The Android Init Language consists of five broad classes of statements: 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.
The init language is used in plain text 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.
One may specify paths in the mount_all command line to have it import .rc files at the specified paths instead of the default ones listed above. This is primarily for supporting factory mode and other non-standard boot modes. The three default paths should be used for the normal boot process.
The intention of these directories is:
All services whose binaries reside on the system, vendor, or odm partitions should have their service entries placed into a corresponding init .rc file, located in the /etc/init/ directory of the partition where they reside. There is a build system macro, LOCAL_INIT_RC, that handles this for developers. Each init .rc file should additionally contain any actions associated with its service.
An example is the logcatd.rc and Android.mk files located in the system/core/logcat directory. The LOCAL_INIT_RC macro in the Android.mk file places logcatd.rc in /system/etc/init/ during the build process. Init loads logcatd.rc during the mount_all command and allows the service to be run and the action to be queued when appropriate.
This break up of init .rc files according to their daemon is preferred to the previously used monolithic init .rc files. This approach ensures that the only service entries that init reads and the only actions that init performs correspond to services whose binaries are in fact present on the file system, which was not the case with the monolithic init .rc files. This additionally will aid in merge conflict resolution when multiple services are added to the system, as each one will go into a separate file.
There are two options "early" and "late" in mount_all command which can be set after optional paths. With "--early" set, the init executable will skip mounting entries with "latemount" flag and triggering fs encryption state event. With "--late" set, init executable will only mount entries with "latemount" flag but skip importing rc files. By default, no option is set, and mount_all will process all entries in the given fstab.
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> [&& <trigger>]* <command> <command> <command>
Services are programs which init launches and (optionally) restarts when they exit. Services take the form of:
service <name> <pathname> [ <argument> ]* <option> <option> ...
Options are modifiers to services. They affect how and when init runs the service.
console [<console>]
This service needs a console. The optional second parameter chooses a specific console instead of the default. The default "/dev/console" can be changed by setting the "androidboot.console" kernel parameter. In all cases the leading "/dev/" should be omitted, so "/dev/tty0" would be specified as just "console tty0".
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. For native executables see libcutils android_get_control_socket().
file <path> <type>
Open a file path and pass its fd to the launched process. type must be "r", "w" or "rw". For native executables see libcutils android_get_control_file().
user <username>
Change to 'username' before exec'ing this service. Currently defaults to root. (??? probably should default to nobody) As of Android M, processes should use this option even if they require Linux capabilities. Previously, to acquire Linux capabilities, a process would need to run as root, request the capabilities, then drop to its desired uid. There is a new mechanism through fs_config that allows device manufacturers to add Linux capabilities to specific binaries on a file system that should be used instead. This mechanism is described on http://source.android.com/devices/tech/config/filesystem.html. When using this new mechanism, processes can use the user option to select their desired uid without ever running as root. As of Android O, processes can also request capabilities directly in their .rc files. See the "capabilities" option below.
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)
capabilities <capability> [ <capability>\* ]
Set capabilities when exec'ing this service. 'capability' should be a Linux capability without the "CAP_" prefix, like "NET_ADMIN" or "SETPCAP". See http://man7.org/linux/man-pages/man7/capabilities.7.html for a list of Linux capabilities.
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.
priority <priority>
Scheduling priority of the service process. This value has to be in range -20 to 19. Default priority is 0. Priority is set via setpriority().
namespace <pid|mnt>
Enter a new PID or mount namespace when forking the service.
oom_score_adjust <value>
Sets the child's /proc/self/oom_score_adj to the specified value, which must range from -1000 to 1000.
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:=' and 'property:=*' respectively. Property triggers are additionally evaluated and triggered accordingly during the initial boot phase of init.
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:
bootchart [start|stop]
Start/stop bootcharting. These are present in the default init.rc files, but bootcharting is only active if the file /data/bootchart/enabled exists; otherwise bootchart start/stop are no-ops.
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 [-f] <path> [<options>]
Install the module at path with the specified options. -f: force installation of the module even if the version of the running kernel and the version of the kernel for which the module was compiled do not match.
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> [ <path> ]\* [--<option>]
Calls fs_mgr_mount_all on the given fs_mgr-format fstab and imports .rc files at the specified paths (e.g., on the partitions just mounted) with optional options "early" and "late". Refer to the section of "Init .rc Files" for detail.
mount <type> <device> <dir> [ <flag>\* ] [<options>]
Attempt to mount the named device at the directory dir _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.
umount <path>
Unmount the filesystem mounted at that path.
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.
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:
Init provides information about the services that it is responsible for via the below properties.
init.svc.<name>
State of a named service ("stopped", "stopping", "running", "restarting")
Init records some boot timing information in system properties.
ro.boottime.init
Time after boot in ns (via the CLOCK_BOOTTIME clock) at which the first stage of init started.
ro.boottime.init.selinux
How long it took the first stage to initialize SELinux.
ro.boottime.init.cold_boot_wait
How long init waited for ueventd's coldboot phase to end.
ro.boottime.<service-name>
Time after boot in ns (via the CLOCK_BOOTTIME clock) that the service was first started.
This version of init contains code to perform "bootcharting": generating log files that can be later processed by the tools provided by http://www.bootchart.org/.
On the emulator, use the -bootchart timeout option to boot with bootcharting activated for timeout seconds.
On a device:
adb shell 'touch /data/bootchart/enabled'
Don't forget to delete this file 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.
A handy script named compare-bootcharts.py can be used to compare the start/end time of selected processes. The aforementioned grab-bootchart.sh 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/compare-bootcharts.py base-bootchart-dir exp-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 (http://developer.android.com/tools/help/systrace.html) 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/systrace.py \
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.
Limitation: 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 affected.
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