Adding to the path environment variable

7 Oktober 2016

Today I played a bit with containers on Windows Server 2016, there is a quick start page on microsoft.com about how to install Docker with step by step instructions. One step involves adding the location of the Docker binaries to your path variable.

The suggested PowerShell code was:

[Environment]::SetEnvironmentVariable("Path", $env:Path + ";C:\Program Files\Docker", [EnvironmentVariableTarget]::Machine)

You see similar code all over the internet to do this, and even my own code I used for years used the same technique which doesn't make it any better.

It kind of works but has several problems.

I point these out using a simplified path variable value.

Consider this my original registry value under HKML\SYSTEM\CurrentControlSet\Control\Session Manager\Environment:

There is a REG_EXPAND_SZ value type with a name of Path and this value:

%SystemRoot%\system32;%ToolsFolder%

at this point the value of $env:path is:

C:\Windows\system32;D:\tools;C:\Users\name\AppData\Local\Microsoft\WindowsApps;

now run the suggest code:

[Environment]::SetEnvironmentVariable("Path", $env:Path + ";C:\Program Files\Docker", [EnvironmentVariableTarget]::Machine)

now $env:path looks like what we expect:

C:\Windows\system32;D:\tools;C:\Users\name\AppData\Local\Microsoft\WindowsApps;C:\Program Files\Docker

but lets look at the registry again:

There is now a REG_SZ value:

C:\Windows\system32;D:\tools;C:\Users\name\AppData\Local\Microsoft\WindowsApps;C:\Program Files\Docker

There are three problems here:

There are three problems with the suggested command, it uses the expanded string in $env:path, not the un-expanded one with its %variables%. $env:Path also has user locations, it merges machine and user path values. And SetEnvironmentVariable always creates a REG_SZ value regardless of the existence of a REG_EXPAND_SZ value.

So what? Is this really a problem? What if I decide to change the environment variable %ToolsFolder% from D:\tools to C:\Util. In the correct setup, everything would work and C:\util would be in the path, but now with this broken path value, none of my tools binaries are found anymore. Any other operation that expects the path value to be REG_EXPAND_SZ may break.

The problem is that using the correct way to add a new location to the path variable requires a few more lines, not just one.

I wrote a small PowerShell script to do this without any of the problems mentioned above, using it you can do:

Set-PathVariable.ps1 -NewLocation "%ProgramFiles%\Docker"

The script is available on my GitHub page.

Tags: Windows

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