After the kernel has booted

In Chapter 4, we saw how the kernel bootstrap code looks for a root filesystem, either initramfs or a filesystem specified by root= on the kernel command line. The kernel bootstrap code then executes a program, which, by default, is /init for initramfs and /sbin/init for a regular filesystem. The init program has root privilege, and since it is the first process to run, it has a process ID (PID) of 1. If, for some reason, init cannot be started, the kernel will panic and the system will fail to boot.

The init program is the ancestor of all other processes, as shown here by the pstree command running on a simple embedded Linux system:

# pstree -gn
init(1)-+-syslogd(63)
        |-klogd(66)
        |-dropbear(99)
        `-sh(100)---pstree(109)

The job of the init program is to take control of the boot process in user space and set it running. It may be as simple as a shell command running a shell script—there is an example of this at the...