Accessing existing Windows environment variables from WSL - bash

Question

I would like to access existing Windows environment variables such as USERPROFILE from withing the WSL bash prompt. There is information from Microsoft on the use of WSLENV here, and I have tried to work with this:
I added WSLENV as a new System variable within the usual Windows "Environment Variables" control panel, setting it to USERPROFILE/u. I then open Ubuntu from the taskbar, and type:
$ echo $USERPROFILE
...but nothing is returned.

Answer 1

Improved Gábor's answer, since as I've discovered it had a small bug, variables obtained that way contain invisible carriage return character, that could cause unexpected issues latter.
Here is Example:
$ cd /mnt/c/
$ mkdir Windows_NT
$ tmpvar=`/mnt/c/Windows/System32/cmd.exe /C "echo %OS%"`
$ echo $tmpvar
Windows_NT
All seems to be ok, but no:
$ cd $tmpvar
: No such file or directory
This is because tmpvar variable contains additional carriage return character (aka ^M or \r) at the end. We can check this via ls command:
$ ls -ld $tmpvar
ls: cannot access 'Windows_NT'$'\r': No such file or directory
In order to remove that character, output could be be additionally processed with sed or tr:
tmpvar=$(cmd.exe /C echo %OS%|sed $'s/\r$//')
or
tmpvar=$(cmd.exe /C echo %OS%|tr -d '\r')
I've also simplified command a bit. Path /mnt/c/Windows/System32 is already included in $PATH WSL variable by default in recent Windows 10 updates, so just cmd.exe should work.
Now, ls and cd commands work without errors:
$ ls -ld $tmpvar
drwxrwxrwx 1 ubuntu ubuntu 512 Feb 12 05:38 Windows_NT
$ cd $tmpvar
$ pwd
/mnt/c/Windows_NT
pwd command confirms that current directory is correct.

Answer 2

Got a workaround for ya.
$ /mnt/c/Windows/System32/cmd.exe /C "echo %OS%"
Windows_NT
$ tmpvar=`/mnt/c/Windows/System32/cmd.exe /C "echo %OS%"`
$ echo $tmpvar
Windows_NT
I haven't checked WSLENV but the upper one should work. Far from being elegant though.

Answer 3

As this question is from early 2018, my first thought is that the Windows 10 build #user2023370 used was not yet 17063.
Other things to point out:
When setting environment variables from the control panel, make sure that the WSL process is not already running.
Because the USERPROFILE environment variable is a path, you need to add the /p option:
WSLENV=USERPROFILE/up
To test how /p option affects the passed variable, try this in cmd.exe:
Microsoft Windows [Version 10.0.19044.1706]
(c) Microsoft Corporation. All rights reserved.
C:\Users\arttu>set WSLENV=USERPROFILE/up
C:\Users\arttu>wsl
$ echo $USERPROFILE
/mnt/c/Users/arttu
$ exit
logout
C:\Users\arttu>set WSLENV=USERPROFILE/u
C:\Users\arttu>wsl
$ echo $USERPROFILE
C:\Users\arttu