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about_Pipelines

Updated: May 8, 2014

Applies To: Windows PowerShell 2.0, Windows PowerShell 3.0, Windows PowerShell 4.0

TOPIC
    about_pipelines

SHORT DESCRIPTION
    Combining commands into pipelines in the Windows PowerShell 

LONG DESCRIPTION
    A pipeline is a series of commands connected by pipeline operators
   (|)(ASCII 124). Each pipeline operator sends the results of the preceding
    command to the next command.
     
    You can use pipelines to send the objects that are output by one command
    to be used as input to another command for processing. And you can send the
    output of that command to yet another command. The result is a very powerful
    command chain or "pipeline" that is comprised of a series of simple commands. 

    For example,    

Command-1 | Command-2 | Command-3  

    In this example, the objects that Command-1 emits are sent to Command-2. 
    Command-2 processes the objects and sends them to Command-3. Command-3 processes
    the objects and send them down the pipeline. Because there are no more commands in
    the pipeline, the results are displayed at the console.

    In a pipeline, the commands are processed from left to right in the order
    that they appear. The processing is handled as a single operation and 
    output is displayed as it is generated.

    Here is a simple example. The following command gets the Notepad process
    and then stops it.

         get-process notepad | stop-process

    The first command uses the Get-Process cmdlet to get an object representing
    the Notepad process. It uses a pipeline operator (|) to send the process object
    to the Stop-Process cmdlet, which stops the Notepad process. Notice that the 
    Stop-Process command does not have a Name or ID parameter to specify the process, 
    because the specified process is submitted through the pipeline.

    Here is a practical example. This command pipeline gets the text files in the 
    current directory, selects only the files that are more than 10,000 bytes long, 
    sorts them by length, and displays the name and length of each file in a table.

        Get-ChildItem -path *.txt | Where-Object {$_.length -gt 10000} | 
        Sort-Object -property Length | Format-Table -property name, length
    
    This pipeline is comprised of four commands in the specified order. The command
    is written horizontally, but we will show the process vertically in the following
    graphic.

       Get-ChildItem -path *.txt

                  |
                  |   (FileInfo objects )
                  |   (    .txt         )
                  |
                  V                   

       Where-Object {$_.length -gt 10000}

                  |
                  |   (FileInfo objects )
                  |   (    .txt         )
                  |   ( Length > 10000  )
                  |
                  V

       Sort-Object -property Length

                  |
                  |   (FileInfo objects  )
                  |   (    .txt          )
                  |   ( Length > 10000   )
                  |   ( Sorted by length )
                  |
                  V

       Format-Table -property name, length

                  |   
                  |   (FileInfo objects     )
                  |   (    .txt             )
                  |   ( Length > 10000      )
                  |   ( Sorted by length    )
                  |   (Formatted in a table )
                  |
                  V
        Name                       Length
        ----                       ------
        tmp1.txt                    82920
        tmp2.txt                   114000
        tmp3.txt                   114000


USING PIPELINES

    The Windows PowerShell cmdlets were designed to be used in pipelines. For example, 
    you can usually pipe the results of a Get cmdlet to an action cmdlet (such as a Set,
    Start, Stop, or Rename cmdlet) for the same noun.

    For example, you can pipe any service from the Get-Service cmdlet to the Start-Service
    or Stop-Service cmdlets (although disabled services cannot be restarted in this way).

    This command pipeline starts the WMI service on the computer:

get-service wmi | start-service

    The cmdlets that get and set objects of the Windows PowerShell providers, such as the
    Item and ItemProperty cmdlets, are also designed to be used in pipelines. 

    For example, you can pipe the results of a Get-Item or Get-ChildItem command in the
    Windows PowerShell registry provider to the New-ItemProperty cmdlet. This command adds
    a new registry entry, NoOfEmployees, with a value of 8124, to the MyCompany registry key.

       get-item -path HKLM:\Software\MyCompany | new-Itemproperty -name NoOfEmployees -value 8124

    Many of the utility cmdlets, such as Get-Member, Where-Object, Sort-Object, Group-Object, 
    and Measure-Object are used almost exclusively in pipelines. You can pipe any objects to 
    these cmdlets.

    For example, you can pipe all of the processes on the computer to the Sort-Object command
    and have them sorted by the number of handles in the process.

get-process | sort-object -property handles
    
    Also, you can pipe any objects to the formatting cmdlets, such as Format-List and 
    Format-Table, the Export cmdlets, such as Export-Clixml and Export-CSV, and the Out
    cmdlets, such as Out-Printer.

    For example, you can pipe the Winlogon process to the Format-List cmdlet to display all 
    of the properties of the process in a list.

get-process winlogon | format-list -property *

    With a bit of practice, you'll find that combining simple commands into pipelines 
    saves time and typing, and makes your scripting more efficient.


HOW PIPELINES WORK

     When you "pipe" objects, that is send the objects in the output of one command to another
     command, Windows PowerShell tries to associate the piped objects with one of the parameters
     of the receiving cmdlet. 

