Before the kernel runs
init, a boot loader starts the kernel. On occasion, you need to tell the boot loader to load different kernels or operating systems, and to start in different modes. This is especially important when trying to fix a system that has a problem that prevents a full boot. To fix such a problem, you may need single-user mode or an alternate kernel.
The boot loader loads a kernel image into memory and hands control of the CPU to the new image, possibly supplying it with some parameters. These parameters are simple text strings like
-s for booting in single-user mode and
partition for using
partition as the root filesystem instead of the default. You can specify a runlevel number as a parameter to make the system ...