Friday, January 27, 2023

PowerShell core compatibility: A lesson learned the hard way

PowerShell core is my preferred scripting language. I've been excited about it since its early days. Here's a tweet from back in 2016 when PowerShell core was still in beta:


I've used PowerShell to automate build steps, deployments, and other tasks on both dev environments and CICD pipelines. It's great to write a script on my Windows machine, test it using PowerShell core, and run it on my docker Linux-based build environments with 100% compatibility. Or so I thought until I learned otherwise!

A few years ago, I was automating a process which required creating a folder if it didn't exist. Out of laziness, this is how I implemented this functionality: 

mkdir $folder -f

When the folder exists and the -f (or --Force) flag is passed, the command will return the existing directory object without errors. I know this is not the cleanest way -more on this later- but it works on my Windows machine, so it should also work in the docker Linux container, except that it didn't. When the script ran, it resulted in this error:

/bin/mkdir: invalid option -- 'f'
Try '/bin/mkdir --help' for more information.

Why did the behavior differ? It turns out that mkdir means different things depending on whether you're running PowerShell on Windows or Linux. And this can be observed using Get-Command Cmdlet:

# Windows:
Get-Command mkdir

The output is:

CommandType     Name                                               Version
-----------     ----                                               -------
Function        mkdir

Under Windows, mkdir is a function, and the definition of this function can be obtained using

(Get-Command mkdir).Definition

And the output is:


    [Parameter(ParameterSetName='nameSet', Position=0, ValueFromPipelineByPropertyName=$true)]
    [Parameter(ParameterSetName='pathSet', Mandatory=$true, Position=0, ValueFromPipelineByPropertyName=$true)]

    [Parameter(ParameterSetName='nameSet', Mandatory=$true, ValueFromPipelineByPropertyName=$true)]

    [Parameter(ValueFromPipeline=$true, ValueFromPipelineByPropertyName=$true)]



begin {
    $wrappedCmd = $ExecutionContext.InvokeCommand.GetCommand('New-Item', [System.Management.Automation.CommandTypes]::Cmdlet)
    $scriptCmd = {& $wrappedCmd -Type Directory @PSBoundParameters }

    $steppablePipeline = $scriptCmd.GetSteppablePipeline()

process {

end {

Which as you can see, wraps the New-Item Cmdlet. However under Linux, it's a different story:

# Linux:
Get-Command mkdir


CommandType     Name                                               Version
-----------     ----                                               -------
Application     mkdir                                    

It's an application, and the source of this applications can be retrieved as:

(Get-Command mkdir).Source

Now that I know the problem, the solution is easy:

New-Item -ItemType Directory $folder -Force

It's generally recommended to use Cmdlets instead of aliases or any kind of shortcuts to improve readability and portability. Unfortunately PSScriptAnalyzer - which integrates well with VSCode- will highlight this issue in scripts but only for aliases (like ls) and not for functions. AvoidUsingCmdletAliases.

I learned my lesson. However, I did it the hard way.