Android Init语言介绍|极客笔记

Android Init 语言

Android Init语言由五部分组成：

Actions
Commands
Services
Options（选项）
Imports

rc文件都是面向行的，由以空白分隔的标记组成。如果要表达空白字符，可以使用C语言中的反斜杠（或双引号）来转义。当反斜杠是一行的最后一个字符时，可用于续行。

注释，以#开头。

System properties can be expanded using the syntax
${property.name}. This also works in contexts where concatenation is
required, such as import /init.recovery.${ro.hardware}.rc.

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.

Services have unique names. 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 文件

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.

/system/etc/init/hw/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.

Init loads all of the files contained within the
/{system,system_ext,vendor,odm,product}/etc/init/ directories immediately after loading
the primary /system/etc/init/hw/init.rc. This is explained in more details in the
Imports section of this file.

Legacy devices without the first stage mount mechanism previously were
able to import init scripts during mount_all, however that is deprecated
and not allowed for devices launching after Q.

The intention of these directories is:

/system/etc/init/ is for core system items such as
SurfaceFlinger, MediaService, and logd.
/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 userdebug 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.

Versioned RC files within APEXs

With the arrival of mainline on Android Q, the individual mainline
modules carry their own init.rc files within their boundaries. Init
processes these files according to the naming pattern /apex/*/etc/*rc.

Because APEX modules must run on more than one release of Android,
they may require different parameters as part of the services they
define. This is achieved, starting in Android T, by incorporating
the SDK version information in the name of the init file. The suffix
is changed from .rc to .#rc where # is the first SDK where that
RC file is accepted. An init file specific to SDK=31 might be named
init.31rc. With this scheme, an APEX may include multiple init files. An
example is appropriate.

For an APEX module with the following files in /apex/sample-module/apex/etc/:

init.rc
init.32rc
init.35rc

The selection rule chooses the highest .#rc value that does not
exceed the SDK of the currently running system. The unadorned .rc
is interpreted as sdk=0.

When this APEX is installed on a device with SDK <=31, the system will
process init.rc. When installed on a device running SDK 32, 33, or 34,
it will use init.32rc. When installed on a device running SDKs >= 35,
it will choose init.35rc

This versioning scheme is used only for the init files within APEX
modules; it does not apply to the init files stored in /system/etc/init,
/vendor/etc/init, or other directories.

This naming scheme is available after Android S.

Actions

Actions are named sequences of commands. Actions have a trigger which
is used to determine when the action is executed. 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  [&& ]*

Actions are added to the queue and executed based on the order that
the file that contains them was parsed (see the Imports section), then
sequentially within an individual file.

For example if a file contains:

on boot
   setprop a 1
   setprop b 2

on boot && property:true=true
   setprop c 1
   setprop d 2

on boot
   setprop e 1
   setprop f 2

Then when the boot trigger occurs and assuming the property true
equals true, then the order of the commands executed will be:

setprop a 1
setprop b 2
setprop c 1
setprop d 2
setprop e 1
setprop f 2

Services

Services是系统启动时由init启动起来的进程，当进程异常退出时会自动重启（可选）。

Services 的语法如下:

service <name> <pathname> [ <argument> ]*
   <option>
   <option>
   ...

Options（选项）

Options are modifiers to services. They affect how and when init
runs the service.

capabilities [ <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.
If no capabilities are provided, then all capabilities are removed from this service, even if it
runs as root.

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.
The 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.

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".
This option connects stdin, stdout, and stderr to the console. It is mutually exclusive with the
stdio_to_kmsg option, which only connects stdout and stderr to kmsg.

critical [window=<fatal crash window mins>] [target=<fatal reboot target>]

This is a device-critical service. If it exits more than four times in
fatal crash window mins minutes or before boot completes, the device
will reboot into fatal reboot target.
The default value of fatal crash window mins is 4, and default value
of fatal reboot target is ‘bootloader’.
For tests, the fatal reboot can be skipped by setting property
init.svc_debug.no_fatal.<service-name> to true for specified critical service.

disabled

This service will not automatically start with its class.
It must be explicitly started by name or by interface name.

enter_namespace <type> <path>

Enters the namespace of type type located at path. Only network namespaces are supported with
type set to "net". Note that only one namespace of a given type may be entered.

