Android Init Language

Introduction

簡單說明：

• Android根目錄存在一些重要的配置文件，如：init.rc init.xxx.rc。
• 這些配置文件是由Android Init Language語法所撰寫，解析這些語法的程式碼位於system/core/init。
• init.rc為核心文件。

接下介紹init.c語法，它主要提供底下這些功能：

• Runs one-time scripts at startup
• Sets up the runtime environment
• Keeps critical processes running
• Launches the Android framework
• Responds to device additions/removals

Android Init Language主要是由四個類別組成：

• Actions
• Commands
• Services
• Options

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>
   ...

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 is set to the specific value .
• device-added-<path>
• device-removed-<path> - Triggers of these forms occur when a device node is added or removed.

Options

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

選項	說明
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.

Commands

指令	說明
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.

屬性	說明
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")

其它參考網址：

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