Working with Collections of Objects using PowerShell

Ref: http://jamesmccaffrey.spaces.live.com/blog/cns!504C7CC53E7E7FE8!593.entry

I recently spoke at the Microsoft Management Summit. MMS is a huge event and this year had over 4,000 attendees. All four of my talks were introductory level talks about PowerShell, Microsoft’s new shell and scripting environment. Over the course of delivering training to many hundreds of people, I’ve discovered that beginners are often confused about the difference between the select-object and the where-object cmdlets. In short, if you are dealing with a collection of objects, and you think of that collection as a table with rows and columns, then select-object is used to select columns, and where-object is used to select rows. There are several exceptions that I’ll describe in a moment, but this simplification is a useful mnemonic. Consider these PowerShell commands:

 

(1)> $p = get-process
(2)> $p | more
(3)> $p | get-member | more
(4)> $p[0].Name
(5)> $p[0].ProcessName
(6)> $p[0].get_ProcessName()
(7)> $q = $p | select-object name,id,handles
(8)> $q
(9)> $q | format-list
(10)> $r = $q | where-object { $_.handles –gt 500 }
(11)> $r | format-list

 

The (1) command fetches process information into a $p array of objects. The (2) command displays the $p array as a table. Each row represents one process and each column is a property. The (3) command shows that objects can have properties, property aliases, and underlying methods. The (4),(5), and (6) commands shows how to get at the ProcessName “column” of the $p “table”. The (7) command uses select-object to conceptually select just the Name, ID, and Handles columns of the $p “table” and store into a $q object. The (8) command displays the result but illustrates that results can scroll off to the right of a PowerShell shell. The (9) command shows how to print collections of objects nicely. The (10) command uses the where-object cmdlet to select just those rows where the Handles column is greater than 500. Notice that where-object is an implied loop and so the curly brace notation is required. The $_ means “the current object in the loop.”

 

Now there are a few details. Confusingly, the select-object cmdlet supports –first n and –last n parameters; this means you can select rows using select-object. Also some cmdlets support parameters such as –filter, and –include which can be used to select rows. Furthermore, instead of using the implicit loop of the where-object cmdlet, you can use the explicit foreach-object cmdlet. Well, all these (and other) exceptions aside, if you remember the little rule, “select-object selects columns and where-object selects rows”, you’ll be fine in most situations.