Init

我们可以在源码/system/core/init/init.c查看init进程的实现

init.rc是什么

他是一个文本文件，他是一个启动脚本，Init进程会根据他定义的变量，系统进程启动，分区挂载，属性等多项配置来进行启动。下面先来看看一个完整的启动脚本：

nexus 5 5.1.1启动脚本文件

init.rc

# Copyright (C) 2012 The Android Open Source Project
#
# IMPORTANT: Do not create world writable files or directories.
# This is a common source of Android security bugs.
#

import /init.environ.rc
import /init.usb.rc
import /init.${ro.hardware}.rc
import /init.${ro.zygote}.rc
import /init.trace.rc

on early-init
    # Set init and its forked children's oom_adj.
    write /proc/1/oom_score_adj -1000

    # Apply strict SELinux checking of PROT_EXEC on mmap/mprotect calls.
    write /sys/fs/selinux/checkreqprot 0

    # Set the security context for the init process.
    # This should occur before anything else (e.g. ueventd) is started.
    setcon u:r:init:s0

    # Set the security context of /adb_keys if present.
    restorecon /adb_keys

    start ueventd

    # create mountpoints
    mkdir /mnt 0775 root system

on init
    sysclktz 0

    loglevel 3

    # Backward compatibility
    symlink /system/etc /etc
    symlink /sys/kernel/debug /d

    # Right now vendor lives on the same filesystem as system,
    # but someday that may change.
    symlink /system/vendor /vendor

    # Create cgroup mount point for cpu accounting
    mkdir /acct
    mount cgroup none /acct cpuacct
    mkdir /acct/uid

    # Create cgroup mount point for memory
    mount tmpfs none /sys/fs/cgroup mode=0750,uid=0,gid=1000
    mkdir /sys/fs/cgroup/memory 0750 root system
    mount cgroup none /sys/fs/cgroup/memory memory
    write /sys/fs/cgroup/memory/memory.move_charge_at_immigrate 1
    chown root system /sys/fs/cgroup/memory/tasks
    chmod 0660 /sys/fs/cgroup/memory/tasks
    mkdir /sys/fs/cgroup/memory/sw 0750 root system
    write /sys/fs/cgroup/memory/sw/memory.swappiness 100
    write /sys/fs/cgroup/memory/sw/memory.move_charge_at_immigrate 1
    chown root system /sys/fs/cgroup/memory/sw/tasks
    chmod 0660 /sys/fs/cgroup/memory/sw/tasks

    mkdir /system
    mkdir /data 0771 system system
    mkdir /cache 0770 system cache
    mkdir /config 0500 root root

    # See storage config details at http://source.android.com/tech/storage/
    mkdir /mnt/shell 0700 shell shell
    mkdir /mnt/media_rw 0700 media_rw media_rw
    mkdir /storage 0751 root sdcard_r

    # Directory for putting things only root should see.
    mkdir /mnt/secure 0700 root root

    # Directory for staging bindmounts
    mkdir /mnt/secure/staging 0700 root root

    # Directory-target for where the secure container
    # imagefile directory will be bind-mounted
    mkdir /mnt/secure/asec  0700 root root

    # Secure container public mount points.
    mkdir /mnt/asec  0700 root system
    mount tmpfs tmpfs /mnt/asec mode=0755,gid=1000

    # Filesystem image public mount points.
    mkdir /mnt/obb 0700 root system
    mount tmpfs tmpfs /mnt/obb mode=0755,gid=1000

    # memory control cgroup
    mkdir /dev/memcg 0700 root system
    mount cgroup none /dev/memcg memory

    write /proc/sys/kernel/panic_on_oops 1
    write /proc/sys/kernel/hung_task_timeout_secs 0
    write /proc/cpu/alignment 4
    write /proc/sys/kernel/sched_latency_ns 10000000
    write /proc/sys/kernel/sched_wakeup_granularity_ns 2000000
    write /proc/sys/kernel/sched_compat_yield 1
    write /proc/sys/kernel/sched_child_runs_first 0
    write /proc/sys/kernel/randomize_va_space 2
    write /proc/sys/kernel/kptr_restrict 2
    write /proc/sys/vm/mmap_min_addr 32768
    write /proc/sys/net/ipv4/ping_group_range "0 2147483647"
    write /proc/sys/net/unix/max_dgram_qlen 300
    write /proc/sys/kernel/sched_rt_runtime_us 950000
    write /proc/sys/kernel/sched_rt_period_us 1000000

