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Android Init Language

06/18/2009

Android Init Language

{Android Root}/system/core/init/readme.txt

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” or “stream”.
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.

chmod
Change file access permissions.

chown
Change file owner and group.

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

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