How to get the value of the PATH environment variable without expanding tokens?

Question

I'm writing a PowerShell script to convert folder names to short names in the PATH environment variable and save the changes. It works correctly for explicit paths, but it expands tokens, so when I save my changes the tokens are replaced with explicit paths. I'd like to preserve the tokens.

For example, if my PATH is this: %SystemRoot%;%SystemRoot%\system32;C:\Program Files\TortoiseSVN\bin;C:\ProgramData\chocolatey\bin

I want this result: %SystemRoot%;%SystemRoot%\system32;C:\PROGRA~1\TORTOI~1\bin;C:\PROGRA~3\CHOCOL~1\bin

But instead I get this: C:\Windows;C:\Windows\system32;C:\PROGRA~1\TORTOI~1\bin;C:\PROGRA~3\CHOCOL~1\bin

Here's the full script that illustrates the problem.

# get the current path.
# I ended up not using either of these approaches because they appear to
# be contextual; on my system, at least, they include paths that aren't
# seen when I view the PATH value from the System Properties -> Environment
# Variables dialog box.
$current_path = $env:Path
$current_path = [System.Environment]::GetEnvironmentVariable("Path")

# Instead, I get the PATH value directly from the registry, like so:
$current_path = (Get-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\Session Manager\Environment' -Name Path).Path

# The problem is that PowerShell expands the tokens even when reading from the 
# registry, so I can't detect the tokens and ignore them.  What I really want
# is just the literal string value, with no token evaluation.
# Here's a sample value; this is what I see in regedt32 and the system dialogs,
# but it's not what I get from the line above.
$current_path = '%SystemRoot%;%SystemRoot%\system32;C:\Program Files\TortoiseSVN\bin;C:\ProgramData\chocolatey\bin'

$new_path = ''

# the FileSystemObject has a method to convert the path using short names for the folders.
$fso = New-Object -ComObject Scripting.FileSystemObject

# Individual paths are delimited by a semicolon.  Skip paths with tokens, 
# and preserve trailing slashes in each path.
$current_path.Split(';') |
    ForEach-Object { 
        if ($_.StartsWith('%'))
            { $_ }
        elseif ($_.EndsWith('\'))
            { "$($fso.GetFolder($_).ShortPath)\" }
        else
            { "$($fso.GetFolder($_).ShortPath)" } 
    } |
    ForEach-Object { $new_path += "$_;" }

# remove the extra semicolon from the end the new PATH.
$new_path = $new_path.TrimEnd(";")

"Current PATH length: $($current_path.Length)"
"New PATH length: $($new_path.Length)"

$new_path

# commented out so you don't accidentally update your path if you try out this script
#[System.Environment]::SetEnvironmentVariable("Path", $new_path, "Machine")

It seems like it should be easy enough if I could just get the literal string value from the registry, but I so far have not been able to figure out how to do that.

Mike Zboray · Accepted Answer · 2015-07-21 21:54:46Z

14

This isn't PowerShell per se, but a "feature" of the underlying Windows registry. The Path variable is of type REG_EXPAND_SZ which automatically expands environment variables on retrieval. I don't think you can get around it with the built-in cmdlets, but you can with the .NET Microsoft.Win32.Registry APIs. Use the RegistryKey.GetValue overload with RegistryValueOptions.DoNotExpandEnvironmentNames:

$regKey = [Microsoft.Win32.Registry]::LocalMachine.OpenSubKey('SYSTEM\CurrentControlSet\Control\Session Manager\Environment', $true)
$regKey.GetValue('Path', $null, "DoNotExpandEnvironmentNames")

when it is time to save the variable, use the overload of SetValue with RegistryValueKind.ExpandString to save it with the correct type:

$regKey.SetValue("Path", $new_path, "ExpandString")

edited Jul 21, 2015 at 21:54

answered Jul 21, 2015 at 19:36

Mike Zboray

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3 Comments

Matt Over a year ago

I had to add a parameter to the call to OpenSubkey(), like so OpenSubKey('SYSTEM\CurrentControlSet\Control\Session Manager\Environment', 'ReadWriteSubTree'), before it would let me write back to the registry using SetValue(), but otherwise that was exactly what I was looking for. Many thanks!

Mike Zboray Over a year ago

@Matt Oh yea the registry keys are not writable by default. You need to call that overload or the one that takes a bool (as in my edit).

CMCDragonkai Over a year ago

Used your answer to create a powershell script that automated idempotent addition of unexpanded paths: gist.github.com/CMCDragonkai/a02d77c2d7c0799dd42fd2aab26a3cd5

mklement0 · Accepted Answer · 2018-10-24 21:17:31Z

8

Without using the .NET [Microsoft.Win32.Registry] type: one-liners for user and system Path entries.

(Get-Item -path "HKCU:\Environment" ).GetValue('Path', '', 'DoNotExpandEnvironmentNames')


(Get-Item -Path "HKLM:\SYSTEM\CurrentControlSet\Control\Session Manager\Environment" ).
  GetValue('Path', '', 'DoNotExpandEnvironmentNames')

edited Oct 24, 2018 at 21:17

mklement0

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answered Oct 3, 2018 at 7:09

Christopher Plewright

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1 Comment

Ed Haack Over a year ago

After all this time... this continues to be the best answer. Surprising that the helper function built into posh doesn't allow something as basic as whether or not to mess with the existing values

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