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().

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)

interface <interface name> <instance name>

Associates this service with a list of the AIDL or HIDL services that it provides. The interface
name must be a fully-qualified name and not a value name. For instance, this is used to allow
servicemanager or hwservicemanager to lazily start services. When multiple interfaces are served,
this tag should be used multiple times. An example of an entry for a HIDL
interface is interface vendor.foo.bar@1.0::IBaz default. For an AIDL interface, use
interface aidl <instance name>. The instance name for an AIDL interface is
whatever is registered with servicemanager, and these can be listed with adb shell dumpsys -l.

ioprio <class> <priority>

Sets the IO priority and IO priority class for this service via the SYS_ioprio_set syscall.
class must be one of "rt", "be", or "idle". priority must be an integer in the range 0 – 7.

keycodes <keycode> [ <keycode>\* ]

Sets the keycodes that will trigger this service. If all of the keys corresponding to the passed
keycodes are pressed at once, the service will start. This is typically used to start the
bugreport service.

This option may take a property instead of a list of keycodes. In this case, only one option is
provided: the property name in the typical property expansion format. The property must contain
a comma separated list of keycode values or the text ‘none’ to indicate that
this service does not respond to keycodes.

For example, keycodes ${some.property.name:-none} where some.property.name expands
to "123,124,125". Since keycodes are handled very early in init,
only PRODUCT_DEFAULT_PROPERTY_OVERRIDES properties can be used.

memcg.limit_in_bytes <value> and memcg.limit_percent <value>

Sets the child’s memory.limit_in_bytes to the minimum of limit_in_bytes
bytes and limit_percent which is interpreted as a percentage of the size
of the device’s physical memory (only if memcg is mounted).
Values must be equal or greater than 0.

memcg.limit_property <value>

Sets the child’s memory.limit_in_bytes to the value of the specified property
(only if memcg is mounted). This property will override the values specified
via memcg.limit_in_bytes and memcg.limit_percent.

memcg.soft_limit_in_bytes <value>

Sets the child’s memory.soft_limit_in_bytes to the specified value (only if memcg is mounted),
which must be equal or greater than 0.

memcg.swappiness <value>

Sets the child’s memory.swappiness to the specified value (only if memcg is mounted),
which must be equal or greater than 0.

namespace <pid|mnt>

Enter a new PID or mount namespace when forking the service.

oneshot

Do not restart the service when it exits.

onrestart

Execute a Command (see below) when service restarts.

oom_score_adjust <value>

Sets the child’s /proc/self/oom_score_adj to the specified value,
which must range from -1000 to 1000.

override

Indicates that this service definition is meant to override a previous definition for a service
with the same name. This is typically meant for services on /odm to override those defined on
/vendor. The last service definition that init parses with this keyword is the service definition
will use for this service. Pay close attention to the order in which init.rc files are parsed,
since it has some peculiarities for backwards compatibility reasons. The ‘imports’ section of
this file has more details on the order.

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().

reboot_on_failure <target>

If this process cannot be started or if the process terminates with an exit code other than
CLD_EXITED or an status other than ‘0’, reboot the system with the target specified in
target. target takes the same format as the parameter to sys.powerctl. This is particularly
intended to be used with the exec_start builtin for any must-have checks during boot.

restart_period <seconds>

If a non-oneshot service exits, it will be restarted at its start time plus
this period. It defaults to 5s to rate limit crashing services.
This can be increased for services that are meant to run periodically. For
example, it may be set to 3600 to indicate that the service should run every hour
or 86400 to indicate that the service should run every day.

rlimit <resource> <cur> <max>

This applies the given rlimit to the service. rlimits are inherited by child
processes, so this effectively applies the given rlimit to the process tree
started by this service.
It is parsed similarly to the setrlimit command specified below.

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.

setenv <name> <value>

Set the environment variable name to value in the launched process.

shutdown <shutdown_behavior>

Set shutdown behavior of the service process. When this is not specified,
the service is killed during shutdown process by using SIGTERM and SIGKILL.
The service with shutdown_behavior of "critical" is not killed during shutdown
until shutdown times out. When shutdown times out, even services tagged with
"shutdown critical" will be killed. When the service tagged with "shutdown critical"
is not running when shut down starts, it will be started.

sigstop

Send SIGSTOP to the service immediately before exec is called. This is intended for debugging.
See the below section on debugging for how this can be used.