    # reflect fwmark from incoming packets onto generated replies
    write /proc/sys/net/ipv4/fwmark_reflect 1
    write /proc/sys/net/ipv6/fwmark_reflect 1

    # set fwmark on accepted sockets
    write /proc/sys/net/ipv4/tcp_fwmark_accept 1

    # Create cgroup mount points for process groups
    mkdir /dev/cpuctl
    mount cgroup none /dev/cpuctl cpu
    chown system system /dev/cpuctl
    chown system system /dev/cpuctl/tasks
    chmod 0666 /dev/cpuctl/tasks
    write /dev/cpuctl/cpu.shares 1024
    write /dev/cpuctl/cpu.rt_runtime_us 800000
    write /dev/cpuctl/cpu.rt_period_us 1000000

    mkdir /dev/cpuctl/bg_non_interactive
    chown system system /dev/cpuctl/bg_non_interactive/tasks
    chmod 0666 /dev/cpuctl/bg_non_interactive/tasks
    # 5.0 %
    write /dev/cpuctl/bg_non_interactive/cpu.shares 52
    write /dev/cpuctl/bg_non_interactive/cpu.rt_runtime_us 700000
    write /dev/cpuctl/bg_non_interactive/cpu.rt_period_us 1000000

    # qtaguid will limit access to specific data based on group memberships.
    #   net_bw_acct grants impersonation of socket owners.
    #   net_bw_stats grants access to other apps' detailed tagged-socket stats.
    chown root net_bw_acct /proc/net/xt_qtaguid/ctrl
    chown root net_bw_stats /proc/net/xt_qtaguid/stats

    # Allow everybody to read the xt_qtaguid resource tracking misc dev.
    # This is needed by any process that uses socket tagging.
    chmod 0644 /dev/xt_qtaguid

    # Create location for fs_mgr to store abbreviated output from filesystem
    # checker programs.
    mkdir /dev/fscklogs 0770 root system

    # pstore/ramoops previous console log
    mount pstore pstore /sys/fs/pstore
    chown system log /sys/fs/pstore/console-ramoops
    chmod 0440 /sys/fs/pstore/console-ramoops

# Healthd can trigger a full boot from charger mode by signaling this
# property when the power button is held.
on property:sys.boot_from_charger_mode=1
    class_stop charger
    trigger late-init

# Load properties from /system/ + /factory after fs mount.
on load_all_props_action
    load_all_props

# Indicate to fw loaders that the relevant mounts are up.
on firmware_mounts_complete
    rm /dev/.booting

# Mount filesystems and start core system services.
on late-init
    trigger early-fs
    trigger fs
    trigger post-fs
    trigger post-fs-data

    # Load properties from /system/ + /factory after fs mount. Place
    # this in another action so that the load will be scheduled after the prior
    # issued fs triggers have completed.
    trigger load_all_props_action

    # Remove a file to wake up anything waiting for firmware.
    trigger firmware_mounts_complete

    trigger early-boot
    trigger boot


on post-fs
    # once everything is setup, no need to modify /
    mount rootfs rootfs / ro remount
    # mount shared so changes propagate into child namespaces
    mount rootfs rootfs / shared rec

    # We chown/chmod /cache again so because mount is run as root + defaults
    chown system cache /cache
    chmod 0770 /cache
    # We restorecon /cache in case the cache partition has been reset.
    restorecon_recursive /cache

    # This may have been created by the recovery system with odd permissions
    chown system cache /cache/recovery
    chmod 0770 /cache/recovery

    #change permissions on vmallocinfo so we can grab it from bugreports
    chown root log /proc/vmallocinfo
    chmod 0440 /proc/vmallocinfo

    chown root log /proc/slabinfo
    chmod 0440 /proc/slabinfo

    #change permissions on kmsg & sysrq-trigger so bugreports can grab kthread stacks
    chown root system /proc/kmsg
    chmod 0440 /proc/kmsg
    chown root system /proc/sysrq-trigger
    chmod 0220 /proc/sysrq-trigger
    chown system log /proc/last_kmsg
    chmod 0440 /proc/last_kmsg

    # make the selinux kernel policy world-readable
    chmod 0444 /sys/fs/selinux/policy

    # create the lost+found directories, so as to enforce our permissions
    mkdir /cache/lost+found 0770 root root

on post-fs-data
    # We chown/chmod /data again so because mount is run as root + defaults
    chown system system /data
    chmod 0771 /data
    # We restorecon /data in case the userdata partition has been reset.
    restorecon /data

