PowerShell 3.0 simplifies the
'Where' clause by not requiring either the {curly} brackets or the
placeholder $_.xyz. The benefit is that it makes easier for beginners
to write scripts, and for everyone to understand 'Where' clauses.
Scenario: You want to list the dynamic link library
files in the Windows System folder.
Example 1 New Style PowerShell 3.0 Where
Get-Childitem C:\Windows\System | Where extension -eq .dll
Note 1: What's new in PowerShell 3.0 is the simpler Where
extension -eq .dll
In PowerShell 2.0 the above command would require three extra syntax elements:
{Curly brackets}, the $_. variable and speech marks around the 'value': where
{$_.extension -eq '.dll'}
Example 2 PowerShell 3.0 Where Lists System .exe Files
# PowerShell 3.0 'Where' filter for listing .exe in system folder
Clear-Host Get-ChildItem $Env:WinDir | Where extension -eq
.exe
Note 2: $Env:WinDir is an environmental variable, which just
get's your Windows folder even if it's on the D:\ and even if you
call it
WindowsOld.
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You can simplify the filter further by substituting the question mark
for the command 'Where', for example: ? extension -eq .exe (Instead
of: Where extension -eq .exe) The use of the question mark is the same
as in PowerShell 2.0, as is the use of the alias gci for Get-ChildItem.
# Windows PowerShell Where script to list exe files C:\Program Files
$FilesExe = gci "C:\Program Files" -recurse $List = $FilesExe | ?
extension -eq .exe $List | Sort-Object -unique | Format-Table Name,
Directory -auto
Learning Points for PowerShell's Where Statement
Note 3:
Most people who use PowerShell prefer to type the question mark character '?' instead of 'Where'. However, I prefer to stick with the
full word 'Where' because it makes the script easier to read especially
with complex statements.
Note
4: If you like these abbreviations, PowerShell has numerous aliases for common commands, for example
in the above script note, gci
for Get-ChildItem. Furthermore, we could employ 'ft' instead of
Format-Table. Even I usually omit the Object from Sort-Object and
Where-Object.
Note 5: What I often forget is to introduce the where statement with a pipe, hence, .....
$FilesExe | ? ...... Please learn from my mistake!
Another situation that benefits from a 'Where' statement is when we research PowerShell's objects.
To take a network example, imagine that our goal is to display the TCP/IP properties, but
we have a problem - what is the WMI object called? Let us begin our quest by researching WMI objects. We already
know that we can use, WmiObject -List, but let us refine our search by adding a where statement.
Where Example
4a
# Windows PowerShell where clause in WMI script Get-WmiObject -List |
where name -Match Network
Where Example 4b
Once again, we can substitute the question mark for 'Where'
# PowerShell ? (where) filter to list Network WMI objects
Clear-Host Get-WmiObject -List | ? name -Match Network | FT name
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PowerShell's where statement is so common that we tend to forget the '?'
and even 'Where' are aliases for the Where-Object cmdlet. The benefit
of this realization is that we can call for Get-Help.
Get-Help Where-Object
Clear-Host Get-Help Where-Object -full # Plain 'help where' also works.
Help reminds us that Where-Object does its filtering by examining a {Scriptblock}.
I have never seen 'where' in isolation, it always receives its input via a pipe (|).
One more point to note, PowerShell uses its own comparison objects such
as -gt or -le, and not > or =<.
Note 6: One of the intesting new parameter in version
3.0 is 'In'. For example try: 2 -In 1..10
PowerShell 3.0 Where-Object with Get-ChildItem
I am assuming that you can research a cmdlet's properties by appending gm
(Get-Member). For the first experiment we need the 'Extension'
property, and later we need LastAccessTime.
Experiment 1: We want a list of the executable (.exe)
files in the Windows folder.
Method: We can use PowerShell 3.0's simple 'Where'
clause.
# PowerShell 3.0 Get-Childitem Where Get-Childitem C:\Windows | Where
Extension -eq .exe
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Following the above example you would think that | Where LastTime.Year
-gt 2010 would provide a list of files that were accessed after 1/1/2011,
but it doesn't.
# PowerShell 3.0 Where Problem Get-Childitem C:\Windows\System | Where
LastAccessTime.Year -gt 2010
Although the above example completes without error it does not display
any files. Just in case there are no files, change -gt to -lt, still
no files appear.
Solution: Revert to the PowerShell 2.0 Where
syntax. Remember the full syntax: Curly brackets}, $_. variable, and 'single speech mark'.
# PowerShell 3.0 Where Clause Problem Get-Childitem C:\Windows\System
| Where {$_.LastAccessTime.Year -gt '2010' }
PowerShell 3.0 simplifies the
syntax of the
'Where' statement. We no longer need either the {curly} brackets or the
placeholder $_.xyz. As a result it's easier for beginners
to learn PowerShell, and it's easier for everyone to understand 'Where' clauses.
See more examples of PowerShell's 'Where' clause in action:
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