You want every future boot of a GRUB 2 system to pass an extra kernel parameter, and you want the change to survive the next kernel update. Which file do you edit before regenerating the configuration?

LPIC-1 Exam 101-500, objective 102. Linux installation and package management medium

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The options

Not correct /boot/grub/menu.lst, adding the parameter to the kernel line

Wrong for GRUB 2. menu.lst is the GRUB Legacy configuration file and GRUB 2 does not read it; on a GRUB 2 system the file usually does not exist at all.

Not correct /boot/grub/grub.cfg, editing each linux line directly

Wrong. The edit works until the next regeneration, which any kernel package update triggers, and then it is silently overwritten.

Not correct /etc/fstab, adding the parameter to the root filesystem line

Wrong. /etc/fstab lists filesystems to mount and their mount options; it is read by userspace long after the kernel has started and cannot pass kernel parameters.

Correct /etc/default/grub, then regenerate grub.cfg

Correct. Adding the parameter to GRUB_CMDLINE_LINUX (or GRUB_CMDLINE_LINUX_DEFAULT) there means every generated menu entry, including entries for kernels installed later, carries it.

Why

Anything you want to persist in GRUB 2 belongs in an input file — /etc/default/grub for global settings, /etc/grub.d/ scripts for custom entries — followed by grub-mkconfig (update-grub on Debian, grub2-mkconfig on Red Hat family). Editing the generated grub.cfg is only appropriate as a temporary fix, and the interactive 'e' editor at the menu is the right tool for a genuinely one-off boot.

Where this comes from

Cited
LPI exam objective 102.2
What it says
Configure the boot loader, including kernel options passed at boot.

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