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Android Init Language
The Android Init Language consists of five broad classes of statements,
which are Actions, Commands, Services, Options, and Imports.
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 is defined
with the same name as an existing one, its commands are appended to
the commands of the existing action. If a second Service is defined
with the same name as an existing one, it is ignored and an error
message is logged.
Init .rc Files
The init language is used in plaintext files that take the .rc file
extension. These are typically multiple of these in multiple
locations on the system, described below.
/init.rc is the primary .rc file and is loaded by the init executable
at the beginning of its execution. It is responsible for the initial
set up of the system. It imports /init.${ro.hardware}.rc which is the
primary vendor supplied .rc file.
During the mount_all command, the init executable loads all of the
files contained within the /{system,vendor,odm}/etc/init/ directories.
These directories are intended for all Actions and Services used after
file system mounting.
The intention of these directories is as follows
1) /system/etc/init/ is for core system items such as
SurfaceFlinger and MediaService.
2) /vendor/etc/init/ is for SoC vendor items such as actions or
daemons needed for core SoC functionality.
3) /odm/etc/init/ is for device manufacturer items such as
actions or daemons needed for motion sensor or other peripheral
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
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> [&& <trigger>]*
Services are programs which init launches and (optionally) restarts
when they exit. Services take the form of:
service <name> <pathname> [ <argument> ]*
Options are modifiers to services. They affect how and when init
runs the service.
This is a device-critical service. If it exits more than four times in
four minutes, the device will reboot into recovery mode.
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.
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.
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 are strings which can be used to match certain kinds of
events and used to cause an action to occur.
Triggers are subdivided into event triggers and property triggers.
Event triggers are strings triggered by the 'trigger' command or by
the QueueEventTrigger() function within the init executable. These
take the form of a simple string such as 'boot' or 'late-init'.
Property triggers are strings triggered when a named property changes
value to a given new value or when a named property changes value to
any new value. These take the form of 'property:<name>=<value>' and
'property:<name>=*' respectively. Property triggers are additionally
evaluated and triggered accordingly during the initial boot phase of
An Action can have multiple property triggers but may only have one
event trigger.
For example:
'on boot && property:a=b' defines an action that is only executed when
the 'boot' event trigger happens and the property a equals b.
'on property:a=b && property:c=d' defines an action that is executed
at three times,
1) During initial boot if property a=b and property c=d
2) Any time that property a transitions to value b, while property
c already equals d.
3) Any time that property c transitions to value d, while property
a already equals b.
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.
insmod <path>
Install the module at <path>
Loads properties from /system, /vendor, et cetera.
This is included in the default init.rc.
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
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
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>.
The import keyword is not a command, but rather its own section and is
handled immediately after the .rc file that contains it has finished
being parsed. It takes the below form:
import <path>
Parse an init config file, extending the current configuration.
If <path> is a directory, each file in the directory is parsed as
a config file. It is not recursive, nested directories will
not be parsed.
There are only two times where the init executable imports .rc files,
1) When it imports /init.rc during initial boot
2) When it imports /{system,vendor,odm}/etc/init/ during mount_all
Init provides information about the services that it is responsible
for via the below properties.
State of a named service ("stopped", "stopping", "running", "restarting")
This version of init contains code to perform "bootcharting": generating log
files that can be later processed by the tools provided by
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
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.
Comparing two bootcharts
A handy script named can be used to compare the
start/end time of selected processes. The aforementioned
will leave a bootchart tarball named bootchart.tgz at /tmp/android-bootchart.
If two such barballs are preserved on the host machine under different
directories, the script can list the timestamps differences. For example:
Usage: system/core/init/ base_bootchart_dir
process: baseline experiment (delta)
- Unit is ms (a jiffy is 10 ms on the system)
/init: 50 40 (-10)
/system/bin/surfaceflinger: 4320 4470 (+150)
/system/bin/bootanimation: 6980 6990 (+10)
zygote64: 10410 10640 (+230)
zygote: 10410 10640 (+230)
system_server: 15350 15150 (-200)
bootanimation ends at: 33790 31230 (-2560)
Systrace [1] can be used for obtaining performance analysis reports during boot
time on userdebug or eng builds.
Here is an example of trace events of "wm" and "am" categories:
$ANDROID_BUILD_TOP/external/chromium-trace/ wm am --boot
This command will cause the device to reboot. After the device is rebooted and
the boot sequence has finished, the trace report is obtained from the device
and written as trace.html on the host by hitting Ctrl+C.
Recording trace events is started after persistent properties are loaded, so
the trace events that are emitted before that are not recorded. Several
services such as vold, surfaceflinger, and servicemanager are affected by this
limitation since they are started before persistent properties are loaded.
Zygote initialization and the processes that are forked from the zygote are not
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).