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". type
may end with "+passcred" to enable SO_PASSCRED on the socket. 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().

stdio_to_kmsg

Redirect stdout and stderr to /dev/kmsg_debug. This is useful for services that do not use native
Android logging during early boot and whose logs messages we want to capture. This is only enabled
when /dev/kmsg_debug is enabled, which is only enabled on userdebug and eng builds.
This is mutually exclusive with the console option, which additionally connects stdin to the
given console.

task_profiles <profile> [ <profile>\* ]

Set task profiles for the process when it forks. This is designed to replace the use of
writepid option for moving a process into a cgroup.

timeout_period <seconds>

Provide a timeout after which point the service will be killed. The oneshot keyword is respected
here, so oneshot services do not automatically restart, however all other services will.
This is particularly useful for creating a periodic service combined with the restart_period
option described above.

updatable

Mark that the service can be overridden (via the ‘override’ option) later in
the boot sequence by APEXes. When a service with updatable option is started
before APEXes are all activated, the execution is delayed until the activation
is finished. A service that is not marked as updatable cannot be overridden by
APEXes.

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.

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.
The use of this option for moving a process into a cgroup is obsolete. Please
use task_profiles option instead.

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.

Trigger Sequence

Init uses the following sequence of triggers during early boot. These are the
built-in triggers defined in init.cpp.

early-init – The first in the sequence, triggered after cgroups has been configured
but before ueventd’s coldboot is complete.
init – Triggered after coldboot is complete.
charger – Triggered if ro.bootmode == "charger".
late-init – Triggered if ro.bootmode != "charger", or via healthd triggering a boot
from charging mode.

Remaining triggers are configured in init.rc and are not built-in. The default sequence for
these is specified under the "on late-init" event in init.rc. Actions internal to init.rc
have been omitted.

early-fs – Start vold.
fs – Vold is up. Mount partitions not marked as first-stage or latemounted.
post-fs – Configure anything dependent on early mounts.
late-fs – Mount partitions marked as latemounted.
post-fs-data – Mount and configure /data; set up encryption. /metadata is
reformatted here if it couldn’t mount in first-stage init.
zygote-start – Start the zygote.
early-boot – After zygote has started.
boot – After early-boot actions have completed.

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. See the start entry for more information on
starting services.

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.

class_restart [--only-enabled] <serviceclass>

Restarts all services of the specified class. If --only-enabled is
specified, then disabled services are skipped.

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.

copy_per_line <src> <dst>

Copies a file line by line. Similar to copy, but useful for dst is a sysfs node
that doesn’t handle multiple lines 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. Properties are expanded
within argument.
Init halts executing commands until the forked process exits.

exec_background [ <seclabel> [ <user> [ <group>\* ] ] ] -- <command> [ <argument>\* ]

Fork and execute command with the given arguments. This is handled similarly
to the exec command. The difference is that init does not halt executing
commands until the process exits for exec_background.

exec_start <service>

Start a given service and halt the processing of additional init commands
until it returns. The command functions similarly to the exec command,
but uses an existing service definition in place of the exec argument vector.

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.

interface_start <name> \
interface_restart <name> \
interface_stop <name>

Find the service that provides the interface name if it exists and run the start, restart,
or stop commands on it respectively. name may be either a fully qualified HIDL name, in which
case it is specified as <interface>/<instance>, or an AIDL name, in which case it is specified as
aidl/<interface> for example android.hardware.secure_element@1.1::ISecureElement/eSE1 or
aidl/aidl_lazy_test_1.

Note that these commands only act on interfaces specified by the interface service option, not
on interfaces registered at runtime.