    # Avoid predictable entropy pool. Carry over entropy from previous boot.
    copy /data/system/entropy.dat /dev/urandom

    # Create dump dir and collect dumps.
    # Do this before we mount cache so eventually we can use cache for
    # storing dumps on platforms which do not have a dedicated dump partition.
    mkdir /data/dontpanic 0750 root log

    # Collect apanic data, free resources and re-arm trigger
    copy /proc/apanic_console /data/dontpanic/apanic_console
    chown root log /data/dontpanic/apanic_console
    chmod 0640 /data/dontpanic/apanic_console

    copy /proc/apanic_threads /data/dontpanic/apanic_threads
    chown root log /data/dontpanic/apanic_threads
    chmod 0640 /data/dontpanic/apanic_threads

    write /proc/apanic_console 1

    # create basic filesystem structure
    mkdir /data/misc 01771 system misc
    mkdir /data/misc/adb 02750 system shell
    mkdir /data/misc/bluedroid 0770 bluetooth net_bt_stack
    mkdir /data/misc/bluetooth 0770 system system
    mkdir /data/misc/keystore 0700 keystore keystore
    mkdir /data/misc/keychain 0771 system system
    mkdir /data/misc/net 0750 root shell
    mkdir /data/misc/radio 0770 system radio
    mkdir /data/misc/sms 0770 system radio
    mkdir /data/misc/zoneinfo 0775 system system
    mkdir /data/misc/vpn 0770 system vpn
    mkdir /data/misc/shared_relro 0771 shared_relro shared_relro
    mkdir /data/misc/systemkeys 0700 system system
    mkdir /data/misc/wifi 0770 wifi wifi
    mkdir /data/misc/wifi/sockets 0770 wifi wifi
    mkdir /data/misc/wifi/wpa_supplicant 0770 wifi wifi
    mkdir /data/misc/ethernet 0770 system system
    mkdir /data/misc/dhcp 0770 dhcp dhcp
    mkdir /data/misc/user 0771 root root
    # give system access to wpa_supplicant.conf for backup and restore
    chmod 0660 /data/misc/wifi/wpa_supplicant.conf
    mkdir /data/local 0751 root root
    mkdir /data/misc/media 0700 media media

    # For security reasons, /data/local/tmp should always be empty.
    # Do not place files or directories in /data/local/tmp
    mkdir /data/local/tmp 0771 shell shell
    mkdir /data/data 0771 system system
    mkdir /data/app-private 0771 system system
    mkdir /data/app-asec 0700 root root
    mkdir /data/app-lib 0771 system system
    mkdir /data/app 0771 system system
    mkdir /data/property 0700 root root

    # create dalvik-cache, so as to enforce our permissions
    mkdir /data/dalvik-cache 0771 root root
    mkdir /data/dalvik-cache/profiles 0711 system system

    # create resource-cache and double-check the perms
    mkdir /data/resource-cache 0771 system system
    chown system system /data/resource-cache
    chmod 0771 /data/resource-cache

    # create the lost+found directories, so as to enforce our permissions
    mkdir /data/lost+found 0770 root root

    # create directory for DRM plug-ins - give drm the read/write access to
    # the following directory.
    mkdir /data/drm 0770 drm drm

    # create directory for MediaDrm plug-ins - give drm the read/write access to
    # the following directory.
    mkdir /data/mediadrm 0770 mediadrm mediadrm

    mkdir /data/adb 0700 root root

    # symlink to bugreport storage location
    symlink /data/data/com.android.shell/files/bugreports /data/bugreports

    # Separate location for storing security policy files on data
    mkdir /data/security 0711 system system

    # Reload policy from /data/security if present.
    setprop selinux.reload_policy 1

    # Set SELinux security contexts on upgrade or policy update.
    restorecon_recursive /data

    # If there is no fs-post-data action in the init.<device>.rc file, you
    # must uncomment this line, otherwise encrypted filesystems
    # won't work.
    # Set indication (checked by vold) that we have finished this action
    #setprop vold.post_fs_data_done 1

on boot
    # basic network init
    ifup lo
    hostname localhost
    domainname localdomain

