Android Init Language

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.

chmod <octal-mode> <path>

   Change file access permissions.

chown <owner> <group> <path>

   Change file owner and group.

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 [email protected] 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 [email protected] /system

   mount yaffs2 [email protected] /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