A systems administrator applies a kernel update to a Linux server. After rebooting, the server fails to load the OS completely and network services do not start, preventing SSH access. The administrator has direct console access. The last messages on the console indicate a service is failing to start, which is halting the boot sequence. Which of the following boot options is the BEST first step to diagnose and potentially revert the problematic update?
Boot into safe mode.
Reload the OS from the latest backup.
Boot into single-user mode.
Use a live CD to mount the filesystem and run chroot.
The correct answer is to boot into single-user mode. In Linux, single-user mode (also known as runlevel 1 or rescue mode in systemd) starts the system with a minimal set of services and provides a root command-line shell. This mode is specifically designed for administrative tasks and troubleshooting, such as repairing a system after a failed update, as it bypasses the standard boot sequence and the services (like networking) that are causing the failure.
'Safe Mode' is the term for a similar diagnostic mode in Windows operating systems, not Linux, making it an incorrect choice.
Reloading the OS from a backup is a valid disaster recovery step, but it is too drastic for an initial troubleshooting action. An administrator should first attempt to diagnose and repair the issue before resorting to a full system restore, which can be time-consuming and may result in data loss.
Using a live CD with chroot is a more complex recovery technique. While it can be used to repair a system, it is typically reserved for situations where the system is unable to boot even to single-user mode, such as a corrupted bootloader. For the scenario described, single-user mode is the more direct and appropriate first step.