Implemented in usr/bin/init.c. First user space program to run.
First it opens the console character device as /dev/console so that file descriptor 0,1 and 2 are set to console. These file descriptors are defined to map to stdin, stdout and stderr in C (see stdio).
Then it forks, the parent will wait on the child to close and fork again if that happens. The child will execv into the shell.
init is not expected to return
Init starts a shell and restarts it in case it closes. The shell uses fork and execv to start user applications. For details see sh and from the kernel perspective: life_cycle_user_application.
Note: If the file /tests/autoexec.sh exists, the shell will be started with this file as a parameter. This script will be executed in a loop so it should call shutdown for automated testing.
Up: user space