PowerShell 101, Lesson 2

Executive Summary: Windows PowerShell provides far more power and flexibility than traditional Windows command shells, such as cmd.exe. To help you take advantage of that power, this six-article series explains how to use PowerShell to perform various tasks. In this lesson, you’ll learn how to link cmdlets into a pipeline to create PowerShell statements. You’ll also learn how to format and sort output from those statements.

In “PowerShell 101, Lesson 1” (February 2008, InstantDoc ID 97742), I introduced you to the concept of cmdlets and how to run basic PowerShell commands. I also showed you how to use aliases and how to use PowerShell’s Get- cmdlets to get help when creating commands. For example, you can use the Get-ChildItem cmdlet to retrieve a list of items in a folder or the Get-Content cmdlet to retrieve the content of a text file. With cmdlets and their parameters, you can run a wide variety commands that display system information or carry out tasks.

However, a cmdlet alone might not always provide the full functionality you require. For this reason, PowerShell lets you create pipelines that link cmdlets together to carry out complex operations and refine the system information you retrieve. In this lesson, you’ll learn how to link cmdlets into a pipeline to create PowerShell statements. You’ll also learn how to format and sort statement output.

Implementing a Pipeline
A PowerShell pipeline is a series of cmdlets that pass objects from one cmdlet to the next. Each cmdlet generates an object and passes it down the pipeline, where it is received by the next cmdlet. The receiving cmdlet uses that object as input and generates its own output as an object. You connect cmdlets into a pipeline by using the pipe (|) operator.

Pipelining in PowerShell is different from pipelining in other command shell environments, such as the Windows command shell. In traditional environments, a command’s results are returned as a single result set, which means that the entire result set must be generated before any information is passed down the pipeline. The first result is returned at the same time as the last result. In PowerShell, however, the results are streamed through the pipeline. As soon as a command returns a result, it passes it down the pipeline, and that result is immediately available to the next command in the pipeline.

Let’s look at an example that will help you understand how a pipeline works. If you run the cmdlet

Get-Service

you’ll receive a list of the services installed your system, similar to the list in Figure 1. Notice that the cmdlet returns the status, name, and display name of each service. Suppose you want to retrieve a list of running services only. You can pipe the output from the Get- Service cmdlet to the Where-Object cmdlet, which filters the output based on the specified criteria, as shown in the statement

Get-Service |
 Where-Object {$_.status -eq ‘running’}

As you can see, you use a pipe operator to connect the two cmdlets. The Get-Service cmdlet generates an object that contains the servicerelated information. The object is passed down the pipeline to the Where-Object cmdlet. The Where-Object cmdlet receives the object and uses the information as input. The Where-Object cmdlet filters the information based on the Status property value. Notice that the Where-Object cmdlet includes an expression enclosed in braces ({ }). If the expression evaluates to true, the Where-Object passes that object down the pipeline and filters out any other object.

In this case, the Where-Object expression states that the Status property value must be equal to (specified by the -eq operator) the string running. Status is one of the properties available to the object generated by the Get- Service cmdlet. When an object is passed down the pipeline, you can access its properties, as I’ve done in the Where-Object expression. To access a property in the pipeline in this manner, you use the $_ built-in variable. This variable holds the current object within the pipeline each time the Where-Object cmdlet loops through the pipeline results. You can then reference the object’s properties, as in $_.Status. The output now looks similar to that in Figure 2. (You’ll learn more about the Where-Object cmdlet, object properties, and operators in later lessons.)

Note that you’d typically enter the statement just given on one line in the Power- Shell console window. However, column widths in the magazine force us to print this statement on more than one line. Also note in Figure 2 the >> character sequence at the beginning of some of the lines in the command. This character sequence constitutes a multiline prompt. For information about when you’d want to enter a statement on multiple lines in the PowerShell console window and how to properly do so.

Now suppose you want to list only the display name of each running service. You can pipe the output of the Where-Object cmdlet to the Select-Object cmdlet:

Get-Service |
where {$_.status -eq ‘running’} |    
select displayname

In this statement, the Select-Object cmdlet receives the object from the Where-Object cmdlet. In this case, the statement uses the where alias to reference the Where-Object cmdlet and the select alias to reference the Select-Object cmdlet. In the select cmdlet, you specify the name of the property (or properties) that you want to display. For this example, I’ve specified the displayname property. The statement will now output results similar to those in Figure 3.

The key to using pipelines is to remember that you’re always working with objects. Each cmdlet generates an object that the next cmdlet in the pipeline receives. Even the final cmdlet generates an object that outputs the statement results. As you progress through the lessons, you’ll learn how to use those objects and their properties to refine your PowerShell statements.

Formatting Statement Output
By default, PowerShell formats a statement’s output based on the type of data in that output. For example, the following statement returns data about the PowerShell process:

Get-Process powershell

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