    # set RLIMIT_NICE to allow priorities from 19 to -20
    setrlimit 13 40 40

    # Memory management.  Basic kernel parameters, and allow the high
    # level system server to be able to adjust the kernel OOM driver
    # parameters to match how it is managing things.
    write /proc/sys/vm/overcommit_memory 1
    write /proc/sys/vm/min_free_order_shift 4
    chown root system /sys/module/lowmemorykiller/parameters/adj
    chmod 0220 /sys/module/lowmemorykiller/parameters/adj
    chown root system /sys/module/lowmemorykiller/parameters/minfree
    chmod 0220 /sys/module/lowmemorykiller/parameters/minfree

    # Tweak background writeout
    write /proc/sys/vm/dirty_expire_centisecs 200
    write /proc/sys/vm/dirty_background_ratio  5

    # Permissions for System Server and daemons.
    chown radio system /sys/android_power/state
    chown radio system /sys/android_power/request_state
    chown radio system /sys/android_power/acquire_full_wake_lock
    chown radio system /sys/android_power/acquire_partial_wake_lock
    chown radio system /sys/android_power/release_wake_lock
    chown system system /sys/power/autosleep
    chown system system /sys/power/state
    chown system system /sys/power/wakeup_count
    chown radio system /sys/power/wake_lock
    chown radio system /sys/power/wake_unlock
    chmod 0660 /sys/power/state
    chmod 0660 /sys/power/wake_lock
    chmod 0660 /sys/power/wake_unlock

    chown system system /sys/devices/system/cpu/cpufreq/interactive/timer_rate
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/timer_rate
    chown system system /sys/devices/system/cpu/cpufreq/interactive/timer_slack
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/timer_slack
    chown system system /sys/devices/system/cpu/cpufreq/interactive/min_sample_time
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/min_sample_time
    chown system system /sys/devices/system/cpu/cpufreq/interactive/hispeed_freq
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/hispeed_freq
    chown system system /sys/devices/system/cpu/cpufreq/interactive/target_loads
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/target_loads
    chown system system /sys/devices/system/cpu/cpufreq/interactive/go_hispeed_load
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/go_hispeed_load
    chown system system /sys/devices/system/cpu/cpufreq/interactive/above_hispeed_delay
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/above_hispeed_delay
    chown system system /sys/devices/system/cpu/cpufreq/interactive/boost
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/boost
    chown system system /sys/devices/system/cpu/cpufreq/interactive/boostpulse
    chown system system /sys/devices/system/cpu/cpufreq/interactive/input_boost
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/input_boost
    chown system system /sys/devices/system/cpu/cpufreq/interactive/boostpulse_duration
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/boostpulse_duration
    chown system system /sys/devices/system/cpu/cpufreq/interactive/io_is_busy
    chmod 0660 /sys/devices/system/cpu/cpufreq/interactive/io_is_busy

    # Assume SMP uses shared cpufreq policy for all CPUs
    chown system system /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
    chmod 0660 /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq

    chown system system /sys/class/timed_output/vibrator/enable
    chown system system /sys/class/leds/keyboard-backlight/brightness
    chown system system /sys/class/leds/lcd-backlight/brightness
    chown system system /sys/class/leds/button-backlight/brightness
    chown system system /sys/class/leds/jogball-backlight/brightness
    chown system system /sys/class/leds/red/brightness
    chown system system /sys/class/leds/green/brightness
    chown system system /sys/class/leds/blue/brightness
    chown system system /sys/class/leds/red/device/grpfreq
    chown system system /sys/class/leds/red/device/grppwm
    chown system system /sys/class/leds/red/device/blink
    chown system system /sys/class/timed_output/vibrator/enable
    chown system system /sys/module/sco/parameters/disable_esco
    chown system system /sys/kernel/ipv4/tcp_wmem_min
    chown system system /sys/kernel/ipv4/tcp_wmem_def
    chown system system /sys/kernel/ipv4/tcp_wmem_max
    chown system system /sys/kernel/ipv4/tcp_rmem_min
    chown system system /sys/kernel/ipv4/tcp_rmem_def
    chown system system /sys/kernel/ipv4/tcp_rmem_max
    chown root radio /proc/cmdline