     To do so, the Windows PowerShell "parameter binding" component, which associates input objects
     with cmdlet parameters, tries to find a parameter that meets the following criteria:
    
     -- The parameter must accept input from a pipeline (not all do)
     -- The parameter must accept the type of object being sent or a type that the object
        can be converted to.
     -- The parameter must not already be used in the command.

     For example, the Start-Service cmdlet has many parameters, but only two of them, Name and InputObject
     accept pipeline input. The Name parameter takes strings and the InputObject parameter takes
     service objects. Therefore, you can pipe strings and service objects (and objects with properties
     that can be converted to string and service objects) to Start-Service.

     If the parameter binding component of Windows PowerShell cannot associate the piped objects
     with a parameter of the receiving cmdlet, the command fails and Windows PowerShell prompts you
     for the missing parameter values.

     You cannot force the parameter binding component to associate the piped objects with a particular
     parameter -- you cannot even suggest a parameter. Instead, the logic of the component manages
     the piping as efficiently as possible.


ONE-AT-A-TIME PROCESSING

     Piping objects to a command is much like using a parameter of the command to submit the 
     objects.

     For example, piping objects representing the services on the computer to a Format-Table command,
     such as:

  get-service | format-table -property name, dependentservices

     is much like saving the service objects in a variable and using the InputObject parameter
     of Format-Table to submit the service object.

  $services = get-service
                  format-table -inputobject $services -property name, dependentservices

     or imbedding the command in the parameter value

                  format-table -inputobject (get-service wmi) -property name, dependentservices

     However, there is an important difference. When you pipe multiple objects to a command, 
     Windows PowerShell sends the objects to the command one at a time. When you use a
     command parameter, the objects are sent as a single array object.

     This seemingly technical difference can have interesting, and sometimes useful, consequences.

     For example, if you pipe multiple process objects from the Get-Process cmdlet to the
     Get-Member cmdlet, Windows PowerShell sends each process object, one at a time, to Get-Member.
     Get-Member displays the .NET class (type) of the process objects, and their properties and methods.
     (Get-Member eliminates duplicates, so if the objects are all of the same type, it displays only
     one object type.)

     In this case, Get-Member displays the properties and methods of each process object, that is, a 
     System.Diagnostics.Process object.

                 get-process | get-member

                    TypeName: System.Diagnostics.Process

                 Name                           MemberType     Definition
                 ----                           ----------     ----------
                 Handles                        AliasProperty  Handles = Handlecount
                 Name                           AliasProperty  Name = ProcessName
                 NPM                            AliasProperty  NPM = NonpagedSystemMemorySize
                 ...
    

      However, if you use the InputObject parameter of Get-Member, then Get-Member receives an 
      array of System.Diagnostics.Process objects as a single unit, and it displays the properties
      of an array of objects. (Note the array symbol ([]) after the System.Object type name.)


                get-member -inputobject (get-process)


                TypeName: System.Object[]

                Name               MemberType    Definition
                ----               ----------    ----------
                Count              AliasProperty Count = Length
                Address            Method        System.Object& Address(Int32 )
                Clone              Method        System.Object Clone()
                ...

     This result might not be what you intended, but after you understand it, you can use it. For
     example, an array of process objects has a Count property that you can use to count the number
     of processes on the computer.

(get-process).count
                
     This distinction can be important, so remember that when you pipe objects to a cmdlet, they
     are delivered one at a time. 


ACCEPTS PIPELINE INPUT

    In order to receive objects in a pipeline, the receiving cmdlet must have a parameter
    that accepts pipeline input. You can use a Get-Help command with the Full or Parameter
    parameters to determine which, if any, of a cmdlet's parameters accepts pipeline input.

    In the Get-Help default display, the "Accepts pipeline input" item appears in a table
    of parameter attributes. This table is displayed only when you use the Full or Parameter
    parameters of the Get-Help cmdlet.

    For example, to determine which of the parameters of the Start-Service cmdlet accepts
    pipeline input, type:
       
        get-help start-service -full

        get-help start-service -parameter *

    For example, the help for the Start-Service cmdlet shows that the Name and InputObject
    parameters accept pipeline input ("true"). All other parameters have a value of "false"
    in the "Accept pipeline input?" row.

        -name <string[]>
           Specifies the service names for the service to be started.
           The parameter name is optional. You can use "-Name" or its alias, 
           "-ServiceName", or you can omit the parameter name.

           Required?                    true
           Position?                    1
           Default value
      -->  Accept pipeline input?       true (ByValue, ByPropertyName)
           Accept wildcard characters?  true

        -inputObject <ServiceController[]>
           Specifies ServiceController objects representing the services to be started. Enter
           a variable that contains the objects or type a command or expression that gets the
           objects.

           Required?                    false
           Position?                    named
           Default value
      -->  Accept pipeline input?       true (ByValue)
           Accept wildcard characters?  false

     This means that you can send objects (PsObjects) through the pipeline to the
     Where-Object cmdlet and Windows PowerShell will associate the object with the
     InputObject parameter.


