这篇文章是从8.0的安卓源码中拿出来的，源码位置是o-base/system/core/init/README.md
Android Init Language
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.
Init .rc Files
The init language is used in plain text files that take the .rc file
extension. There 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:
- /system/etc/init/ is for core system items such as
SurfaceFlinger, MediaService, and logcatd. - /vendor/etc/init/ is for SoC vendor items such as actions or
daemons needed for core SoC functionality. - /odm/etc/init/ is for device manufacturer items such as
actions or daemons needed for motion sensor or other peripheral
functionality.
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
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
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.
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> [ <name>\* ]
Specify class names 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. Additional classnames beyond the (required) first
one are used to group services.
animation class
‘animation’ class should include all services necessary for both
boot animation and shutdown animation. As these services can be
launched very early during bootup and can run until the last stage
of shutdown, access to /data partition is not guaranteed. These
services can check files under /data but it should not keep files opened
and should work when /data is not available.
onrestart
Execute a Command (see below) when service restarts.
writepid <file> [ <file>\* ]
Write the child’s pid to the given files when it forks. Meant for
cgroup/cpuset usage. If no files under /dev/cpuset/ are specified, but the
system property ‘ro.cpuset.default’ is set to a non-empty cpuset name (e.g.
‘/foreground’), then the pid is written to file /dev/cpuset/cpuset_name/tasks.
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
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:
- During initial boot if property a=b and property c=d.
- Any time that property a transitions to value b, while property c already equals d.
- Any time that property c transitions to value d, while property a already equals b.
Commands
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 usingclass_start.
class_restart <serviceclass>
Restarts all services of the specified class.
copy <src> <dst>
Copies a file. Similar to write, but useful for binary/large
amounts of data.
Regarding to the src file, copying from symbolic link file and world-writable
or group-writable files are not allowed.
Regarding to the dst file, the default mode created is 0600 if it does not
exist. And it will be truncated if dst file is a normal regular file and
already exists.
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. Properties are expanded
within argument.
Init halts executing commands until the forked process exits.
exec_start <service>
Start service a given service and halt processing of additional init commands
until it returns. It functions similarly to theexeccommand, but uses an
existing service definition instead of providing them as arguments.
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
restart <service>
Stops and restarts a running service, does nothing if the service is currently
restarting, otherwise, it just starts 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.
wait_for_prop <name> <value>
Wait for system property name to be value. Properties are expanded
within value. If property name is already set to value, continue
immediately.
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.
Imports
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:
- When it imports /init.rc during initial boot
- When it imports /{system,vendor,odm}/etc/init/ or .rc files at specified
paths during mount_all
Properties
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”)
Boot timing
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.
Bootcharting
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.
Comparing two bootcharts
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
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.
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