看懂android下init.rc文件

init.rc相当于是init程序的配置文件。如何看懂它对于了解Android挺有帮助的。
文章的末尾是摘自init的readme原文
不翻译了，英语太蹩脚，翻了估计更看不懂。
以下是自己总结的几个，方便查看而已。

Actions由一系列commands组成，不过有一定的trigger条件，只要条件满足，将执行Actions下面的所有commands。它的形式是：

on

Services由Init程序启动。它后面一般都跟着好多options。它的形式是：

service   [  ]*


      ...

而option又有以下几种：
critical:有该option的service，表示当service挂掉的时候，在一定时间内还没发重启，那么设备将进入recovery模式。个人体验就是机子死机了，过会儿自个重启，但是在手机状态中查看开机时间时又不是从零开始的。
disabled：service不会随着它所在的class自动重启
oneshot：仅运行一次，不会有重启service的可能。
onrestart：service重启后，运行后面的command
其他等等

至于command参见下面的英语原文。
-----------------------


Android Init Language
---------------------

The Android Init Language consists of four broad classes of statements,
which are Actions, Commands, Services, and Options.

All of these are line-oriented, consisting of tokens separated by
whitespace.  The c-style backslash escapes may be used to insert
whitespace into a token.  Double quotes may also be used to prevent
whitespace from breaking text into multiple tokens.  The backslash,
when it is the last character on a line, may be used for line-folding.

Lines which start with a # (leading whitespace allowed) are comments.

Actions and Services implicitly declare a new section.  All commands
or options belong to the section most recently declared.  Commands
or options before the first section are ignored.

Actions and Services have unique names.  If a second Action or Service
is declared with the same name as an existing one, it is ignored as
an error.  (??? should we override instead)


Actions
-------
Actions are named sequences of commands.  Actions have a trigger which
is used to determine when the action should occur.  When an event
occurs which matches an action's trigger, that action is added to
the tail of a to-be-executed queue (unless it is already on the
queue).

Each action in the queue is dequeued in sequence and each command in
that action is executed in sequence.  Init handles other activities
(device creation/destruction, property setting, process restarting)
"between" the execution of the commands in activities.

Actions take the form of:

on
  
  
  


Services
--------
Services are programs which init launches and (optionally) restarts
when they exit.  Services take the form of:

service [ ]*
  
  
   ...


Options
-------
Options are modifiers to services.  They affect how and when init
runs the service.

critical
   This is a device-critical service. If it exits more than four times in
   four minutes, the device will reboot into recovery mode.

disabled
   This service will not automatically start with its class.
   It must be explicitly started by name.

setenv
   Set the environment variable to in the launched process.

socket [ [ ] ]
   Create a unix domain socket named /dev/socket/ and pass
   its fd to the launched process.  must be "dgram", "stream" or "seqpacket".
   User and group default to 0.

user
   Change to username before exec'ing this service.
   Currently defaults to root.  (??? probably should default to nobody)
   Currently, if your process requires linux capabilities then you cannot use
   this command. You must instead request the capabilities in-process while
   still root, and then drop to your desired uid.

group [ ]*
   Change to groupname before exec'ing this service.  Additional
   groupnames beyond the (required) first one are used to set the
   supplemental groups of the process (via setgroups()).
   Currently defaults to root.  (??? probably should default to nobody)

oneshot
   Do not restart the service when it exits.

class
   Specify a class name for the service.  All services in a
   named class may be started or stopped together.  A service
   is in the class "default" if one is not specified via the
   class option.

onrestart
    Execute a Command (see below) when service restarts.

Triggers
--------
   Triggers are strings which can be used to match certain kinds
   of events and used to cause an action to occur.

boot
   This is the first trigger that will occur when init starts
   (after /init.conf is loaded)

=
   Triggers of this form occur when the property is set
   to the specific value .

device-added-
device-removed-
   Triggers of these forms occur when a device node is added
   or removed.

service-exited-
   Triggers of this form occur when the specified service exits.


Commands
--------

exec [ ]*
   Fork and execute a program ().  This will block until
   the program completes execution.  It is best to avoid exec
   as unlike the builtin commands, it runs the risk of getting
   init "stuck". (??? maybe there should be a timeout?)

export
   Set the environment variable equal to in the
   global environment (which will be inherited by all processes
   started after this command is executed)

ifup
   Bring the network interface online.

import
   Parse an init config file, extending the current configuration.

hostname
   Set the host name.

chdir
   Change working directory.

chmod
   Change file access permissions.

chown
   Change file owner and group.

chroot
  Change process root directory.

class_start
   Start all services of the specified class if they are
   not already running.

class_stop
   Stop all services of the specified class if they are
   currently running.

domainname
   Set the domain name.

insmod
   Install the module at

mkdir [mode] [owner] [group]
   Create a directory at , optionally with the given mode, owner, and
   group. If not provided, the directory is created with permissions 755 and
   owned by the root user and root group.

mount [ ]*
   Attempt to mount the named device at the directory
   may be of the form mtd@name to specify a mtd block
   device by name.
   s include "ro", "rw", "remount", "noatime", ...

setkey
   TBD

setprop
   Set system property to .

setrlimit
   Set the rlimit for a resource.

start
   Start a service running if it is not already running.

stop
   Stop a service from running if it is currently running.

symlink
   Create a symbolic link at with the value

sysclktz
   Set the system clock base (0 if system clock ticks in GMT)

trigger
   Trigger an event.  Used to queue an action from another
   action.

write [ ]*
   Open the file at and write one or more strings
   to it with write(2)


Properties
----------
Init updates some system properties to provide some insight into
what it's doing:

init.action
   Equal to the name of the action currently being executed or "" if none

init.command
   Equal to the command being executed or "" if none.

init.svc.
   State of a named service ("stopped", "running", "restarting")


Example init.conf
-----------------

# not complete -- just providing some examples of usage
#
on boot
   export PATH /sbin:/system/sbin:/system/bin
   export LD_LIBRARY_PATH /system/lib

   mkdir /dev
   mkdir /proc
   mkdir /sys

   mount tmpfs tmpfs /dev
   mkdir /dev/pts
   mkdir /dev/socket
   mount devpts devpts /dev/pts
   mount proc proc /proc
   mount sysfs sysfs /sys

   write /proc/cpu/alignment 4

   ifup lo

   hostname localhost
   domainname localhost

   mount yaffs2 mtd@system /system
   mount yaffs2 mtd@userdata /data

   import /system/etc/init.conf

   class_start default

service adbd /sbin/adbd
   user adb
   group adb

service usbd /system/bin/usbd -r
   user usbd
   group usbd
   socket usbd 666

service zygote /system/bin/app_process -Xzygote /system/bin --zygote
   socket zygote 666

service runtime /system/bin/runtime
   user system
   group system

on device-added-/dev/compass
   start akmd

on device-removed-/dev/compass
   stop akmd

service akmd /sbin/akmd
   disabled
   user akmd
   group akmd

Debugging notes
---------------
By default, programs executed by init will drop stdout and stderr into
/dev/null. To help with debugging, you can execute your program via the
Andoird program logwrapper. This will redirect stdout/stderr into the
Android logging system (accessed via logcat).

For example
service akmd /system/bin/logwrapper /sbin/akmd