Example usage of these commands: \
interface_start android.hardware.secure_element@1.1::ISecureElement/eSE1
will start the HIDL Service that provides the android.hardware.secure_element@1.1 and eSI1
instance. \
interface_start aidl/aidl_lazy_test_1 will start the AIDL service that
provides the aidl_lazy_test_1 interface.

load_exports <path>

Open the file at path and export global environment variables declared
there. Each line must be in the format export <name> <value>, as described
above.

load_system_props

(This action is deprecated and no-op.)

load_persist_props

Loads persistent properties when /data has been decrypted.
This is included in the default init.rc.

loglevel <level>

Sets init’s log level to the integer level, from 7 (all logging) to 0
(fatal logging only). The numeric values correspond to the kernel log
levels, but this command does not affect the kernel log level. Use the
write command to write to /proc/sys/kernel/printk to change that.
Properties are expanded within level.

mark_post_data

Used to mark the point right after /data is mounted.

mkdir <path> [<mode>] [<owner>] [<group>] [encryption=<action>] [key=<key>]

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.

action can be one of:

None: take no encryption action; directory will be encrypted if parent is.

Require: encrypt directory, abort boot process if encryption fails

Attempt: try to set an encryption policy, but continue if it fails

DeleteIfNecessary: recursively delete directory if necessary to set
encryption policy.

key can be one of:

ref: use the systemwide DE key

per_boot_ref: use the key freshly generated on each boot.

mount_all [ <fstab> ] [--<option>]

Calls fs_mgr_mount_all on the given fs_mgr-format fstab with optional
options "early" and "late".
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. By default,
no option is set, and mount_all will process all entries in the given fstab.
If the fstab parameter is not specified, fstab. ${ro.boot.fstab_suffix}, fstab.$ {ro.hardware} or fstab.${ro.hardware.platform} will be scanned for
under /odm/etc, /vendor/etc, or / at runtime, in that order.

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, e.g. barrier=1,noauto_da_alloc

perform_apex_config

Performs tasks after APEXes are mounted. For example, creates data directories
for the mounted APEXes, parses config file(s) from them, and updates linker
configurations. Intended to be used only once when apexd notifies the mount
event by setting apexd.status to ready.

restart [--only-if-running] <service>

Stops and restarts a running service, does nothing if the service is currently
restarting, otherwise, it just starts the service. If "–only-if-running" is
specified, the service is only restarted if it is already running.

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.

readahead <file|dir> [--fully]

Calls readahead(2) on the file or files within given directory.
Use option –fully to read the full file content.

setprop <name> <value>

Set system property name to value. Properties are expanded
within value.

setrlimit <resource> <cur> <max>

Set the rlimit for a resource. This applies to all processes launched after
the limit is set. It is intended to be set early in init and applied globally.
resource is best specified using its text representation (‘cpu’, ‘rtio’, etc
or ‘RLIM_CPU’, ‘RLIM_RTIO’, etc). It also may be specified as the int value
that the resource enum corresponds to.
cur and max can be ‘unlimited’ or ‘-1’ to indicate an infinite rlimit.

start <service>

Start a service running if it is not already running.
Note that this is not synchronous, and even if it were, there is
no guarantee that the operating system’s scheduler will execute the
service sufficiently to guarantee anything about the service’s status.
See the exec_start command for a synchronous version of start.

This creates an important consequence that if the service offers
functionality to other services, such as providing a
communication channel, simply starting this service before those
services is not sufficient to guarantee that the channel has
been set up before those services ask for it. There must be a
separate mechanism to make any such guarantees.

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.
If the fstab parameter is not specified, fstab. ${ro.boot.fstab_suffix}, fstab.$ {ro.hardware} or fstab.${ro.hardware.platform} will be scanned for
under /odm/etc, /vendor/etc, or / at runtime, in that order.

symlink <target> <path>

Create a symbolic link at path with the value target

sysclktz <minutes_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.

umount_all [ <fstab> ]

Calls fs_mgr_umount_all on the given fstab file.
If the fstab parameter is not specified, fstab. ${ro.boot.fstab_suffix}, fstab.$ {ro.hardware} or fstab.${ro.hardware.platform} will be scanned for
under /odm/etc, /vendor/etc, or / at runtime, in that order.

verity_update_state

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. This is required since
Android 12, because CtsNativeVerifiedBootTestCases will read property
"partition.${partition}.verified.hash_alg" to check that sha1 is not used.
See https://r.android.com/1546980 for more details.

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. The timeout value can be
fractional seconds, specified in floating point notation.

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

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.

The import keyword is not a command, but rather its own section,
meaning that it does not happen as part of an Action, but rather,
imports are handled as a file is being parsed and follow the below logic.

There are only three times where the init executable imports .rc files:

When it imports /system/etc/init/hw/init.rc or the script indicated by the property
ro.boot.init_rc during initial boot.
When it imports /{system,system_ext,vendor,odm,product}/etc/init/ immediately after
importing /system/etc/init/hw/init.rc.
(Deprecated) When it imports /{system,vendor,odm}/etc/init/ or .rc files
at specified paths during mount_all, not allowed for devices launching
after Q.

The order that files are imported is a bit complex for legacy reasons. The below is guaranteed:

/system/etc/init/hw/init.rc is parsed then recursively each of its imports are
parsed.
The contents of /system/etc/init/ are alphabetized and parsed sequentially, with imports
happening recursively after each file is parsed.
Step 2 is repeated for /system_ext/etc/init, /vendor/etc/init, /odm/etc/init,
/product/etc/init

The below pseudocode may explain this more clearly:

fn Import(file)
  Parse(file)
  for (import : file.imports)
    Import(import)

Import(/system/etc/init/hw/init.rc)
Directories = [/system/etc/init, /system_ext/etc/init, /vendor/etc/init, /odm/etc/init, /product/etc/init]
for (directory : Directories)
  files = 
  for (file : files)
    Import(file)

Actions are executed in the order that they are parsed. For example the post-fs-data action(s)
in /system/etc/init/hw/init.rc are always the first post-fs-data action(s) to be executed in
order of how they appear in that file. Then the post-fs-data actions of the imports of
/system/etc/init/hw/init.rc in the order that they’re imported, etc.

Properties

Init provides state information with the following properties.

init.svc.<name>

State of a named service ("stopped", "stopping", "running", "restarting")

dev.mnt.dev.<mount_point>, dev.mnt.blk.<mount_point>, dev.mnt.rootdisk.<mount_point>

Block device base name associated with a mount_point.
The mount_point has / replaced by . and if referencing the root mount point
"/", it will use "/root".
dev.mnt.dev.<mount_point> indicates a block device attached to filesystems.
(e.g., dm-N or sdaN/mmcblk0pN to access /sys/fs/ext4/${dev.mnt.dev.<mount_point>}/)

dev.mnt.blk.<mount_point> indicates the disk partition to the above block device.
(e.g., sdaN / mmcblk0pN to access /sys/class/block/${dev.mnt.blk.<mount_point>}/)

dev.mnt.rootdisk.<mount_point> indicates the root disk to contain the above disk partition.
(e.g., sda / mmcblk0 to access /sys/class/block/${dev.mnt.rootdisk.<mount_point>}/queue)

Init responds to properties that begin with ctl.. These properties take the format of
ctl.[<target>_]<command> and the value of the system property is used as a parameter. The
target is optional and specifies the service option that value is meant to match with. There is
only one option for target, interface which indicates that value will refer to an interface
that a service provides and not the service name itself.

For example:

SetProperty("ctl.start", "logd") will run the start command on logd.

SetProperty("ctl.interface_start", "aidl/aidl_lazy_test_1") will run the start command on the
service that exposes the aidl aidl_lazy_test_1 interface.

Note that these
properties are only settable; they will have no value when read.

The commands are listed below.

start \
restart \
stop \
These are equivalent to using the start, restart, and stop commands on the service specified
by the value of the property.

oneshot_on and oneshot_off will turn on or off the oneshot
flag for the service specified by the value of the property. This is
particularly intended for services that are conditionally lazy HALs. When
they are lazy HALs, oneshot must be on, otherwise oneshot should be off.

sigstop_on and sigstop_off will turn on or off the sigstop feature for the service
specified by the value of the property. See the Debugging init section below for more details
about this feature.

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.first_stage

How long in ns it took to run first stage.

ro.boottime.init.selinux

How long in ns it took to run SELinux stage.

ro.boottime.init.modules

How long in ms it took to load kernel modules.

