The grub-reboot command is very handy, but has a fatal flaw on config files with submenus. It requires the user to parse grub.cfg to figure out non-obvious MENU_ENTRY numbers separated by greater than signs. It would be nice if it showed a list and asked the user which boot entry is desired.
I've included below a patched version of grub-reboot which does exactly that. It is imperfect, but I believe it shows what could be done without too much more work. The main flaw is a lack of a decent parser for the config file (an awk kludge is included instead). A secondary flaw is that it uses Bash's mapfile and select commands. For maximum portability, it may make sense to try to reimplement these as Bourne shell.
—b9
$ grub-reboot -h
Usage: grub-reboot [OPTION] [MENU_ENTRY]
Set the default boot menu entry for GRUB, for the next boot only.
-h, --help print this message and exit
-V, --version print the version information and exit
-s, --select interactively select entry to boot (DEFAULT)
--boot-directory=DIR expect GRUB images under the directory DIR/grub
instead of the /boot/grub directory
MENU_ENTRY is a number, a menu item title or a menu item identifier.
Please note that menu items in submenus or sub-submenus require
specifying the submenu components and then the menu item component.
The titles should be separated using the greater-than character (>)
with no extra spaces. Depending on your shell some characters
including > may need escaping. More information about this is available
in the GRUB Manual in the section about the 'default' command.
Report bugs to <address@hidden>. $ grub-reboot
Please pick an OS to reboot into (for this next boot only). ^C to cancel.
1) 0>0 Debian GNU/Linux
2) 1 submenu 'Advanced options for Debian GNU/Linux'
3) 1>0 Debian GNU/Linux, with Linux 4.19.0-6-amd64
4) 1>1 Debian GNU/Linux, with Linux 4.19.0-6-amd64 (recovery mode)
5) 1>2 Debian GNU/Linux, with Linux 4.9.0-11-amd64
6) 1>3 Debian GNU/Linux, with Linux 4.9.0-11-amd64 (recovery mode)
7) 1>4 Debian GNU/Linux, with Linux 4.9.0-3-amd64
8) 1>5 Debian GNU/Linux, with Linux 4.9.0-3-amd64 (recovery mode)
9) None of the above.
#? 4.9
Next reboot will be into `Debian GNU/Linux, with Linux 4.9.0-11-amd64'
grub-reboot