以下摘自plka：
Linux employs a hierarchical scheme in which each process depends on a parent process. The kernel starts the init program as the first process that is responsible for further system initialization actions and display of the login prompt or (in more widespread use today) display of a graphical login interface. init is therefore the root from which all processes originate, more or less directly.
init进程是Linux运行的第一个进程，是其它所有进程的“祖先”。
sinit展示了一个最简单的init进程实现：初始化服务和等待清理子进程。