CLI: Adding paths to PATH environment variable in Windows

adding-paths.ps1

#!/usr/bin/env powershell

function Prepend-Idempotent {

    # the delimiter is expected to be just 1 unique character
    # otherwise there may be problems with trimming
    param (
        [string]$InputString, 
        [string]$OriginalString, 
        [string]$Delimiter = '', 
        [bool]$CaseSensitive = $false
    )

    if ($CaseSensitive -and ("$OriginalString" -cnotlike "*${InputString}*")) {

        "$InputString".TrimEnd("$Delimiter") + "$Delimiter" + "$OriginalString".TrimStart("$Delimiter")

    } elseif (! $CaseSensitive -and ("$OriginalString" -inotlike "*${InputString}*")) {

        "$InputString".TrimEnd("$Delimiter") + "$Delimiter" + "$OriginalString".TrimStart("$Delimiter")

    } else {

        "$OriginalString"
    
    }

}

function Append-Idempotent {

    # the delimiter is expected to be just 1 unique character
    # otherwise there may be problems with trimming
    param (
        [string]$InputString, 
        [string]$OriginalString, 
        [string]$Delimiter = '', 
        [bool]$CaseSensitive = $false
    )

    if ($CaseSensitive -and ("$OriginalString" -cnotlike "*${InputString}*")) {

        "$OriginalString".TrimEnd("$Delimiter") + "$Delimiter" + "$InputString".TrimStart("$Delimiter")

    } elseif (! $CaseSensitive -and ("$OriginalString" -inotlike "*${InputString}*")) {
        
        "$OriginalString".TrimEnd("$Delimiter") + "$Delimiter" + "$InputString".TrimStart("$Delimiter")

    } else {

        "$OriginalString"
    
    }

}

function Add-Path {

    param (
        [string]$NewPath, 
        [ValidateSet('Prepend','Append')]$Style = 'Prepend', 
        [ValidateSet('User', 'System')]$Target = 'User'
    )

    try {
        
        # we need to do this to make sure not to expand the environment variables already inside the PATH
        if ($Target -eq 'User') {
            $Key = [Microsoft.Win32.Registry]::CurrentUser.OpenSubKey('Environment', $true)
        } elseif ($Target -eq 'System') {
            $Key = [Microsoft.Win32.Registry]::LocalMachine.OpenSubKey('SYSTEM\CurrentControlSet\Control\Session Manager\Environment', $true)
        }

        $Path = $Key.GetValue('Path', $null, 'DoNotExpandEnvironmentNames')

        # note that system path can only expand system environment variables and vice versa for user environment variables
        # in order to make sure this method is idempotent, we need to check if the new path already exists, this requires having a semicolon at the very end
        if ($Style -eq 'Prepend') {
            $key.SetValue('Path', (Prepend-Idempotent ("$NewPath".TrimEnd(';') + ';') ("$Path".TrimEnd(';') + ';') ';' $false), 'ExpandString')
        } elseif ($Style -eq 'Append') {
            $key.SetValue('Path', (Append-Idempotent ("$NewPath".TrimEnd(';') + ';') ("$Path".TrimEnd(';') + ';') ';' $false), 'ExpandString')
        }

        # update the path for the current process as well
        $Env:Path = $key.GetValue('Path', $null)

    } finally {
        
        $key.Dispose()
    
    }

}

Thanks for publishing this as most examples appear to ignore the fact that the tokens will be expanded and I wanted to leave them intact.

we need to check if the new path already exists, this requires having a semicolon at the very end

Can I just confirm though that the reason you're appending the ; to $NewPath is so that if $NewPath is a substring of one of the existing paths it won't accidentally match? For example if I wanted to add C:\, which is substring of most other paths in PATH, it should still be added because the -notlike "*${InputString}*" has to match the full C:\; and not simply C:\?

I was originally thinking of using a comparison like this as it feels more readable (if less performant):

if ( ($Path -split ';') -notcontains $NewPath )

but I'm wondering if there is an edge case that this misses which your approach solves?

(I've seen a few other examples which get this wrong and I wasn't sure about your version until I read it a little closer.)