in the following example.

workflow Test-Workflow
{
    $a = 3

    # Without $Using, the $a workflow variable is not visible
    # in inline script.
    InlineScript {"Inline A0 = $a"}
    
    # Using imports the variable and its current value.
    InlineScript {"Inline A1 = $Using:a"}
}

PS C:\> Test-Workflow
Inline A0 = 
Inline A1 = 3 

Commands in an InlineScript script block can change the value of variables that were imported from workflow scope, but the changes are not visible in workflow scope. To make them visible, return the changed value to the workflow scope, as shown in the following example.

workflow Test-Workflow
{
    $a = 3

    # Changes to variable value in an InlineScript do not affect
    # the value of the workflow variable.
    InlineScript {$a = $Using:a + 1; "Inline A = $a"}
    "Workflow A = $a"

    # To change the value in workflow scope, return the new value.
     $a = InlineScript {$a = $Using:a+1; $a}
     "Workflow New A = $a"
}
PS C:\> Test-Workflow
Inline A = 4
Workflow A = 3
Workflow New A = 4

noteNote
Troubleshooting Note: A statement with the $Using scope modifier and the imported variable should appear before any use of the variable in the InlineScript script block. In Windows PowerShell 3.0 and newer releases of Windows PowerShell, the $Using scope modifier can appear anywhere in the InlineScript script block.

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