METHODS OF ACCEPTING PIPELINE INPUT

     Cmdlets parameters can accept pipeline input in one of two different ways:

     -- ByValue: Parameters that accept input "by value" can accept piped objects
        that have the same .NET type as their parameter value or objects that can be
        converted to that type. 

        For example, the Name parameter of Start-Service accepts pipeline input
        by value. It can accept string objects or objects that can be converted to
        strings.

     -- ByPropertyName: Parameters that accept input "by property name" can accept piped 
        objects only when a property of the object has the same name as the parameter.

        For example, the Name parameter of Start-Service can accept objects that have
        a Name property. 

        (To list the properties of an object, pipe it to Get-Member.)

     Some parameters can accept objects by value or by property name. These parameters are
     designed to take input from the pipeline easily.


INVESTIGATING PIPELINE ERRORS

     If a command fails because of a pipeline error, you can investigate the failure and
     rewrite the command.

     For example, the following command tries to move a registry entry from one
     registry key to another by using the Get-Item cmdlet to get the destination path and
     then piping the path to the Move-ItemProperty cmdlet.

     Specifically, the command uses the Get-Item cmdlet to get the destination path. It uses
     a pipeline operator to send the result to the Move-ItemProperty cmdlet. The Move-ItemProperty
     command specifies the current path and name of the registry entry to be moved. 

          get-item -path hklm:\software\mycompany\sales | 
          move-itemproperty -path hklm:\software\mycompany\design -name product

     The command fails and Windows PowerShell displays the following error 
     message:    

         Move-ItemProperty : The input object cannot be bound to any parameters for the
         command either because the command does not take pipeline input or the input
         and its properties do not match any of the parameters that take pipeline input.
         At line:1 char:23
         + $a | move-itemproperty <<<<  -path hklm:\software\mycompany\design -name product

    To investigate, use the Trace-Command cmdlet to trace the Parameter Binding component of 
    Windows PowerShell. The following command traces the Parameter Binding component while the
    command is processing. It uses the -pshost parameter to display the results at the console
    and the -filepath command to send them to the debug.txt file for later reference.

         trace-command -name parameterbinding -expression {get-item -path hklm:\software\mycompany\sales |
             move-itemproperty -path hklm:\software\mycompany\design -name product} -pshost -filepath debug.txt

    The results of the trace are lengthy, but they show the values being bound to the Get-Item cmdlet 
    and then the named values being bound to the Move-ItemProperty cmdlet. 

       ... 
        BIND NAMED cmd line args [Move-ItemProperty]
            BIND arg [hklm:\software\mycompany\design] to parameter [Path]
        ...
            BIND arg [product] to parameter [Name]
        ....
        BIND POSITIONAL cmd line args [Move-ItemProperty]
        ...


    Finally, it shows that the attempt to bind the path to the Destination parameter
    of Move-ItemProperty failed.
        ...
        BIND PIPELINE object to parameters: [Move-ItemProperty]
            PIPELINE object TYPE = [Microsoft.Win32.RegistryKey]
            RESTORING pipeline parameter's original values
            Parameter [Destination] PIPELINE INPUT ValueFromPipelineByPropertyName NO COERCION
            Parameter [Credential] PIPELINE INPUT ValueFromPipelineByPropertyName NO COERCION
        ... 

     To investigate the failure, use the Get-Help cmdlet to view the attributes of the
     Destination parameter. The following command gets detailed information about the
     Destination parameter.

get-help move-itemproperty -parameter destination

     The results show that Destination takes pipeline input only "by property name".
     That is, the piped object must have a property named Destination.

        -destination <string>
            Specifies the path to the destination location.

            Required?                    true
            Position?                    2
            Default value
            Accept pipeline input?       true (ByPropertyName)
            Accept wildcard characters?  true    

     To see the properties of the object being piped to the Move-ItemProperty cmdlet,
     pipe it to the Get-Member cmdlet. The following command pipes the results of the
     first part of the command to the Get-Member cmdlet.

          get-item -path hklm:\software\mycompany\sales | get-member
       
     The output shows that the item is a Microsoft.Win32.RegistryKey that does not
     have a Destination property. That explains why the command failed.

     To fix the command, we must specify the destination in the Move-ItemProperty cmdlet. We can 
     use a Get-ItemProperty command to get the path, but the name and destination must be specified
     in the Move-ItemProperty part of the command.
          
         get-item -path hklm:\software\mycompany\design | 
             move-itemproperty -dest hklm:\software\mycompany\design -name product    

     To verify that the command worked, use a Get-ItemProperty command:

get-itemproperty hklm:\software\mycompany\sales

     The results show that the Product registry entry was moved to the Sales key.

        PSPath       : Microsoft.PowerShell.Core\Registry::HKEY_LOCAL_MACHINE\software\mycompany\sales
        PSParentPath : Microsoft.PowerShell.Core\Registry::HKEY_LOCAL_MACHINE\software\mycompany
        PSChildName  : sales
        PSDrive      : HKLM
        PSProvider   : Microsoft.PowerShell.Core\Registry
        Product      : 18

SEE ALSO
    about_objects
    about_parameters
    about_command_syntax
    about_foreach



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