讲init进程的启动过程从/etc/inittab出发。

内核启动init进程时已经打开/dev/console设备作为控制台，一般情况下Busybox init程序就使用/dev/console，

如果内核启动init进程的同时设置了环境变量CONSOLE或console，则使用环境变量所指定的设备。

1、/etc/inittab文件的相关文档和示例代码都在Busybox 的examples/inittab文件中。

vi examples/inittab

内容如下(红色字为格式重要部分)：

# /etc/inittab init(8) configuration for BusyBox
#
# Copyright (C) 1999-2004 by Erik Andersen <andersen@codepoet.org>
#
#
# Note, BusyBox init doesn't support runlevels. The runlevels field is
# completely ignored by BusyBox init. If you want runlevels, use sysvinit.
#
#
# Format for each entry: <id>:<runlevels>:<action>:<process>
#
# <id>: WARNING: This field has a non-traditional meaning for BusyBox init!
#
# The id field is used by BusyBox init to specify the controlling tty for
# the specified process to run on. The contents of this field are
# appended to "/dev/" and used as-is. There is no need for this field to
# be unique, although if it isn't you may have strange results. If this
# field is left blank, it is completely ignored. Also note that if
# BusyBox detects that a serial console is in use, then all entries
# containing non-empty id fields will be ignored. BusyBox init does
# nothing with utmp. We don't need no stinkin' utmp.
<runlevels>: The runlevels field is completely ignored.
#
# <action>: Valid actions include: sysinit, respawn, askfirst, wait, once,
# restart, ctrlaltdel, and shutdown.
#
# Note: askfirst acts just like respawn, but before running the specified
# process it displays the line "Please press Enter to activate this
# console." and then waits for the user to press enter before starting
# the specified process.
#
# Note: unrecognised actions (like initdefault) will cause init to emit
# an error message, and then go along with its business.
#
# <process>: Specifies the process to be executed and it's command line.
#
# Note: BusyBox init works just fine without an inittab. If no inittab is
# found, it has the following default behavior:
# ::sysinit:/etc/init.d/rcS
# ::askfirst:/bin/sh
# ::ctrlaltdel:/sbin/reboot
# ::shutdown:/sbin/swapoff -a
# ::shutdown:/bin/umount -a -r
# ::restart:/sbin/init
#
# if it detects that /dev/console is _not_ a serial console, it will
# also run:
# tty2::askfirst:/bin/sh
# tty3::askfirst:/bin/sh
# tty4::askfirst:/bin/sh
#
# Boot-time system configuration/initialization script.
# This is run first except when booting in single-user mode.
#
::sysinit:/etc/init.d/rcS
# /bin/sh invocations on selected ttys
#
# Note below that we prefix the shell commands with a "-" to indicate to the
# shell that it is supposed to be a login shell. Normally this is handled by
# login, but since we are bypassing login in this case, BusyBox lets you do
# this yourself...
#
# Start an "askfirst" shell on the console (whatever that may be)
::askfirst:-/bin/sh
# Start an "askfirst" shell on /dev/tty2-4
tty2::askfirst:-/bin/sh
tty3::askfirst:-/bin/sh
tty4::askfirst:-/bin/sh

# /sbin/getty invocations for selected ttys
tty4::respawn:/sbin/getty 38400 tty5
tty5::respawn:/sbin/getty 38400 tty6

# Example of how to put a getty on a serial line (for a terminal)
#::respawn:/sbin/getty -L ttyS0 9600 vt100
#::respawn:/sbin/getty -L ttyS1 9600 vt100
#
# Example how to put a getty on a modem line.
#::respawn:/sbin/getty 57600 ttyS2

# Stuff to do when restarting the init process
::restart:/sbin/init

# Stuff to do before rebooting
::ctrlaltdel:/sbin/reboot
::shutdown:/bin/umount -a -r
::shutdown:/sbin/swapoff -a
2、仿照Busybox 的examples/inittab 文件，创建一个inittab文件，内容如下：

#/etc/inittab

#init进程启动的第一个子进程，它是一个脚本，可以在里面指定用户想执行的操作

#比较挂接其他文件系统、配置网络等

::sysinit:/etc/init.d/rcS

#启动shell，以/dev/ttySAC0作为控制台

ttySAC0::askfirst:-/bin/sh

#按下Ctrl＋Alt＋Del之后执行的程序，不过在串口控制台中无法输入Ctrl＋Alt＋Del组合键

::ctrlaltdel:/sbin/reboot

#重启、关机前执行的程序

::shutdown:/bin/umount -a -r