Selecting data
Running a command can be powerful, you get data from your local machine or from across the network. To be even more effective, you need to learn how to get the data that you want. Most commands operate on objects, as input or as output, or both. Objects have properties and you may want to access a subset of those properties and present them in a report. You might also want to sort the data based on one or more properties. But how do you get there?
Use Get-Member to inspect output
When you pass the results of a command to Get-Member, Get-Member returns information about an object, like:
- The type of object being passed to Get-Member.
- The Properties of the object that may be evaluated.
- The Methods of the object that may be executed.
Let's demonstrate this fact by running Get-Member on the command Get-Process.
Get-Process | Get-Member
Note how you're using the pipe | and that by calling Get-Member, you are in fact creating a pipeline already. The first few lines of output from the preceding statement look like so:
TypeName: System.Diagnostics.Process
Name MemberType Definition
---- ---------- ----------
Handles AliasProperty Handles = Handlecount
Name AliasProperty Name = ProcessName
NPM AliasProperty NPM = NonpagedSystemMemorySize64
PM AliasProperty PM = PagedMemorySize64
SI AliasProperty SI = SessionId
VM AliasProperty VM = VirtualMemorySize64
WS AliasProperty WS = WorkingSet64
...
The output shows the type of object that the Get-Process command returns (System.Diagnostics.Process). The rest of the response shows the name, type, and definition of the object's members. You can see that If you want to fit Get-Process with another command in a pipeline, pairing it with Get-Member is a good first step.
Select-Object
By default, when you run a command that is going to output to the screen, PowerShell automatically adds the command Out-Default. When the output data is a collection of objects, PowerShell looks at the object type to determine if there's a registered view for that object type. If it finds one, it uses that view.
The view generally doesn't contain all the properties of an object because it wouldn't display properly on screen, so only some of the most common properties are included in the view.
You can override the default view by using Select-Object and choosing your own list of properties. You can then send those properties to Format-Table or Format-List, to display the table however you like.
Consider the result of running Get-Process on the process zsh:
NPM(K) PM(M) WS(M) CPU(s) Id SI ProcessName
------ ----- ----- ------ -- -- -----------
0 0.00 0.01 0.38 644 620 zsh
0 0.00 0.01 0.38 727 727 zsh
0 0.00 0.01 0.38 731 731 zsh
0 0.00 0.01 0.38 743 743 zsh
0 0.00 0.01 0.38 750 750 zsh
0 0.00 0.88 0.91 15747 …47 zsh
0 0.00 0.01 0.29 41983 …83 zsh
0 0.00 1.16 0.31 68298 …98 zsh
What you see is a view that represents what you most likely want to see from this command. However, this view doesn't show you a complete set of information. In order to see something different, you can explicitly specify which properties you want to see in the result.
Getting the full response
What you've seen so far is a limited response. To present the full response, you use a wildcard *, like so:
Get-Process zsh | Format-List -Property *
The * character shows you every attribute and its value, which allows you to investigate the values you're interested in. The full response also uses presentation names for properties instead of the actual property names, and presentation names look good in a report.
Despite these benefits, you may not want a full output of data, but you may not be content with the default response either.
Selecting specific columns
To limit the response and find a middle ground between the default response and the full response, you want to select some properties you're interested in and have that as parameter input to Select-Object. But, and here's a problem, you need to use the real names for the columns. How do you find out the real names? Use Get-Member. A call to Get-Member gives you all the properties and their actual names.
Finding the real property name
Let's quickly recap on the default response, with this subset:
NPM(K) PM(M) WS(M) CPU(s) Id SI ProcessName
------ ----- ----- ------ -- -- -----------
0 0.00 0.01 0.38 644 620 zsh
From the default response, the properties Id and ProcessName are most likely called the same, but CPU(s) is a presentation name, real property names tend to consist of only text characters and no spaces. To find out the real name for a specific property, you can use Get-Member:
Get-Process zsh | Get-Member -Name C*
You now get a list of all members with names that start with a C. Among them is CPU, which is likely what you want:
TypeName: System.Diagnostics.Process
Name MemberType Definition
---- ---------- ----------
CancelErrorRead Method void CancelErrorRead()
CancelOutputRead Method void CancelOutputRead()
Close Method void Close()
CloseMainWindow Method bool CloseMainWindow()
Container Property System.ComponentModel.IContainer Container {get;}
CommandLine ScriptProperty System.Object CommandLine {get=…
Company ScriptProperty System.Object Company {get=$this.Mainmodule.FileVersionInfo.CompanyName;}
CPU ScriptProperty System.Object CPU {get=$this.TotalProcessorTime.TotalSeconds;}
You now know how to use Select-Object to ask for exactly what you need with the correct property names, like so:
Get-Process zsh | Select-Object -Property Id, Name, CPU
And here it is:
Id Name CPU
-- ---- ---
644 zsh 0.3812141
727 zsh 0.3826498
731 zsh 0.3784953
743 zsh 0.3776352
750 zsh 0.3824036
15747 zsh 0.9097993
41983 zsh 0.2934763
68298 zsh 0.3121695
This sequence of commands gives you an output that differs from the default output but contains properties that you care about.
Sorting
When you use Sort-Object in a pipeline, PowerShell sorts the output data by using the default properties first. If no such properties exist, it then tries to compare the objects themselves. The sorting is either by ascending or descending order.
By providing properties, you can choose to sort by specific columns, like so:
Get-Process | Sort-Object -Descending -Property Name
In the preceding command, we're sorting by the column Name in descending order. To sort by more than one column, separate the column names with a comma, like so:
Get-Process | Sort-Object -Descending -Property Name, CPU
In addition to sorting by column name, you can also provide your own custom expression. In this example, we use a custom expression to sort by the columns Name and CPU and control the sort order for each column.
Get-Process 'some process' | Sort-Object -Property @{Expression = "Name"; Descending = $True}, @{Expression = "CPU"; Descending = $False}
The preceding example demonstrates how powerful and flexible Sort-Object can be. This subject is a bit advanced and out of scope for this module, but is revisited in more advanced modules.
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