Master Linux Boot Failures: Single-User Mode, GRUB Fixes & Rescue Mode Guide

I discovered various Linux boot failures and compiled several case studies on single‑user mode, GRUB commands, and rescue mode to help readers resolve these issues.

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 systems, restoring configuration files, moving user data, and other maintenance tasks.

Case 1: Forgotten root password

In many distributions (e.g., Red Hat) single‑user mode does not require the root password, making password reset straightforward. To enter single‑user mode:

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 to set a new password, then exit to reboot.

Other typical 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., setting id:3:initdefault:).

Case 2: Disk sector errors

Bad sectors or corrupted partitions often appear after power loss. The system may stop with a prompt like “Press root password or ctrl+D”. Enter the root password to drop into single‑user mode and run: fsck -y /dev/hda6 Replace /dev/hda6 with the affected partition. After the check finishes, reboot with reboot.

Case 3: GRUB configuration typo

An “Error 15” usually means the kernel filename in grub.conf is misspelled (e.g., vmlinux instead of vmlinuz). Return to the GRUB edit screen, correct the typo, save, and boot with b. Then fix the typo permanently in grub.conf.

2. GRUB Boot Troubleshooting

Sometimes the system boots directly to a grub> prompt. The two most common causes are:

Incorrect options in grub.conf.

Missing or corrupted grub.conf (or kernel/image files).

If the configuration is wrong, you can edit it from the GRUB command line; if the file is missing, you need to use Linux rescue mode.

Key entries in grub.conf include:

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

GRUB first loads the /boot partition, then the kernel, then the initrd.

Example: Missing title line

If the title line is removed, the system drops to grub>. To recover:

Locate grub.conf with find /boot/grub/grub.conf (hd0,0).

Inspect the file using cat (hd0,0)/boot/grub/grub.conf (or a backup).

Correct the entries, then boot with b.

3. Linux Rescue Mode

If neither single‑user nor GRUB recovery works, boot from a Linux installation CD and choose “linux rescue”. Follow the prompts to select language, keyboard, and network settings (network is usually unnecessary). The installer will mount the root filesystem under /mnt/sysimage. Run chroot /mnt/sysimage to operate on the installed system.

Case 1: Restoring dual‑boot after Windows overwrites MBR

Enter rescue mode, then:

chroot /mnt/sysimage
grub-install /dev/hda   # replace hda with the appropriate disk
exit
exit   # leave chroot and rescue mode
reboot

Case 2: Recovering a lost /etc/inittab

If a backup exists, copy it back:

chroot /mnt/sysimage
cp /etc/inittab.bak /etc/inittab
exit
reboot

If no backup is available, reinstall the package that provides inittab:

rpm -qf /etc/inittab   # find the owning package
rpm -ivh --replacepkgs --root /mnt/sysimage /mnt/source/Fedora/RPMS/initscripts-8.45.3-1.i386.rpm

Alternatively, extract the file from the RPM:

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

Rescue mode is a powerful tool for fixing boot problems; understanding the Linux boot process is essential for effective troubleshooting.