Pipelines

 

SHORT DESCRIPTION

Combining commands into pipelines in the Windows PowerShell

 

LONG DESCRIPTION

PowerShell allows you to combine two or more commands into a single custom command that creates an operation known as a pipeline. When commands are chained together in a pipeline, they pass data to each other as objects. The first command returns one or more objects that it passes down the pipeline to the second command. The second command processes those objects and then passes on new or revised objects to the third command. This process continues until each command in the pipeline has run.

 

To chain commands together into a pipeline, you specify each command in the order that they should run. You must separate the commands with a pipe symbol (|), also known as a vertical bar. The commands will run from left to right, passing the necessary objects down the pipeline as part of a single operation.

 

The following example demonstrates how commands can be combined into a pipeline:

 

 

The first command (Get-ChildItem *.txt) retrieves all text files from the current working location. The command passes data about each file as objects to the second command (where {$_.length -gt 100}). The second command receives these objects and filters them out based on the length property. The command then passes each object with a length value greater than 100 bytes down the pipeline to the third command (Format-Table name). The third command receives those objects and displays the name of the files in a table format.

 

Notice in the second command that the length property is preceded by the $_ variable. This is a variable created automatically by PowerShell to store the current pipeline object. As is done here, you can use the variable to call properties of the pipeline object. (You must separate the variable name and the property name with a period.)

 

In order to chain commands together in a pipeline, the applicable parameter of the receiving Cmdlet must be able to accept pipeline input. You can determine whether a parameter accepts pipeline input by using a Get-Help command to retrieve information about a specific Cmdlet. For example, the following command retrieves information about the Where-Object Cmdlet:

 

 

From the information returned by this command, you can determine whether the applicable parameter accepts pipeline input. If the parameter accepts pipeline input, the "Accepts pipeline input?" setting of the parameter must show a value of True. In the case of the Where-Object Cmdlet, the applicable parameter that accepts pipeline input is ScriptToApply.

 

If the applicable parameter does not accept pipeline input, you must use other strategies to achieve the results you're looking for. Often the best way to do this is to put your commands into a script and use variables to pass data from one command to the next. For example, you can re-write the earlier Get-ChildItem example as a foreach loop, as shown in the following script:

 

 

In this example, the results from the Get-ChildItem command are put into the $a variable, which is then used by the foreach loop to iterate through the file information. As you can see, a flow control statement such as foreach can be a useful way to access information that cannot be passed easily through a pipeline. Fortunately, the Where-Object Cmdlet and the Format-Table Cmdlet both support parameters that accept pipeline input, which is the simplest way to pass data from one command to the next.

 

SEE ALSO

For information about objects, enter the following command at the PowerShell command prompt:

 

 

For information about parameters, enter the following command:

 

 

For information about command syntax, enter the following command:

 

 

For information about foreach statements, enter the following command: