The coding blog of Alastair Smith, a software developer based in Cambridge, UK. Interested in DevOps, Azure, Kubernetes, .NET Core, and VueJS.
PowerShell is Microsoft’s new command line shell for Windows. Version 1 was released in 2006, and version 2 has been in CTP for about a year now; it will finally RTM with Windows 7. It’s a truly fantastic shell, and if you haven’t looked into it already, I would urge you to do so. There’s a wealth of information out there, not least on the PowerShell blog, that will get you started very quickly.
I’m such a fan of PowerShell that I’m quite surprised I haven’t posted about it here in any detail before. One of the key advantages is that it is a first-class .NET language, giving you access to the entire .NET framework from the command-line; this arises from the basic data item being an object (rather than plain text as it is in UNIX shells). I hope to post an introduction/tutorial to PowerShell sometime in the near future.
The subject of this post, however, is a specific feature of PowerShell, the -whatif switch. I assume familiarity with basic PowerShell features, and the PowerShell syntax.
Recently at work, I was writing some PowerShell scripts to automate some XML manipulation. Unfortunately, I got the script a bit wrong, and so the XML manipulation didn’t work as expected. This could have been avoided had I not forgotten about -whatif. Additionally, I was running the script in a sandbox location, which meant duplication of files and directory structure; this too could have been avoided if by using the -whatif switch.
More of a magic shield than a magic bullet, -whatif does a dry run of your script. Supplying -whatif to your command line runs the script exactly as it would be run, but without making any actual modifications. As such, it is only valid on cmdlets that make modifications; supplying it to a get-* cmdlet will throw an error.
What follows is a slightly contrived example, but bear with me :-) Say my log directory (e.g., C:\logs) was getting a bit full and hard to manage, I might want to delete all old log files, where “old” means more than one week old. The following code looks good, but there’s a not-so-subtle error in it.
Running this one-liner (formatted to reduce scrolling) as-is will cause a huge screw-up: I’ve got the logic wrong for the date comparison, and suddenly I’ve deleted all my most recent log files.
Adding that one flag does a dry-run, and gives me the following verbose output:
“Wait a minute”, I think, “that file is dated with today’s date. I must have got the logic wrong!” And so my ass and my log files are saved. Flicking the comparison from a -gt to a -lt and leaving the -whatif switch enabled confirms my suspicions, and I now have the confidence to let PowerShell remove those old log files for me.
This is a really important and useful switch, and if you write PowerShell artefacts (scripts, cmdlets, etc.) that you share with others, you really should support this switch. And don’t forget to implement the other standard switches while you’re at it.
Now all I need to do is train myself to test out my PowerShell artefacts with this switch before I run them for real.