After modifying GNU GRUB's configuration file, which command must be run for the changes to take effect?

`kill -HUP $(pidof grub)` is irrelevant because GRUB is a bootloader that executes before the operating system and is not a running process to be signaled within the OS.

`grub-install` is used to install GRUB to the Master Boot Record (MBR) or UEFI partition, not to apply configuration changes.

The command `grub` by itself typically enters a GRUB shell for interactive configuration, but does not apply changes from a configuration file to the bootloader without further actions.

When the GRUB configuration file, typically `/boot/grub/grub.cfg` (generated from `/etc/default/grub` and scripts in `/etc/grub.d/`), is directly modified, GRUB reads this file each time the system boots. Therefore, no additional command is needed for the changes to take effect on the next reboot.