Param() Tricks

One of the new features of PowerShell v5 is support for creating hard links, symbolic links and junctions. This is long overdue, and much appreciated. Before, I'd been forced to the workaround of using "cmd /c mklink" to create links, and I'm always glad to find a way to get rid of any vestige of cmd. Plus, having it as a part of PowerShell gives me way more flexibility in creating scripts.

 

As I was looking at some of my existing scripts, it occurred to me that I should be taking advantage of hard links in more scripts. I already use hard links for my various RDP connects, using a long switch statement. (I'll write that up one of these days, it's actually pretty cool.) But what caught my eye today was the script I wrote to create virtual machines for my labs -- New-myVM.ps1. I have a -Client parameter to the script that is a Boolean, defaulted to $False:

[CmdletBinding()]
Param([Parameter(Mandatory = $True,Position = 0)]
      [alias("Name")]
      [string]
      $VmName,
      [Parameter(Mandatory=$False)]
      [Boolean]
      $Client=$False
      )

Which is OK, but it occurred to me that I could do better. So, first, I created a new, hard-linked file with:

New-Item -Type HardLink -Name New-myClientVM.ps1 -Path .\New-myVM.ps1

Now I have one file with two names. Cool. So, let's take that a step further. I can tell which version of it I called from the command line by taking advantage of the automatic PowerShell variable $myInvocation:

$myInvocation.mycommand.name

This returns the filename (".name") of the command (".mycommand") that was executed ($myInvocation). So now, I can use:

$client =  ($myInvocation.mycommand.name -match "client")

I put that near the top of the script, and now I could branch depending on whether I was creating a server VM or a client VM. Which was definitely better, but still left me thinking it could be improved.

 

So, how about making the whole thing a lot cleaner by getting rid of that extra line? After all, I'm creating a variable and defaulting its value to $false, but why not default its value more intelligently, controlled by which file I executed to create the VM? I can still override it with the parameter (so no scripts that call this script will break), but now, I can set it automatically without using a parameter at all.

[CmdletBinding()]
Param([Parameter(Mandatory = $True,Position = 0)]
      [alias("Name")]
      [string]
      $VmName,
      [Parameter(Mandatory=$False)]
      [Boolean]
      $Client=($myInvocation.myCommand.Name -match "Client")
      )

Now that pleases me. It feels "cleaner", it's clear what I'm doing, and it doesn't take any longer to evaluate than it would as a standalone line.

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