摘自《Android系统原理及开发要点详解》
Android启动脚本init.rc
在 Android中使用启动脚本init.rc,可以在系统的初始化过程中进行一些简单的初始化操作。这个脚本被直接安装到目标系统的根文件系统中，被 init可执行程序解析。 init.rc是在init启动后被执行的启动脚本，其语法主要包含了以下内容：
Commands：命令
Actions： 动作
Triggers:触发条件
Services：服务
Options： 选项
Propertise：属性
(1) Commands是一些基本的操作，例如：
mkdir /sdcard 0000 system system
mkdir /system
mkdir /data 0771 system system
mkdir /cache 0770 system cache
mkdir /config 0500 root root
mkdir /sqlite_stmt_journals 01777 root root
mount tmpfs tmpfs /sqlite_stmt_journals size=4m
这些命令在init可执行程序中被解析，然后调用相关的函数来实现。
(2) Actions(动作)表示一系列的命令，通常在Triggers（触发条件）中调用，动作和触发条件的形式为：
on <trigger>
<command>
<command>
<command>
动作的使用示例如下：
on init
export PATH /sbin:/system/sbin:/system/bin:/system/xbin
mkdir /system
init表示一个触发条件，这个触发事件发生后，进行设置环境变量和建立目录的操作称为一个“动作”
(3) Services（服务）通常表示启动一个可执行程序，Options（选项）是服务的附加内容，用于配合服务使用。
service vold /system/bin/vold
socket vold stream 0660 root mount
service bootsound /system/bin/playmp3
user media
group audio
oneshot
vold和bootsound分别是两个服务的名称，/system/bin/vold和/system /bin/playmp3分别是他们所对应的可执行程序。
socket、user、group、oneshot就是配合服务使用的选项。其中oneshot选项表示该服务只启动一次，而如果没有oneshot选项，
这个可执行程序会一直存在--如果可执行程序被杀死，则会重新启动。
(4) Properties（属性）是系统中使用的一些值，可以进行设置和读取。
setprop ro.FOREGROUND_APP_MEM 1536
setprop ro.VISIBLE_APP_MEM 2048
on property:ro.kernel.qemu=1
start adbd
setprop 用于设置属性，on property可以用于判断属性，这里的属性在整个Android系统运行中都是一致的。
init脚本的关键字可以参考init进程的system/core/init/keyword.h文件。
init.rc的使用方法，可以参考说明文件system/core/init/readme.txt
如果想要修改启动过程只需要修改init.c（system/core/init）或者init.rc里的内容即可.
附录：system/core/init/readme.txt
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 <trigger>
<command>
<command>
<command>
Services
--------
Services are programs which init launches and (optionally) restarts
when they exit. Services take the form of:
service <name> <pathname> [ <argument> ]*
<option>
<option>
...
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 <name> <value>
Set the environment variable <name> to <value> in the launched process.
socket <name> <type> <perm> [ <user> [ <group> ] ]
Create a unix domain socket named /dev/socket/<name> and pass
its fd to the launched process. <type> must be "dgram" or "stream".
User and group default to 0.
user <username>
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 <groupname> [ <groupname> ]*
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 <name>
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)
<name>=<value>
Triggers of this form occur when the property <name> is set
to the specific value <value>.
device-added-<path>
device-removed-<path>
Triggers of these forms occur when a device node is added
or removed.
service-exited-<name>
Triggers of this form occur when the specified service exits.
Commands
--------
exec <path> [ <argument> ]*
Fork and execute a program (<path>). 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 <name> <value>
Set the environment variable <name> equal to <value> in the
global environment (which will be inherited by all processes
started after this command is executed)
ifup <interface>
Bring the network interface <interface> online.
import <filename>
Parse an init config file, extending the current configuration.
hostname <name>
Set the host name.
chdir <directory>
Change working directory.
chmod <octal-mode> <path>
Change file access permissions.
chown <owner> <group> <path>
Change file owner and group.
chroot <directory>
Change process root directory.
class_start <serviceclass>
Start all services of the specified class if they are
not already running.
class_stop <serviceclass>
Stop all services of the specified class if they are
currently running.
domainname <name>
Set the domain name.
insmod <path>
Install the module at <path>
mkdir <path> [mode] [owner] [group]
Create a directory at <path>, 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 <type> <device> <dir> [ <mountoption> ]*
Attempt to mount the named device at the directory <dir>
<device> may be of the form mtd@name to specify a mtd block
device by name.
<mountoption>s include "ro", "rw", "remount", "noatime", ...
setkey
TBD
setprop <name> <value>
Set system property <name> to <value>.
setrlimit <resource> <cur> <max>
Set the rlimit for a resource.
start <service>
Start a service running if it is not already running.
stop <service>
Stop a service from running if it is currently running.
symlink <target> <path>
Create a symbolic link at <path> with the value <target>
sysclktz <mins_west_of_gmt>
Set the system clock base (0 if system clock ticks in GMT)
trigger <event>
Trigger an event. Used to queue an action from another
action.
write <path> <string> [ <string> ]*
Open the file at <path> 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.<name>
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