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 tarballs 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

When a service starts from init, it may fail to execv() the service. This is not typical, and may
point to an error happening in the linker as the new service is started. The linker in Android
prints its logs to logd and stderr, so they are visible in logcat. If the error is encountered
before it is possible to access logcat, the stdio_to_kmsg service option may be used to direct
the logs that the linker prints to stderr to kmsg, where they can be read via a serial port.

Launching init services without init is not recommended as init sets up a significant amount of
environment (user, groups, security label, capabilities, etc) that is hard to replicate manually.

If it is required to debug a service from its very start, the sigstop service option is added.
This option will send SIGSTOP to a service immediately before calling exec. This gives a window
where developers can attach a debugger, strace, etc before continuing the service with SIGCONT.

This flag can also be dynamically controlled via the ctl.sigstop_on and ctl.sigstop_off properties.

Below is an example of dynamically debugging logd via the above:

stop logd
setprop ctl.sigstop_on logd
start logd
ps -e | grep logd
> logd          4343     1   18156   1684 do_signal_stop 538280 T init
gdbclient.py -p 4343
b main
c
c
c
> Breakpoint 1, main (argc=1, argv=0x7ff8c9a488) at system/core/logd/main.cpp:427

Below is an example of doing the same but with strace

stop logd
setprop ctl.sigstop_on logd
start logd
ps -e | grep logd
> logd          4343     1   18156   1684 do_signal_stop 538280 T init
strace -p 4343

(From a different shell)
kill -SIGCONT 4343

> strace runs

Host Init Script Verification

Init scripts are checked for correctness during build time. Specifically the below is checked.

1) Well formatted action, service and import sections, e.g. no actions without a preceding ‘on’
line, and no extraneous lines after an ‘import’ statement.
2) All commands map to a valid keyword and the argument count is within the correct range.
3) All service options are valid. This is stricter than how commands are checked as the service
options’ arguments are fully parsed, e.g. UIDs and GIDs must resolve.

There are other parts of init scripts that are only parsed at runtime and therefore not checked
during build time, among them are the below.

1) The validity of the arguments of commands, e.g. no checking if file paths actually exist, if
SELinux would permit the operation, or if the UIDs and GIDs resolve.
2) No checking if a service exists or has a valid SELinux domain defined
3) No checking if a service has not been previously defined in a different init script.

Early Init Boot Sequence

The early init boot sequence is broken up into three stages: first stage init, SELinux setup, and
second stage init.

First stage init is responsible for setting up the bare minimum requirements to load the rest of the
system. Specifically this includes mounting /dev, /proc, mounting ‘early mount’ partitions (which
needs to include all partitions that contain system code, for example system and vendor), and moving
the system.img mount to / for devices with a ramdisk.

Note that in Android Q, system.img always contains TARGET_ROOT_OUT and always is mounted at / by the
time first stage init finishes. Android Q will also require dynamic partitions and therefore will
require using a ramdisk to boot Android. The recovery ramdisk can be used to boot to Android instead
of a dedicated ramdisk as well.

First stage init has three variations depending on the device configuration:
1) For system-as-root devices, first stage init is part of /system/bin/init and a symlink at /init
points to /system/bin/init for backwards compatibility. These devices do not need to do anything to
mount system.img, since it is by definition already mounted as the rootfs by the kernel.

2) For devices with a ramdisk, first stage init is a static executable located at /init. These
devices mount system.img as /system then perform a switch root operation to move the mount at
/system to /. The contents of the ramdisk are freed after mounting has completed.

3) For devices that use recovery as a ramdisk, first stage init it contained within the shared init
located at /init within the recovery ramdisk. These devices first switch root to
/first_stage_ramdisk to remove the recovery components from the environment, then proceed the same
as 2). Note that the decision to boot normally into Android instead of booting
into recovery mode is made if androidboot.force_normal_boot=1 is present in the
kernel commandline, or in bootconfig with Android S and later.

Once first stage init finishes it execs /system/bin/init with the "selinux_setup" argument. This
phase is where SELinux is optionally compiled and loaded onto the system. selinux.cpp contains more
information on the specifics of this process.

Lastly once that phase finishes, it execs /system/bin/init again with the "second_stage"
argument. At this point the main phase of init runs and continues the boot process via the init.rc
scripts.