LPIC-2 201-450: System Startup

System Startup covers the path from firmware through the boot loader and kernel to PID 1, plus the targets and recovery modes an administrator uses when normal boot fails.

Boot loader and targets

• Firmware loads the boot loader, which loads kernel and initramfs; the kernel then starts init or systemd.
• GRUB 2 configuration is generated: /boot/grub/grub.cfg on Debian, /boot/grub2/grub.cfg on RHEL. Edit /etc/default/grub and /etc/grub.d/, then run update-grub or grub2-mkconfig -o /boot/grub2/grub.cfg.
• GRUB_CMDLINE_LINUX adds kernel parameters; GRUB_DEFAULT selects the default entry; grub-install writes the boot loader to disk.
• systemd targets replace runlevels: multi-user.target, graphical.target, rescue.target, emergency.target. Use systemctl get-default and set-default.
• systemd.unit=rescue.target, init=/bin/bash, and RHEL's rd.break are recovery paths with different mount states.

Worked example

Make a serial console permanent: add console=ttyS0,115200 to GRUB_CMDLINE_LINUX in /etc/default/grub, run grub2-mkconfig -o /boot/grub2/grub.cfg, then reboot. Reasoning: the generated grub.cfg is recreated from /etc/default/grub and /etc/grub.d; editing grub.cfg directly survives only until the next regeneration. Confirm the running command line in /proc/cmdline.

Traps

• Editing grub.cfg instead of the source files.
• Forgetting grub-install after replacing a disk.
• Confusing rescue (root password, local filesystems mounted) with emergency (read-only root, minimal services).
• Expecting /etc/inittab to control systemd.