This a collection of notes and files used in my quest to create "Reboot to Windows" and "Reboot to Linux" scripts (and desktop shortcuts) for Linux and Windows respectively that automatically reboot my system and instruct rEFInd to auto-select the appropriate OS entry.
The key for achieving this is to modify the EFI Variable PreviousBoot
with GUID 36d08fa7-cf0b-42f5-8f14-68df73ed3740
, which rEFInd uses to store the last entry selected in the menu and, if using the +
default entry, will be used to select the default OS. By doing this, we trick rEFInd into booting the OS we choose without having to be physically there to press the keyboard.
This variable seems to use the following format:
- 4 bytes,
07 00 00 00
(although Windows ignores this) - The text string of the entry, in UTF-16 Little Endian (no BOM)
- 4 bytes,
20 00 00 00
(effectively: a space and a NUL character)
The variable doesn't need to contain the full text of the entry, either: Any substring will match. I don't know what rEFInd does in case of multiple matches; I believe it stops after the first. It's up to you to put everything in there or just a substring.
Linux exposes all EFI variables via efivarfs
in the directory /sys/firmware/efi/efivars/
, with file names {NAME}-{GUID}
. Specifically, the relevant variable is at /sys/firmware/efi/efivars/PreviousBoot-36d08fa7-cf0b-42f5-8f14-68df73ed3740
. These files contain the value of the variable in NVRAM and can be modified (by root
only). Most of them will have the immutable flag set, to prevent errors, so you must call chattr -i /path/to/efivar
before attempting to modify them.
This is enough to edit the default rEFInd entry: Write to the efivar file with the format specified in the previous point and the name of the entry you want selected and you're done. See the linux.refind-next-boot.py
file below for a ready to use script that will set the value to this variable to its first command line argument with the appropriate format.
If you place that script (renamed to refind-next-boot
) in your $PATH
and give it the appropriate file permissions, you can just run:
sudo refind-next-boot 'Microsoft'
systemctl reboot
Those two commands can be conviniently placed in a script or desktop launcher so that you can reboot to Windows directly. You might want to add yourself to the sudoers file so that you can run that command with no password, in wich case remember to adequately secure the script: Set root
as its owner and group and set permissions to 0755
or more restrictive.
And this is it. That was the easy part.
Ok, this is where it gets tricky. Windows has no way of giving you access to the EFI variables other than using the Windows API, specifically via GetFirmwareEnvironmentVariable
/SetFirmwareEnvironmentVariable
. These functions bot receive the name of the variable, its GUID surrounded by curly braces, a buffer to read/write from/to, respectively, and the length of the buffer or the data.
To call those two functions, the running process needs elevated privileges and a modification to the user access token, which aparently is a thing in Windows. All of this is only available via the Windows API, of course, so you'll need to write some C/C++ code.
Below is a script program that works essentially like the python script but for Windows. It needs to be compiled, which I painfully did using Visual Studio, a experience I wouldn't want to repeat. It works the same: Just call it with the name of the entry you want to boot or a substring of that set. Afterwards, you are free to power off or shut down your system using whatever method and rEFInd will just select the correct entry.
Of course, Windows being Windows, creating a desktop shortcut that has an icon and is just double-click-and-forget is a bit more tricky than the Linux equivalent. First, you'll need to place the compiled program someplace and set it to run as administrator (right click, Propertied, Compatibility, check Run as administrator). After that, in that same folder, create a .bat
file that calls our program and restarts:
refind-next-boot "linux"
shutdown -t 0 -r
Now create a shortcut to that .bat
file, place it in your desktop, give it a proper icon and name and voilà, a "Reboot to Linux" button! It will bother you with a few console windows and a UAC dialog, yes, but it's better than nothing.
I can't get this to work in Windows 10, I tried first compiling in linux with the following command.
x86_64-w64-mingw32-g++ -municode -static -o refind-next-boot.exe refind-next-boot.cpp
but I had to change main to wmainIt compiled without issue but after getting no errors when running rewind-next-boot "ubuntu" I restarted only to find out that I was back in windows like the program did nothing.
I then tried installing VS on a different machine and changing the character set to Unicode and this required no changes to the code. copy the compiled exe to the machine I was using and still nothing (restarted back into windows).
I even went a step further and made a binary copy of the efivars from linux and modified the code in windows to actully write that same binary file to the efivars using this program and even verified the value using GetFirmwareEnvironmentVariable and same thing restarted into Windows again. Right now the only way to get back to Ubuntu is to have direct access to the machine :-( sad face.
If anyone has any ideas why this won't work I'm up for suggestions at this point I've spent to much time screwing around with this. I'm giving up.