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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