Android Init Language

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[From: http://www.kandroid.org/android_pdk/bring_up.html]

The Android Init Language consists of four broad classes of statements:

  • Actionn
  • Commands
  • Services
  • Options

The language syntax includes the following conventions:

  • All classes are line-oriented and consist of tokens separated by whitespace. 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. A backslash
    appearing as the last character on a line is used for line-folding.
  • Lines that start with a # (leading whitespace allowed) are comments.
  • Actions and Services implicitly declare new sections. 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.

Actions

Actions are named sequences of commands. Actions have a trigger 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. Each command in an action is executed in sequence. Init handles other activities (such as, 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 that 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 that affect how and when init runs a service. Options are described in the table below:

Option Description
disabled This service will not automatically start with its class. It must be explicitly started by name.
socket <type> <name> <perm> [ <user> [ <group> ] ] Create a unix domain socket named /dev/socket/<name> and pass its fd to the launched process. Valid <type> values include dgram and stream. user and group default to 0.
user <username> Change to username before exec'ing this service. Currently defaults to root.
group <groupname> [ <groupname> ]* Change to groupname before exec'ing this service.  Additional  groupnames beyond the first, which is required, are used to set additional groups of the process (with setgroups()). Currently defaults to root.
capability [ <capability> ]+ Set linux capability before exec'ing this service
oneshot Do not restart the service when it exits.
class <name> Specify a class name for the service.  All services in a named class must start and stop together. A service is considered of class "default" if one is not specified via the class option.

Triggers

Triggers are strings used to match certain kinds of events that cause an action to occur.

Trigger Description
boot This is the first trigger that occurs 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

Command Description
exec <path> [ <argument> ]* Fork and execute a program (<path>). This will block until the program completes execution. Try to avoid exec. Unlike the builtin commands, it runs the risk of getting init "stuck".
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.
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> Make a directory at <path>.
mount <type> <device> <dir> [ <mountoption> ]* Attempt to mount the named device at the directory <dir> <device>. This may be of the form mtd@name to specify a mtd block device by name.
setkey - currenlty undefined -
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>.
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:

Property Description
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", or "restarting").

Example init.conf

The following snippet is an incomplete example of the init.conf file, simply meant to give you an idea of what a proper configuration resembles.

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 defaultservice adbd /sbin/adbd  user adb  group adbservice usbd /system/bin/usbd -r  user usbd  group usbd  socket usbd 666service zygote /system/bin/app_process -Xzygote /system/bin --zygote  socket zygote 666service runtime /system/bin/runtime  user system  group systemon device-added-/dev/compass  start akmdon device-removed-/dev/compass  stop akmdservice akmd /sbin/akmd  disabled  user akmd  group akmd

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