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 commandsor options belong to the section most recently declared.  Commandsor 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> [ <seclabel> ] ] ]

  Create a unix domain socket named /dev/socket/<name> and pass  its fd to the launched process.  <type> must be "dgram", "stream" or "seqpacket".User and group default to 0.

  'seclabel' is the SELinux security context for the socket.It defaults to the service security context, as specified by seclabel or computed based on the service executable file security context.

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)

seclabel <seclabel>

  Change to 'seclabel' before exec'ing this service.

  Primarily for use by services run from the rootfs, e.g. ueventd, adbd.

  Services on the system partition can instead use policy-defined transitions

  based on their file security context.

  If not specified and no transition is defined in policy, defaults to the init context.

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.

writepid <file...>

  Write the child's pid to the given files when it forks. Meant for

  cgroup/cpuset usage.

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>.

   One can also test multiple properties to execute a group

   of commands. For example:

   on property:test.a=1 && property:test.b=1

       setprop test.c 1

   The above stub sets test.c to 1 only when

   both test.a=1 and test.b=1

Commands

bootchart_init   Start bootcharting if configured (see below). This is included in the default init.rc.

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 and disable all services of the specified class if they are

   currently running.

class_reset <serviceclass>

   Stop all services of the specified class if they are currently running, without disabling them. They can be restarted

   later using class_start.

copy <src> <dst>

   Copies a file. Similar to write, but useful for binary/large amounts of data.

domainname <name>

   Set the domain name.

enable <servicename>

   Turns a disabled service into an enabled one as if the service did not specify disabled. If the service is supposed to be running, it will be started now. Typically used when the bootloader sets a variable that indicates a specific

   service should be started when needed. E.g.on property:ro.boot.myfancyhardware=1  enable my_fancy_service_for_my_fancy_hardware

exec [ <seclabel> [ <user> [ <group> ]* ] ] -- <command> [ <argument> ]*

   Fork and execute command with the given arguments. The command starts  after "--" so that an optional security context, user, and supplementary  groups can be provided. No other commands will be run until this one

   finishes. <seclabel> can be a - to denote default.

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)

hostname <name>

   Set the host name.

ifup <interface>

   Bring the network interface <interface> online.

import <filename>

   Parse an init config file, extending the current configuration.

insmod <path>

   Install the module at <path>


load_all_props

   Loads properties from /system, /vendor, et cetera.This is included in the default init.rc.

load_persist_props

   Loads persistent properties when /data has been decrypted.

   This is included in the default init.rc.

loglevel <level>

   Sets the kernel log level to level. Properties are expanded within <level>.

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. If provided, the mode, owner and group

   will be updated if the directory exists already.

mount_all <fstab>

   Calls fs_mgr_mount_all on the given fs_mgr-format fstab.

mount <type> <device> <dir> [ <flag> ]* [<options>]

   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.

   <flag>s include "ro", "rw", "remount", "noatime", ...

   <options> include "barrier=1", "noauto_da_alloc", "discard", ... as

   a comma separated string, eg: barrier=1,noauto_da_alloc

powerctl

   Internal implementation detail used to respond to changes to the

   "sys.powerctl" system property, used to implement rebooting.

restart <service>

   Like stop, but doesn't disable the service.

restorecon <path> [ <path> ]*

   Restore the file named by <path> to the security context specified

   in the file_contexts configuration.

   Not required for directories created by the init.rc as these are

   automatically labeled correctly by init.

restorecon_recursive <path> [ <path> ]*

   Recursively restore the directory tree named by <path> to the

   security contexts specified in the file_contexts configuration.

rm <path>

   Calls unlink(2) on the given path. You might want to

   use "exec -- rm ..." instead (provided the system partition is

   already mounted).

rmdir <path>

   Calls rmdir(2) on the given path.

setprop <name> <value>

   Set system property <name> to <value>. Properties are expanded

   within <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.

swapon_all <fstab>

   Calls fs_mgr_swapon_all on the given fstab file.

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.

verity_load_state

   Internal implementation detail used to load dm-verity state.

verity_update_state <mount_point>

   Internal implementation detail used to update dm-verity state and

   set the partition.<mount_point>.verified properties used by adb remount

   because fs_mgr can't set them directly itself.

wait <path> [ <timeout> ]

   Poll for the existence of the given file and return when found,

   or the timeout has been reached. If timeout is not specified it

   currently defaults to five seconds.

write <path> <content>

   Open the file at <path> and write a string to it with write(2).

   If the file does not exist, it will be created. If it does exist,

   it will be truncated. Properties are expanded within <content>.

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")

Bootcharting

This version of init contains code to perform "bootcharting": generating log

files that can be later processed by the tools provided by www.bootchart.org.

On the emulator, use the -bootchart <timeout> option to boot with bootcharting

activated for <timeout> seconds.

On a device, create /data/bootchart/start with a command like the following:

  adb shell 'echo $TIMEOUT > /data/bootchart/start'

Where the value of $TIMEOUT corresponds to the desired bootcharted period in

seconds. Bootcharting will stop after that many seconds have elapsed.

You can also stop the bootcharting at any moment by doing the following:

  adb shell 'echo 1 > /data/bootchart/stop'

Note that /data/bootchart/stop is deleted automatically by init at the end of

the bootcharting. This is not the case with /data/bootchart/start, so don't

forget to delete it when you're done collecting data.

The log files are written to /data/bootchart/. A script is provided to

retrieve them and create a bootchart.tgz file that can be used with the

bootchart command-line utility:

  sudo apt-get install pybootchartgui

  # grab-bootchart.sh uses $ANDROID_SERIAL.

  $ANDROID_BUILD_TOP/system/core/init/grab-bootchart.sh

One thing to watch for is that the bootchart will show init as if it started

running at 0s. You'll have to look at dmesg to work out when the kernel

actually started init.

Debugging init

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

Android program logwrapper. This will redirect stdout/stderr into the

Android logging system (accessed via logcat).

For example

service akmd /system/bin/logwrapper /sbin/akmd

For quicker turnaround when working on init itself, use:

  mm -j

  m ramdisk-nodeps

  m bootimage-nodeps

  adb reboot bootloader

  fastboot boot $ANDROID_PRODUCT_OUT/boot.img

Alternatively, use the emulator:

  emulator -partition-size 1024 -verbose -show-kernel -no-window

You might want to call klog_set_level(6) after the klog_init() call

so you see the kernel logging in dmesg (or the emulator output).

参考资料:

http://cecs.wright.edu/~pmateti/Courses/4900/Lectures/Internals/Embedded-Android-228-247-pm.pdf