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Windows PowerShell: Peering Through the Pipeline

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You've probably done at least a little pipeline work with Windows PowerShell. Did you get the expected results every time? Timothy Warner, author of Sams Teach Yourself Windows PowerShell 5 in 24 Hours, points out how most of us go wrong when piping. Learn more powerful ways to use the pipeline.
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Okay, PowerShell enthusiast. You say you're ready to embrace the "secret sauce" that gives Windows PowerShell its power? I've found that once an IT pro begins to understand the pipeline, his or her effectiveness with PowerShell makes dramatic gains.

By the time you finish reading this article, you should have the following PowerShell skills under your belt:

  • Listing and using the properties and methods of PowerShell objects
  • Intelligently piping the output of one PowerShell command to another
  • Troubleshooting when PowerShell gives you unexpected output

Let's begin!

The 'Pipeline' in Cmd.exe

The Windows command processor, Cmd.exe, provides a tremendously rudimentary pipeline—specifically, what's called "standard in" (STDIN) and "standard out" (STDOUT). There's also a STDERR output, but we won't worry about that here.

In a limited number of cases, we can redirect the current command output (STDIN stream) to another output in addition to the screen (STDOUT stream):

dir "C:\Windows\System32" > "C:\sys32contents.txt"

The preceding example redirects a directory listing from the console to a text file. Perhaps you've done the following to display long directory listings one screen at a time:

dir "C:\Windows\System32" | more

While redirection like these examples can be useful, it's important to understand that in both Windows and Linux pipelines, you're dealing only with text—nothing more. To be sure, you can construct some complex pipelines in Linux. Imagine being able to do the following:

Generate a table showing the top five processes, ordered by CPU consumption in descending order, listing only the process name and CPU value.

Believe it or not, performing this task in Linux is quite difficult, involving a bunch of totally separate command-line utilities like ls, grep, awk, and sed. Even so, all you have in the pipeline is "dumb" text. To do the above in Windows without bringing VBScript to the table is inconceivable.

In PowerShell, Everything Is an Object

I can't stress enough the importance of understanding PowerShell's object orientation. We can look at an object as a "three-dimensional" data structure that has descriptive attributes (called properties), as well as actions that the object can perform (called methods).

In PowerShell, we pipe output to Get-Member to view the members of an object. Check this out:

Get-Service | Get-Member

   TypeName: System.ServiceProcess.ServiceController

Name                      MemberType    Definition
----                      ----------    ----------
Name                      AliasProperty Name = ServiceName
Close                     Method        void Close()
Pause                     Method        void Pause()
Refresh                   Method        void Refresh()
Start                     Method        void Start(), void Start(string[] args)
Stop                      Method        void Stop()
DependentServices         Property      System.ServiceProcess.ServiceControl...
DisplayName               Property      string DisplayName {get;set;}
MachineName               Property      string MachineName {get;set;}
ServiceHandle             Property      System.Runtime.InteropServices.SafeH...
ServiceName               Property      string ServiceName {get;set;}
Status                    Property      System.ServiceProcess.ServiceControl...
ToString                  ScriptMethod  System.Object ToString();

Don't be blown away by the member list. What you see above is actually a truncation of the full output. I just want you to get the feel for this now.

Let me draw your attention to the object data type that's produced by Get-Service:

System.ServiceProcess.ServiceController

IT pros tend to use the last part of an object name. Thus, we can say that Get-Service spits out ServiceController objects. Under the hood, ServiceController is a .NET Framework class. You can look it up on the Microsoft Developer Network (MSDN) website.

When you run Get-Service normally, you see a data table like the following (again, I'm truncating output to save space and spare your eyes):

Get-Service

Status   Name               DisplayName
------   ----               -----------
Running  AdobeARMservice    Adobe Acrobat Update Service
Stopped  AeLookupSvc        Application Experience
Stopped  ALG                Application Layer Gateway Service
Running  AMD External Ev... AMD External Events Utility
Stopped  AppIDSvc           Application Identity
Running  Appinfo            Application Information
Running  Apple Mobile De... Apple Mobile Device Service

This is a list of ServiceController objects that the Microsoft folks call a collection. The three columns (Status, Name, and DisplayName) are selected properties from each ServiceController object. The output shows seven ServiceController instances from the entire collection. Do you see how the vocabulary fits together?

