How to Fix Linux Boot Failures: Single‑User Mode, GRUB & Rescue Techniques

1. Single‑User Mode

Linux provides a single‑user mode (runlevel 1), similar to Windows safe mode, where the system boots into a root shell with networking disabled and only essential processes running. It is useful for repairing file system damage, restoring configuration files, or moving user data.

Case 1: Forgotten root password

In single‑user mode most Linux distributions do not require the root password (Fedora Core 6 example). To reset the password:

During boot, press any key to access the GRUB menu.

Press e to edit the selected entry and move to the kernel line.

Append single to the end of the line, press Enter, then b to boot.

When the root shell appears, run passwd root, then exit to reboot.

Other common fixes in single‑user mode include disabling problematic services (e.g., chkconfig smb off) and changing the default runlevel by editing /etc/inittab (e.g., set id:3:initdefault:).

Case 2: Disk sector errors

Bad sectors or corrupted partitions often appear after an improper shutdown. The system may stop at a prompt like “Press root password or ctrl+D”. After entering the root password, run: fsck -y /dev/hda6 Replace /dev/hda6 with the appropriate partition, then reboot with reboot.

Case 3: Incorrect GRUB configuration

An “Error 15” indicates that GRUB cannot find the kernel specified in grub.conf, often due to a typo such as “vmlinux” instead of “vmlinuz”. Edit the entry in GRUB, correct the typo, save, and boot.

2. GRUB Boot Troubleshooting

When the system drops to a grub> prompt, the most common causes are:

Incorrect options in grub.conf.

Missing or corrupted grub.conf (or other boot files).

If the configuration file is wrong, you can boot manually from the GRUB prompt, then fix grub.conf from within the running system. The typical grub.conf layout is:

title Fedora Core (2.6.18-1.2798.fc6)
root (hd0,0)
kernel /boot/vmlinuz-2.6.18-1.2798.fc6 ro root=LABEL=/ rhgb quiet
initrd /boot/initrd-2.6.18-1.2798.fc6.img

The boot sequence loads the /boot partition, then the kernel, and finally the initrd image.

Example: Missing title line

If the title line is missing, the system stops at the grub> prompt. Use the following commands to locate and inspect grub.conf:

GRUB> find /boot/grub/grub.conf (hd0,0)
GRUB> cat (hd0,0)/boot/grub/grub.conf
GRUB> cat (hd0,0)/boot/grub/grub.conf.bak   # if a backup exists

After correcting the file, reboot.

3. Linux Rescue Mode

If neither single‑user mode nor GRUB commands can restore the system, boot from a Linux installation CD into rescue mode:

Boot from the CD, at the boot: prompt type linux rescue.

Follow the prompts to select language, keyboard, and optionally network settings.

The installer mounts the root partition under /mnt/sysimage. Choose “continue” for read‑write access.

Run chroot /mnt/sysimage to work inside the installed system.

Case A: Restoring a dual‑boot GRUB

After reinstalling Windows, the MBR may be overwritten. From rescue mode run:

chroot /mnt/sysimage
grub-install /dev/hda   # adjust device name as needed
exit
exit

The system will boot both Linux and Windows again.

Case B: Recovering a lost /etc/inittab

If /etc/inittab is missing, restore it from a backup or reinstall the package that provides it. Example using RPM:

chroot /mnt/sysimage
rpm -ivh --replacepkgs --root /mnt/sysimage /mnt/source/Fedora/RPMS/initscripts-8.45.3-1.i386.rpm

Or extract the file directly:

rpm2cpio /mnt/source/Fedora/RPMS/initscripts-8.45.3-1.i386.rpm | cpio -idv ./etc/inittab
cp etc/inittab /mnt/sysimage/etc

After restoring the file, exit the chroot and reboot.

Source: csghjnbj, blog.chinaunix.net/uid-26567710-id-3050269.html