Quick Learn – Cmdlets of the Same Name (VMware & Hyper-V)

Update: This post was submitted to PowerShell.org for their PowerShell TechLetter. For the most update to date version of this post, please read it here: http://powershell.org/techletter/issues/2015-01-january.php#Article1

One of the first things I did when I moved to Windows 8.1 (and Windows PowerShell 4.0, by default), was to add the Hyper-V feature. If you didn’t already know, yes, you can run virtual machines in Windows 8.1 without the need for third-party software. I can’t be 100% certain, but I’m fairly certain I installed this feature a year ago on Windows 8, as well. In both operating systems, you will need to meet some hardware requirements to install Hyper-V, otherwise the feature will be grayed out. In addition, it may not even be listed in non-Pro and non-Enterprise versions of Windows 8.1.

Adding Hyper-V was twofold. One, I wanted to gain experience with the Hyper-V cmdlets and two, I wanted to add at least a couple virtual machines to my computer – Windows 8.1 to test PowerShell 5.0, and Windows 7 with PowerShell 3.0. After a day or so of playing with Hyper-V, it occurred to me that my future work with PowerCLI, VMware’s vSphere PowerShell PSSnapin, meant there would be name overlap between cmdlets in Hyper-V and cmdlets in VMware. This includes popular cmdlets, such as Get-VM and Get-VMHost.

In previous experience, I had learned about command precedence (Get-Help about_Command_Precedence). These are the precedence rules that are used when a command runs. If we have an alias, a function, a cmdlet, and a native Windows command, all with the same name, it will run the alias first when the name is entered into the PowerShell console. If we don’t have an alias with that name, but do have a function, a cmdlet, and native Windows command, then it will run the function. If the name is the same, but the command type is different, then they will always run in order from alias to function to cmdlet to native Windows command.

If we have a two of the same command type and they have the same name, such as the cmdlet Get-VM, it will run the one that was added most recently. This is unless we provide the cmdlet a path. For instance, if the Hyper-V module was loaded and then the VMware PSSnapin was loaded, when we run Get-VM it will run the cmdlet from VMware. If we wanted to use the Get-VM cmdlet from the Hyper-V module, we would need to enter the full path that includes the module name: Hyper-V\Get-VM. Just because one cmdlet was loaded more recently, doesn’t mean the other one is gone.

In my mind, there’s a couple ways to fix the problem, or rather, make it easier to use cmdlets with the same name. The first option I tried, was to determine if the Add-PSSnapin cmdlet, that is used by VMware, had a -Prefix parameter. It didn’t, but had it, I could have added a prefix to all the VMware cmdlets with an option like the example below.

Add-PSSnapin -Name VMware.VimAutomation.Core -Prefix VMware

This doesn’t actually work. Read the post.

An option like this wouldn’t have clobbered my Hyper-V cmdlets, and therefore, Hyper-V’s Get-VM cmdlet would still work without a path, and VMware’s Get-VM cmdlet, for example, would have been Get-VMwareVM. The Import-Module cmdlet does have a -Prefix parameter so I could have changed the Hyper-V cmdlets to use a prefix, but I didn’t think the Hyper-V cmdlets should be the ones to suffer. I’m a PowerShell enthusiast, and I didn’t want to change the naming of Microsoft-built cmdlets, and therefore, learn them incorrectly.

Here’s what I did. I added a function to my profile ($PROFILE) that would allow me to choose which set of cmdlets would be loaded first, and which would be loaded second. The cmdlets I loaded second wouldn’t need the path to use them. This meant I could set the Hyper-V cmdlets to not need a path when I work with those, or I could set VMware cmdlets to not need a path when I work with those.

I started by declaring an empty function called Add-VMCs (Virtual Machine Cmdlets) that included a single parameter called Default.