'Bookend' Cmdlets

In my opinion, the easiest way to start understanding the PowerShell pipeline is to combine what I call "bookend" cmdlets, which operate directly on the same kinds of objects. For instance, let's say that we want to stop the Spooler service on our computer. We can do this with one line of PowerShell code:

Get-Service -Name spooler | Stop-Service

Likewise, we can start the service:

Get-Service -Name spooler | Start-Service

Interesting. In this case, we already know what object type is produced by Get-Service, but if you aren't sure, you can use the GetType() built-in method:

$spool = Get-Service -Name spooler
$spool.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     False    ServiceController                        System.ComponentM...

Let's turn to the Stop-Service help to investigate how this cmdlet might accept inbound ServiceController objects:

Get-Help Stop-Service -Parameter InputObject

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

    Required?                    true
    Position?                    1
    Default value
    Accept pipeline input?       true (ByValue)
    Accept wildcard characters?  false

Admittedly, I'm "cherry picking" my attributes. I knew in advance that Stop-Service had a parameter named InputObject that accepts ServiceController objects. But the point is the same.

I want you to notice this line:

-InputObject <ServiceController[]>

What this tells us is that the Stop-Service parameter named InputObject expects ServiceController objects as input. Now look at the following line in the parameter's attribute table:

Accept pipeline input?       true (ByValue)

This is a crucial parameter attribute to understand when we face the PowerShell pipeline. We read that InputObject can in fact accept input (not all parameters can), and it does so ByValue, which really means "by object type." Thus, we see that feeding ServiceController objects to Get-Service fits hand-in-glove with the Stop-Service and Start-Service cmdlets by virtual of the target cmdlet's InputType parameter.

More About Parameter Binding

If you understand what I've explained thus far, you are most of the way toward mastering the Windows PowerShell pipeline. We've learned that PowerShell uses parameter binding to take input from the previous cmdlet and feed it to the next cmdlet in the pipeline.

PowerShell takes care of the binding itself, although you can "help it along" manually if necessary. Although I've found that the ByValue binding type is by far the most common, some cmdlet parameters can accept input ByPropertyName.

I've discovered that PowerShell tends to fall back on PropertyName binding when or if it encounters problems in doing so by value. When I say that PowerShell attempts a bind by property name, I mean that the receiving cmdlet only looks for an output data field (property) with a name that's understandable. Yes, it's that simple!

Let's try a practical example. Create a simple comma-separated value (CSV) file named spooler.csv that contains the following data:

SvcName,SvcStatus
spooler,running

Now let's import the CSV and feed our spooler service data to Stop-Service:

Import-Csv -Path D:\spooler.csv | Stop-Service -PassThru

What happened? You should have seen what Windows PowerShell MVP Jason Helmick calls "blood on the screen." In other words, PowerShell first looked to bind on a ServiceController object; failing that, it looked for an output object property called "Name." No go there, either, so we are hosed.

Let's look at the -Name parameter of the Stop-Service cmdlet:

Get-Help Stop-Service -Parameter Name

-Name <String[]>
    Specifies the service names of the services to be stopped. Wildcards are
    permitted.

    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 (ByPropertyName, ByValue)
    Accept wildcard characters?  true

By investigating the previous output, we see that the Stop-Service -Name parameter a) expects to receive string values; and b) can accept pipeline input either by property name or by value.

I can tell you that the Import-Csv import gives you a PSCustomObject collection that consists of (drum roll, please) string data. Therefore, to fix the problem, we simply need to rename the column heading in our CSV file:

Name,SvcStatus
spooler,running

Done and done.

Troubleshooting and Conclusion

I'm rapidly running out of "white space," so I need to wrap this up. PowerShell MVP Don Jones gave me the best advice concerning how to troubleshoot unexpected pipeline output. He said that you should a) remove the last element from the pipeline; and b) run Get-Member to see what kind of object you're getting.

For example, the following pipeline doesn't give me the CSV I want:

Get-Service | Format-Table -Property ServiceName, Status | Export-Csv -Path "D:\services.csv"

When I open the CSV, all I see is a bunch of formatting codes. Blech! Let's remove the last part of the pipeline and run a Get-Member. Now I get two .NET object classes:

Microsoft.PowerShell.Commands.Internal.Format.GroupEndData
Microsoft.PowerShell.Commands.Internal.Format.FormatEndData

Aha! So this is why Don Jones taught me to "filter left, format right"!

I hope you found this article informative. Happy PowerShelling!

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