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Windows PowerShell: Peering Through the Pipeline

Jun 11, 2015

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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 ofSams 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

让我们开始吧!

的'Pipeline' in`用于cmd . exe`

的Windows command processor,用于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"

的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 likels,grep,awk, andsed。即便如此,所有你有管道“哑巴”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 (calledproperties), as well as actions that the object can perform (calledmethods).

In PowerShell, we pipe output toGet-Memberto 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 byGet-Service:

System.ServiceProcess.ServiceController

IT pros tend to use the last part of an object name. Thus, we can say thatGet-Servicespits outServiceControllerobjects. Under the hood,ServiceControlleris a .NET Framework class. You can look it up on theMicrosoft Developer Network (MSDN) website。

When you runGet-Servicenormally, 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 ofServiceControllerobjects that the Microsoft folks call acollection。的three columns (Status,Name, andDisplayName) are selected properties from eachServiceControllerobject. The output shows sevenServiceControllerinstances 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 byGet-Service, but if you aren't sure, you can use theGetType()built-in method:

$spool = Get-Service -Name spooler $spool.GetType() IsPublic IsSerial Name BaseType -------- -------- ---- -------- True False ServiceController System.ComponentM...

Let's turn to theStop-Servicehelp to investigate how this cmdlet might accept inboundServiceControllerobjects:

Get-Help Stop-Service -Parameter InputObject -InputObject  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 thatStop-Servicehad a parameter namedInputObjectthat acceptsServiceControllerobjects. But the point is the same.

I want you to notice this line:

-InputObject

What this tells us is that theStop-Serviceparameter namedInputObjectexpectsServiceControllerobjects 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 thatInputObjectcan in fact accept input (not all parameters can), and it does soByValue, which really means "by object type." Thus, we see that feedingServiceControllerobjects toGet-Servicefits hand-in-glove with theStop-ServiceandStart-Servicecmdlets by virtual of the target cmdlet'sInputTypeparameter.

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 theByValuebinding type is by far the most common, some cmdlet parameters can accept inputByPropertyName。

I've discovered that PowerShell tends to fall back onPropertyNamebinding 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 namedspooler.csvthat contains the following data:

SvcName,SvcStatus spooler,running

现在让我们导入CSV和饲料our spooler service data toStop-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 aServiceControllerobject; failing that, it looked for an output object property called "Name。" No go there, either, so we are hosed.

Let's look at the-Nameparameter of theStop-Servicecmdlet:

Get-Help Stop-Service -Parameter Name -Name  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 theStop-Service-Nameparameter a) expects to receive string values; and b) can accept pipeline input either by property name or by value.

I can tell you that theImport-Csvimport gives you aPSCustomObjectcollection that consists of (drum roll, please) string data. Therefore, to fix the problem, we simply need to rename the column heading in ourCSVfile:

Name,SvcStatus spooler,running

完成了,完成了。

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) runGet-Memberto 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 aGet-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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