Editing GNU GRUB (section)

==== After startup ====
GRUB presents a menu where the user can choose from operating systems (OS) found by grub-install. GRUB can be configured to automatically load a specified OS after a user-defined timeout. If the timeout is set to zero seconds, pressing and holding {{Keypress|Shift}}, or in some modern GRUB versions loaded using UEFI, pressing {{Keypress|Esc}} rapidly while the computer is booting makes it possible to access the boot menu.<ref name="howtogeek.com">{{cite web|url=http://www.howtogeek.com/196655/how-to-configure-the-grub2-boot-loaders-settings/|title=How to Configure the GRUB2 Boot Loader's Settings|website=HowToGeek.com|first=Chris|last=Hoffman|date=September 22, 2014|access-date=April 25, 2015|archive-date=April 28, 2015|archive-url=https://web.archive.org/web/20150428233135/http://www.howtogeek.com/196655/how-to-configure-the-grub2-boot-loaders-settings/|url-status=live}}</ref>

In the operating system selection menu GRUB accepts a couple of commands:
* By pressing {{key press|e}}, it is possible to edit kernel parameters of the selected menu item ''before the operating system is started''. The reason for doing this in GRUB (i.e. not editing the parameters in an already booted system) can be an emergency case: the system has failed to boot. Using the kernel parameters line it is possible, among other things, to specify a module to be disabled (blacklisted) for the kernel. This could be required if the specific kernel module is broken and thus prevents boot-up. For example, to blacklist the kernel module <code>nvidia-current</code>, one could append <code>modprobe.blacklist=nvidia-current</code> at the end of the kernel parameters.
* By pressing {{key press|c}}, the user enters the GRUB command line. The GRUB command line resembles GNU Bash, but only implements a subset of the line editing functions and GRUB-specific commands.<ref>{{cite web |url=https://www.gnu.org/software/grub/grub-documentation.html |title=GNU GRUB documentation |access-date=April 16, 2014 |archive-date=April 18, 2014 |archive-url=https://web.archive.org/web/20140418191412/http://www.gnu.org/software/grub/grub-documentation.html |url-status=live }}</ref>

Once boot options have been selected, GRUB loads the selected kernel into memory and passes control to the kernel. Alternatively, GRUB can pass control of the boot process to another boot loader, using ''[[chain loading]]''. This is the method used to load operating systems that do not support the [[Multiboot Specification]] or are not supported directly by GRUB.