使用linux的目的是為了啟動應用程序
init進程：

讀取配置文件

解析配置文件

根據配置文件執行用戶的程序

busybox --> init_main
parse_inittab
file = open(INITTAB, “r”) ; //打開配置文件　　/etc/inittab

inittab格式：
:::
id => /dev/id , 用作終端：　stdin, stdout, stderr: printf scanf ,…

inittab文件如下：

# /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> # inittab的格式： <id>:<runlevels>:<action>:<process><id>用法說明的地方 # <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>可以完全被忽略 # <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

總結

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