Have you ever ran a powershell cmdlet and stare at the output, and ever wondered, what makes Powershell display the results in such a way? Is there any hidden configuration that guides the powershell engine to display results in a specific format for one cmdlet and in a different format for another cmdlet??
Read along. This blog will answer your curiosity !
Example: Below is the output of Get-service
Below is the output of Get-Process
The answer is Yes. The Powershell Engine is indeed guided by a configuration file that tells powershell on how to display the results. Or we call it as the “Default rules for Formatting” (not an official name :))
You will find the configuration files in the Powershell Installation Folder. Use the “$pshome” variable to find out the Powershell Installation Folder.
PS E:\Work\Powershell\scripts> $pshome C:\Windows\System32\WindowsPowerShell\v1.0
Change the directory to the Powershell Installation directory and you must be able to find a file named “DotNetTypes.format.ps1xml”.
Please be cautious, not to edit the file. As it will break the signature and the PowerShell will not be able to use it anymore.
If you want to find out the default rules that are applied, then simply open the file and search for the cmdlet.
Example: If I want to know the default rules for the “Get-Service” cmdlet, I search the file for the keyword “service”. Note that the keyword “Service” should be enclosed within the “Name” tags. That is the correct one. As per the below image.
You can double confirm if it is the correct branch, using the “<TypeName>” tag. This value should equate to the TypeName when you do a Get-Member of that cmdlet.
Example: “System.ServiceProcess.ServiceController” for Get-Service
Now what you are looking in the file are set of directions that the Powershell Engine follows. If you can see in the preceding lines, we can see a <TableControl> tag, which says, that the output should be in the form of table. Next few lines specifies the attributes of the table, such as Width and Height.
When you run Get-Service, here’s what happens:
- The cmdlet places objects of the type System.ServiceProcess.ServiceController into the pipeline.
- At the end of the pipeline is an invisible cmdlet called Out-Default. It is always there, and it’s job is to pick up what ever objects are in the pipeline after all the commands have run.
- Out-Default passes the objects to Out-Host, because the PowerShell console is designed to use the screen (called the host) as it’s default form of output.
- Most of the Out- cmdlets are incapable of working with normal objects. Instead, they are designed to work with special formatting instructions. So when Out-Host sees that it has been handed normal objects, it passes them to the formatting system.
- The formatting system looks at the type of the object and follow an internal set of formatting rules. It uses those rules to produce formatting instructions, which are passed back to Out-Host.
- Once Out-Host sees that it has formatting instructions, it follows those instructions to construct the onscreen display.
So when you run the below cmdlet,
Get-Process | Out-File process.txt
The out-File will see the normal objects. It will pass them to the formatting system, which will create formatting instructions and then passes back them to the Out-File. The Out-File then constructs the file based on those formatting instructions.
Below are the formatting rules:
- First Rule: The System looks to see if the type of object it is dealing with has a predefined view. That is what you saw in the DotNetTypes.format.ps1xml. There are other .format.ps1xml files installed with Powershell, and they are loaded when the powershell starts. You can create your own predefined views as well.
- Second Rule: If the system is not able to find a predefined view, then it will look to see if anyone has declared a “default display property set” for that type of object. You can find that in a configuration file called- “Types.ps1xml” (under the Powershell Home directory)
Go back to Powershell and Run:
Get-WmiObject win32_operatingsystem
I guess, now you know from where these entries came from. The properties you see are present because they are listed as defaults in the Types.ps1xml. If the formatting system finds a “default display property set”, it will use those set of properties.
- Third Rule: It is about what kind of output to create. If the formatting system will display four or fewer properties, it will use a table. If there are five or more properties, then it will display the results as a list. This is why the Win32_OperatingSystem object was not displayed as a table, as it contained six properties, triggering a list. The theory is that more than 4 properties might not fit into a table, without truncating information.
Manjunath Rao 