kexec

Additional Notes

kexec loads a new kernel into memory and boots it directly without going through the full firmware or bootloader cycle. This significantly reduces reboot time because the hardware initialization phase is skipped.

It is commonly used for fast reboots during kernel development, testing new kernels without full reboots, and setting up crash dump kernels (kdump) for capturing kernel panic information. The kexec system call is the underlying mechanism that tools like systemd-kexec use for fast reboots.

Syntax

kexec [options] [kernel-image]

Parameters

• kernel-image: The path to the kernel binary to load (e.g., /boot/vmlinuz-linux).

Common Options

• -l kernel, --load kernel: Load the specified kernel for later execution.
• -e, --exec: Immediately execute the currently loaded kernel.
• -u, --unload: Unload the currently loaded kernel image.
• -p, --load-panic: Load a kernel for use as a crash dump kernel (kdump).
• --initrd initrd: Specify the initramfs image to use.
• --command-line=string: Set the kernel command line.
• --reuse-cmdline: Reuse the command line from the running kernel.
• -t type, --type type: Specify the kernel type (e.g., bzImage).

Examples

kexec -l /boot/vmlinuz-linux --initrd=/boot/initramfs-linux.img --reuse-cmdline
kexec -e

Load a new kernel and immediately boot into it.

kexec -l /boot/vmlinuz-linux --initrd=/boot/initramfs-linux.img --command-line="root=/dev/sda1 ro quiet"
kexec -e

Load a kernel with a custom command line.

kexec -p /boot/vmlinuz-linux --initrd=/boot/initramfs-linux-fallback.img --reuse-cmdline

Load a panic/crash dump kernel for kdump.

kexec -u

Unload the currently loaded kexec kernel.

Practical Notes

• Running kexec -e immediately boots the new kernel; save your work first.
• On UEFI systems, kexec may behave differently or require efi-boot support.
• For crash dumping, ensure the crash kernel is configured with the crashkernel= kernel parameter.
• systemctl kexec uses kexec under the hood and handles service shutdown gracefully.