How to use the command 'kexec' (with examples)

The kexec command is a powerful utility in Linux environments that enables direct rebooting into a new kernel without going through the standard bootloader process. By loading a new kernel into memory and directly transitioning control to it, kexec can significantly reduce downtime associated with reboots. This capability is particularly useful in systems requiring high availability or during kernel updates when minimal interruption is desired.

Use case 1: Load a new kernel

Code:

kexec -l path/to/kernel --initrd=path/to/initrd --command-line=arguments

Motivation:

This use case allows system administrators to load a new kernel into memory, preparing it for execution without immediately rebooting the system. This step is crucial when you need to test new kernels or patches but aren’t ready to interrupt the current system operation. Loading the kernel separately ensures that everything is set up correctly before committing to a reboot.

Explanation:

• -l path/to/kernel: The -l (or --load) option specifies the path to the new kernel that you wish to load into memory. It’s the core component of the task.
• --initrd=path/to/initrd: This option points to the initial ramdisk (initrd) associated with the kernel. The initrd contains essential drivers and scripts for initializing the kernel.
• --command-line=arguments: This part allows specifying custom command-line parameters for the new kernel, dictating its behavior on boot.

Example Output:

After executing the command, there might not be any visible output if the command completes successfully. However, you can verify the operation by checking loaded kernels via /proc/kexec or logs. New kernel environments will be ready for execution when the system reboots.

Use case 2: Load a new kernel with current boot parameters

Code:

kexec -l path/to/kernel --initrd=path/to/initrd --reuse-cmdline

Motivation:

Reusing the current boot parameters is particularly useful when you want to maintain the existing boot configuration. This approach is beneficial if the system is already tuned with specific parameters that you want to preserve for the new kernel, ensuring consistency across reboots and reducing the risk of misconfiguration.

Explanation:

• -l path/to/kernel: As before, this sets the new kernel’s memory load path.
• --initrd=path/to/initrd: Identifies the initial ramdisk for kernel initialization.
• --reuse-cmdline: This option instructs the system to use the same command-line arguments as the current running kernel. Ideal for maintaining consistency or avoiding errors in manually typed parameters.

Example Output:

Successful execution might not produce direct output, but the system is correctly set up for reboot into the new kernel with existing parameters preserved. Logs can be checked for detailed progress.

Use case 3: Execute a currently loaded kernel

Code:

kexec -e

Motivation:

This use case immediately executes the previously loaded kernel, effectively rebooting the system into it without having to pass through a bootloader. It’s advantageous when you want to minimize downtime and directly transition from an old kernel to a newly loaded one swiftly.

Explanation:

• -e: This option stands for --exec. It triggers the execution of the kernel that was loaded into memory through a previous kexec -l command. This means the system will reboot immediately using it.

Example Output:

Executing this command will result in the system shutting down processes and rebooting into the new kernel, offering a seamless transition. As this is a critical system operation, no terminal output is usually visible post-execution.

Use case 4: Unload current kexec target kernel

Code:

kexec -u

Motivation:

Unloading a previously loaded kernel is essential if you no longer intend to reboot into it, whether due to configuration changes or recognizing the wrong kernel being loaded. This helps in maintaining system stability and ensuring that the wrong configurations don’t persist.

Explanation:

• -u: This option (--unload) removes the kernel previously set for execution, cleaning up any prepared state in memory and allowing fresh configurations to be set if necessary.

Example Output:

Successfully running this command will remove the loaded kernel from memory. It helps clear any set states, ensuring the system stays ready for the next intended kernel preparation. Again, the absence of terminal output typically signifies successful unloading.

Conclusion:

The kexec command is a versatile tool for administrators, allowing efficient management of kernel transitions with minimal downtime. Through examples provided, users can approach each common use case in a syntax-friendly manner to address scenarios encountered in system administration and kernel testing.