Linux Recovery

Well, this is just a note to myself rather than a bunch of weird links.  So...if you do "prime-select intel" on a hybrid graphics system and everything freezes when the boot sequence gets to X:

1. Hard reboot and wait for the BIOS screen to pass.
2. Hit Esc to bring up the Grub bootloader menu (holding down Shift didn't work on my system).
3. Type 'e' on the first entry to edit the boot commands.
4. On the line with "linux", append " 3" (space, 3).  This will restrict the runlevel to 3, which is multi-user mode.  Alternatively, one could append " single", but single-user mode is perhaps less useful because you can't start X from there, nor do you have network access.
5. Hit F10 to boot with those options.
6. Log in and do your thing.  In multi-user console mode, you can hit Ctrl-Alt-1 through 6 to get tty1-6.  Ctrl-Alt-7 is X, usually.  If you have something to try, like fix the graphics selection: "sudo prime-select nvidia ; systemctl stop lightdm ; systemctl start lightdm".

Of course, that may not completely fix things either, but getting access to the system is better than being locked out.