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Introduction

Booting an instance in runlevel 1, also known as single-user mode, is a troubleshooting technique used in Unix-based systems, including Linux. This mode starts the system with minimal services, providing a controlled environment for diagnosing and fixing problems such as corrupted configuration files, user permissions issues, or failed updates. Runlevel 1 grants root access, so it is often used to reset forgotten passwords or resolve disk-related issues.

Understanding how to boot into this mode is critical for system administrators who need to maintain uptime and recover systems effectively. This guide explains the process for booting an instance into single-user mode, discusses its applications, and provides step-by-step instructions for various bootloader configurations.

Prerequisites

• Access to the server console (physical or virtual, such as VNC or cloud provider console).
• Administrative privileges to modify boot parameters.
• Familiarity with the GRUB bootloader or your system’s bootloader configuration.
• Backup of critical data, as changes made in single-user mode may affect the system.

Step-by-step Guide

Step 1: Access the Bootloader Menu

1. Reboot the system:
   If the instance is running, reboot it to access the bootloader menu. This can be done using the following command:

   sudo reboot

2. Interrupt the boot process:
   During startup, interrupt the bootloader menu by pressing the designated key. For GRUB, this is typically the Esc, Shift, or F2 key, depending on the system’s configuration.

Step 2: Edit Bootloader Parameters

1. Select the desired kernel:
   Use the arrow keys to highlight the kernel entry you wish to modify.
2. Enter edit mode:
   Press the e key to edit the boot parameters for the selected kernel.
3. Modify the kernel command line:
   Locate the line starting with linux or linux16 and append the following:

   single

   Alternatively, use:

   systemd.unit=rescue.target

Step 3: Boot into Single-User Mode

1. Boot with the modified parameters:
   Press Ctrl+X or F10 to boot the system with the modified parameters.
2. Authenticate if required:
   Depending on the system’s configuration, you may be prompted for the root password.

Step 4: Perform Maintenance Tasks

Once in single-user mode, perform the required maintenance tasks, such as:

• Resetting passwords.
• Repairing disk errors using fsck.
• Editing configuration files.

Step 5: Reboot into Normal Mode

1. Exit single-user mode:
   After completing your tasks, reboot the system to return to normal multi-user mode:

   reboot

2. Verify system functionality:
   Ensure that all services and applications are functioning correctly after the reboot.

Conclusion

Booting into runlevel 1 or single-user mode is an essential skill for resolving system-level issues. By following the steps outlined in this guide, you can effectively access and troubleshoot your instance with minimal services running. This approach is particularly useful for diagnosing boot issues, repairing corrupted configurations, and resetting passwords.

Should you have any inquiries about the guidelines, please feel free to open a ticket through your portal account or contact us at support@ipserverone.com. We’ll be happy to assist you further.