Entering Single User Mode in AlmaLinux 8 and 9: Steps and Use Cases

Linux
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Single user mode, also known as rescue mode, is a crucial tool for system administrators using AlmaLinux 8 and 9. This minimal Linux environment allows for critical maintenance tasks, troubleshooting, and system recovery when normal boot processes fail. In this guide, we’ll explore how to enter single user mode, its uses, and best practices for system maintenance.

What is Single User Mode?

Single user mode is a special boot state in Linux distributions like AlmaLinux that provides a minimal environment with the following characteristics:

  • Only the root user has access
  • Networking services are disabled
  • Only essential system processes are started
  • The system runs in command-line mode without a graphical interface

This mode is ideal for:

  • Resetting forgotten root passwords
  • Repairing corrupted filesystems
  • Fixing misconfigurations that prevent normal system boot
  • Performing low-level system maintenance and troubleshooting

Entering Single User Mode

To boot into single user mode, follow these steps:

Step 1: Restart your AlmaLinux system

Step 2: Access the GRUB boot menu by pressing the Esc key (or Shift on some systems) as soon as the system begins to reboot

Step 3: Use the arrow keys to select the kernel version you want to modify

Step 4: Press e to edit the boot configuration

Step 5: Find the line starting with linux or linuxefi (for UEFI systems)

Step 6: Go to the end of this line and add: systemd.unit=rescue.target

Step 7: Press Ctrl + X or F10 to boot with the modified settings

Performing Maintenance Tasks

Once in single user mode, you can perform various maintenance tasks:

Resetting the Root Password

If you’ve forgotten the root password, use the following commands:

passwd root
touch /.autorelabel

The touch /.autorelabel command updates the SELinux policy after changing the password.

Checking and Repairing Filesystems

To check and fix errors on the root partition:

fsck -y /dev/mapper/almalinux-root

Editing Configuration Files

Edit important configuration files as needed:

vi /etc/fstab

Make necessary corrections and save the file.

Emergency Mode: An Alternative to Single User Mode

For situations where single user mode is insufficient, AlmaLinux offers emergency mode. To boot into emergency mode, use this parameter instead:

systemd.unit=emergency.target

Emergency mode provides an even more minimal environment, useful for deep system troubleshooting.

Example: Fixing a Corrupted /etc/fstab File

A common use case for single user or emergency mode is fixing a corrupted /etc/fstab file. If you’ve added an incorrect entry like:

/dev/sdb1 /mnt/data ext4 defaults 0 0

And the partition /dev/sdb1 doesn’t exist, the system may fail to boot normally. In this case:

  1. Boot into emergency mode
  2. Remount the root filesystem as read-write:
    mount -o remount,rw /
  3. Edit the /etc/fstab file and remove or correct the problematic entry
  4. Reboot the system

Differences Between AlmaLinux 8 and 9

While the process for entering single user mode is similar in AlmaLinux 8 and 9, there are some differences to be aware of:

  • AlmaLinux 9 uses GRUB2 with BLS (BootLoader Specification) by default, which can affect how kernel parameters are managed
  • The location of some configuration files may differ between versions
  • AlmaLinux 9 may have additional security features that could impact the single user mode process

Always consult the official AlmaLinux documentation for version-specific details.

Single user mode in AlmaLinux 8 and 9 is a powerful tool for system administrators to perform critical maintenance and troubleshooting tasks. By following these steps and best practices, you can safely navigate system issues and ensure the stability of your AlmaLinux installations.