Boot and Recovery
How a Linux host goes from firmware to a running system, and how to recover when it does not: the boot chain, GRUB2, rescue and password reset, kernel panics, and kernel updates. The theme is localising a failure to one stage and fixing that stage.
Revision Card
| Fact | Value |
| Boot chain | firmware → GRUB2 → kernel + initramfs → switch_root → systemd → target |
| Kernel args | /proc/cmdline shows what the running kernel got |
| grub.cfg | Generated; edit /etc/default/grub, then grub2-mkconfig / update-grub |
| Default target | systemctl get-default; set with set-default |
| Boot timing | systemd-analyze, blame, critical-chain |
| Rescue vs emergency | Rescue mounts local FS and prompts root; emergency is barer |
| Reset root password | GRUB rd.break → remount /sysroot rw → passwd → .autorelabel |
| Bad fstab | Boots to emergency; fix, then mount -a before reboot |
| Failed unit | systemctl --failed, status, journalctl -b -u |
| Oops vs panic | Oops logs and may continue; panic halts the kernel |
| Panic sysctls | kernel.panic (reboot delay), kernel.panic_on_oops |
| Magic SysRq | R E I S U B for a hung box, if enabled beforehand |
| Kernel patch | Installed is not active until reboot; check uname -r |
| Rollback | Boot the previous kernel from the GRUB menu |
| Task | Command |
| Kernel command line | cat /proc/cmdline |
| What made boot slow | systemd-analyze critical-chain |
| Boot without GUI | sudo systemctl set-default multi-user.target |
| Boot into rescue | append systemd.unit=rescue.target at GRUB |
| Reset root password | rd.break, then remount, chroot, passwd, .autorelabel |
| Validate fstab | sudo mount -a |
| List failed units | systemctl --failed |
| Panic auto-reboot | kernel.panic=30 in /etc/sysctl.d |
| Reboot required | needs-restarting -r |
| Rebuild initramfs | sudo dracut -f / sudo update-initramfs -u |
Topic Map
| File | Covers | Track | Weight |
| Boot Process | Firmware, GRUB2, kernel, initramfs, systemd targets, boot timing | Core | High |
| GRUB2 | Generated config, kernel parameters, default kernel, menu password | RHCSA | Med |
| Recovery | Rescue and emergency, root password reset, bad fstab, failed units | RHCSA | Med |
| Kernel Panic | Oops versus panic, triggers, SysRq, kdump and netconsole | Advanced | Med |
| Kernel Updates | Installed versus running, rollback, initramfs, reboot check | Core | Low |
Scenarios and Labs