Richard Siddaway's Blog

Do you understand the difference between do loops and while loops?  On the surface they appear very similar.

£> do{$i++}while($i -lt 10)
£> $i
10

£> $i=0
£> while($i -lt 10){$i++}
£> $i
10

BUT there is a difference

£> $i=11
£> do{$i++}while($i -lt 10)
£> $i
12

£> $i=11
£> while($i -lt 10){$i++}
£> $i

Notice the difference in the final result

A DO loop will ALWAYS execute at least once because the test is at the end.

A while loop has the test at the beginning so may not execute at all.

I’ve seen this cause people real difficulty at times so think about where you need the test to be, if the loop needs to execute at least once and what the final value of any incrementing variables should be

This is a common type of use for Get-ChildItem:

Get-ChildItem -Path c:\temp 

However, you could get files or folders returned.  Very often you just want to see the files

In PowerShell 1.0 & 2.0 you had to do this:

Get-ChildItem -Path c:\temp   | where {-not $_.PSIsContainer}

If you want to see the subfolders only then you reverse the condition:

Get-ChildItem -Path c:\temp  | where {$_.PSIsContainer}

In PowerShell 3.0 Get-ChildItem got some new parameters –File and –Directory. They display files only or folders only respectively. The default action is still to display files and folders.

Get-ChildItem -Path c:\temp  -File
Get-ChildItem -Path c:\temp  -Directory

I still see a lot of people making more work for themselves because they don’t use or don’t know about the new parameters.

I was asked a question about deleting files from a folder based on age.  The requirement was to delete all but the youngest N files.

One solution is a classic PowerShell one-liner. It is actually one PowerShell pipeline though I’ve split it across multiple lines for ease of reading.

Get-ChildItem -Path c:\temp -Filter *.tmp -File |
sort LastWriteTime -Descending |
select -Skip 5 |
Remove-Item

Use Get-Childitem to read the files you’re interested in. I’ve used the –File parameetr to restrict the examination to files.

Sort the files on LastWriteTime – you could use CreationTime if the files haven’t been changed since creation. You want the results sorted in descending order so the latest files are at the top of the list. Younger dates are greater than older dates.

use Select-Object to skip the first N (in this case 5) files and pipe everything else into remove-item.  Add a –whatif parameter to Remove-Item if want to see how its working before letting it loose.

A nice simple answer with a use for Select-Object’s –Skip parameter that I hadn’t thought of before.

You may have seen this at the top of scripts or modules

#Requires –Version 3

This will stop the script running if the PowerShell version is 2.0 or below.

Other options are available

#Requires –PSSnapin      can be used to force the loading of a snapin

#Requires –Modules     can be used to force importing of modules

#Requires –Shellid    controls which shells the script can run in.  Default PowerShell installs have the ISE and console

#Requires –RunAsAdminstrator  - script must be running in instance of PowerShell with elevated privileges

These provide a simple and easy way to control execution without lots of code. I particularly like the last one.

Some times you need to reboot a remote machine as part of your process.  PowerShell provides the Restart-Computer cmdlet to perform that task:

Restart-Computer -ComputerName server03

If you want you process to pause until the reboot has finished then you add the –Wait parameter:

Restart-Computer -ComputerName server03

You final options when pausing are to wait until a specific service – PowerShell, WinRM or WMI is avialable:

Restart-Computer -ComputerName server03 -Wait -For PowerShell

Restart-Computer -ComputerName server03 -Wait -For WinRM

Restart-Computer -ComputerName server03 -Wait -For Wmi

These options should cover most situations as these services are among the last to become available when a machine starts.