path_variables.md

Windows PATH variables

• There are three different sets of environment variables
  • Process (temporary)
  • User (permanent)
  • Machine (aka. system, permanent)
• Windows Path is an Windows environment variable with the name Path.

How the PATH variables are inherited & copied

    User PATH      Machine/System PATH 
            \                  / 
             \                / 
              v              v  
    +-----------------------------------------------------+
    | MainProcess                                         |
    |                                                     |
    |  PATH = Process PATH                                |
    |    (initially copy of User and Machine PATH)        |
    |                                                     |
    +-----------------------------------------------------+
                        |                                  
                        |                                  
                        v                                  
    +-----------------------------------------------------+
    | ChildProcess                                        |
    |                                                     |
    |   PATH = Process PATH                               |
    |     (initially copy of MainProcess PATH)            |
    +-----------------------------------------------------+

Thanks for the diagram tool, textik.com

1) Process environment variables

• These are used for setting temporary environment variables. They are destroyed when the process terminates.
• When process starts, it gets either (a) Copy of the Process PATH from its parent or (b) Copy of union of User and System PATH.

1.1) Getting Process environment variables

• In Python, just read os.environ['PATH'].
• This is the same as reading value of $Env:Path (powershell) or PATH (cmd).

1.2) Setting Process environment variables

• In Python, just edit os.environ['PATH'].

2) User environment variables

• These values are permanent.
• When process starts, the Process PATH receives all items from User PATH.

2.1) Getting User environment variables

When [Environment]::GetEnvironmentVariable(...) is called with EnvironmentVariableTarget.User as the target, then (source)

• Variable is retrieved from the HKEY_CURRENT_USER\Environment key of the local computer's registry

2.2) Setting User environment variables

When [Environment]::SetEnvironmentVariable(...) is called with EnvironmentVariableTarget.User as the target, then

• Variable is stored in the HKEY_CURRENT_USER\Environment key of the local computer's registry. This means that the environment variable is stored permanently. (source)
• Other applications are notified of the set operation by a Windows WM_SETTINGCHANGE message. (source)
• Variable is copied to instances of File Explorer that are running as the current user
• Environment variable is then inherited by any new processes that the user launches from File Explorer.
• Microsoft recommends that the length is kept under 2048 characters. The hard limit is 32766 characters. (source)
• Note: Setting User environment variable does not necessarily update the Process environment variables.

3) System/Machine environment variables

• These values are permanent.
• When process starts, the Process PATH receives all items from System/Machine PATH.
• Need admin rights to edit these.

3.1) Getting System/Machine environment variables

When [Environment]::GetEnvironmentVariable(...) is called with EnvironmentVariableTarget.Machine as the target, then

• Variable is retrieved from the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment[*1] key of the local computer's registry.

3.2) Setting System/Machine environment variables

When [Environment]::SetEnvironmentVariable(...) is called with EnvironmentVariableTarget.Machine as the target, then

• Variable is stored to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment[*1] key of the local computer's registry. This means that the environment variable is stored permanently. (source)
• Other applications are notified of the set operation by a Windows WM_SETTINGCHANGE message. (source)
• Variable is copied to instances of File Explorer that are running as the current user
• Environment variable is then inherited by any new processes that the user launches from File Explorer.
• Microsoft recommends that the length is kept under 2048 characters. The hard limit is 32766 characters. (source)
• Note: Setting Sytem/Machine environment variable does not necessarily update the Process environment variables.

footnotes

[*1] HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment according to this source, HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Session Manager\Environment according to this source. These might be the same thing.