Function Add-VMCs($Default) {

}

I started the function with an If statement. On line 2, below, it checks the parameter that has been provided when calling the function. If it matches the letter h or the letter v it will continue to line 3. If it does not, it will jump to line 7 and run the Else portion – displaying a message that nothing was changed and what parameters can be used. Line 3 and 4 remove the Hyper-V module and VMware PSSnapin, whether or not they are already loaded. While there could have been some logic to first check if they are loaded, I decided I was fine with hiding any errors that might occur (-ErrorAction SilentlyContinue) if I tried to unload a module or PSSnapin that wasn’t already loaded. I usually handle errors better than this, but I didn’t think it was necessary for this function.

The reason this is required is because if we try to load an already loaded module or PSSnapin, it won’t actually bother doing it, or at least that’s what I think is going on. This would prevent the function from ensuring the module and PSSnapin were loaded in my preferred order. Remember, if cmdlet names are the same, the most recently loaded cmdlet will be the one that is used.

Function Add-VMCs($Default) {
    If ($Default -eq 'h' -or $Default -eq 'v') {
        Remove-Module -Name Hyper-V -ErrorAction SilentlyContinue
        Remove-PSSnapin -Name VMware.VimAutomation.Core -ErrorAction SilentlyContinue

    } Else {
        Write-Output -Verbose "INFO: No changes made`r`nUse H to set Hyper-V as the default (Add-VMCs H) or use V to set VMware as the default (Add-VMCs V)."
    }
 }

Like we said previously, if the parameter is equal to the letter v or the letter h, it will remove the module and PSSnapin. The function continues to an If-ElseIf statement that begins on line 5. This bit of logic takes different actions depending on the value of the parameter. If the parameter is equal to the letter h, it will load the VMware PSSnapin and then import the Hyper-V module. This means that Get-VM would be the cmdlet associated with Hyper-V. If it is not equal to the letter h, and instead it is equal to the letter v, then it will load the Hyper-V module and then the VMware PSSnapin. This means that Get-VM would be the cmdlet associated with VMware.

Function Add-VMCs($Default) {
    If ($Default -eq 'h' -or $Default -eq 'v') {
        Remove-Module -Name Hyper-V -ErrorAction SilentlyContinue
        Remove-PSSnapin -Name VMware.VimAutomation.Core -ErrorAction SilentlyContinue
        If ($Default -eq 'h') {
            Add-PSSnapin -Name VMware.VimAutomation.Core
            Import-Module -Name Hyper-V
        } ElseIf ($Default -eq 'v') {
            Import-Module -Name Hyper-V
            Add-PSSnapin -Name VMware.VimAutomation.Core
        }
    } Else {
        Write-Output -Verbose "INFO: No changes made`r`nUse H to set Hyper-V as the default (Add-VMCs H) or use V to set VMware as the default (Add-VMCs V)."
    }
}

Here’s the function in action. At first I set the Hyper-V cmdlets to be the default on line 1. I then verify that my cmdlet is from the correct module by using the Get-Command cmdlet. Once that’s been verified, I run the Get-VM cmdlet on line 8. Then, I do it all again after I change the default to VMware. Note: If you’ve ever used VMware’s PowerCLI then you know I had to use the Connect-VIServer cmdlet to connect to a vCenter system before it would allow me to run the VMWare Get-VM cmdlet.

PS C:\> Add-VMCs -Default H
PS C:\> Get-Command Get-VM

CommandType     Name                                               ModuleName
-----------     ----                                               ----------
Cmdlet          Get-VM                                             Hyper-V

PS C:\> Get-VM -Name Win8.1*

Name        State   CPUUsage(%) MemoryAssigned(M) Uptime   Status
----        -----   ----------- ----------------- ------   ------
Win8.1PS5.0 Running 23          1536              00:26:20 Operating normally

PS C:\> Add-VMCs V
PS C:\> Get-Command Get-VM

CommandType     Name                                               ModuleName
-----------     ----                                               ----------
Cmdlet          Get-VM                                             VMware.VimAutomation.Core

PS C:\> Get-VM -Name Windows*

Name                 PowerState Num CPUs MemoryGB
----                 ---------- -------- --------
Windows 2003 R2 S... PoweredOff 1        1.000

Linked from here:
http://blogs.technet.com/b/heyscriptingguy/archive/2014/09/04/powertip-use-complete-name-for-powershell-cmdlet.aspx

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