about_Pipelines

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

Format-Table -property name, length

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

about_Pipelines

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