    # Define default initial receive window size in segments.
    setprop net.tcp.default_init_rwnd 60

    class_start core

on nonencrypted
    class_start main
    class_start late_start

on property:vold.decrypt=trigger_default_encryption
    start defaultcrypto

on property:vold.decrypt=trigger_encryption
    start surfaceflinger
    start encrypt

on property:sys.init_log_level=*
    loglevel ${sys.init_log_level}

on charger
    class_start charger

on property:vold.decrypt=trigger_reset_main
    class_reset main

on property:vold.decrypt=trigger_load_persist_props
    load_persist_props

on property:vold.decrypt=trigger_post_fs_data
    trigger post-fs-data

on property:vold.decrypt=trigger_restart_min_framework
    class_start main

on property:vold.decrypt=trigger_restart_framework
    class_start main
    class_start late_start

on property:vold.decrypt=trigger_shutdown_framework
    class_reset late_start
    class_reset main

on property:sys.powerctl=*
    powerctl ${sys.powerctl}

# system server cannot write to /proc/sys files,
# and chown/chmod does not work for /proc/sys/ entries.
# So proxy writes through init.
on property:sys.sysctl.extra_free_kbytes=*
    write /proc/sys/vm/extra_free_kbytes ${sys.sysctl.extra_free_kbytes}

# "tcp_default_init_rwnd" Is too long!
on property:sys.sysctl.tcp_def_init_rwnd=*
    write /proc/sys/net/ipv4/tcp_default_init_rwnd ${sys.sysctl.tcp_def_init_rwnd}


## Daemon processes to be run by init.
##
service ueventd /sbin/ueventd
    class core
    critical
    seclabel u:r:ueventd:s0

service logd /system/bin/logd
    class core
    socket logd stream 0666 logd logd
    socket logdr seqpacket 0666 logd logd
    socket logdw dgram 0222 logd logd
    seclabel u:r:logd:s0

service healthd /sbin/healthd
    class core
    critical
    seclabel u:r:healthd:s0

service console /system/bin/sh
    class core
    console
    disabled
    user shell
    group shell log
    seclabel u:r:shell:s0

on property:ro.debuggable=1
    start console

# adbd is controlled via property triggers in init.<platform>.usb.rc
service adbd /sbin/adbd --root_seclabel=u:r:su:s0
    class core
    socket adbd stream 660 system system
    disabled
    seclabel u:r:adbd:s0

# adbd on at boot in emulator
on property:ro.kernel.qemu=1
    start adbd

service lmkd /system/bin/lmkd
    class core
    critical
    socket lmkd seqpacket 0660 system system

service servicemanager /system/bin/servicemanager
    class core
    user system
    group system
    critical
    onrestart restart healthd
    onrestart restart zygote
    onrestart restart media
    onrestart restart surfaceflinger
    onrestart restart drm

service vold /system/bin/vold
    class core
    socket vold stream 0660 root mount
    ioprio be 2

service netd /system/bin/netd
    class main
    socket netd stream 0660 root system
    socket dnsproxyd stream 0660 root inet
    socket mdns stream 0660 root system
    socket fwmarkd stream 0660 root inet

service debuggerd /system/bin/debuggerd
    class main

service debuggerd64 /system/bin/debuggerd64
    class main

service ril-daemon /system/bin/rild
    class main
    socket rild stream 660 root radio
    socket rild-debug stream 660 radio system
    user root
    group radio cache inet misc audio log

service surfaceflinger /system/bin/surfaceflinger
    class core
    user system
    group graphics drmrpc
    onrestart restart zygote

service drm /system/bin/drmserver
    class main
    user drm
    group drm system inet drmrpc

service media /system/bin/mediaserver
    class main
    user media
    group audio camera inet net_bt net_bt_admin net_bw_acct drmrpc mediadrm
    ioprio rt 4

# One shot invocation to deal with encrypted volume.
service defaultcrypto /system/bin/vdc --wait cryptfs mountdefaultencrypted
    disabled
    oneshot
    # vold will set vold.decrypt to trigger_restart_framework (default
    # encryption) or trigger_restart_min_framework (other encryption)

# One shot invocation to encrypt unencrypted volumes
service encrypt /system/bin/vdc --wait cryptfs enablecrypto inplace default
    disabled
    oneshot
    # vold will set vold.decrypt to trigger_restart_framework (default
    # encryption)

service bootanim /system/bin/bootanimation
    class core
    user graphics
    group graphics audio
    disabled
    oneshot

service installd /system/bin/installd
    class main
    socket installd stream 600 system system

service flash_recovery /system/bin/install-recovery.sh
    class main
    seclabel u:r:install_recovery:s0
    oneshot

service racoon /system/bin/racoon
    class main
    socket racoon stream 600 system system
    # IKE uses UDP port 500. Racoon will setuid to vpn after binding the port.
    group vpn net_admin inet
    disabled
    oneshot

service mtpd /system/bin/mtpd
    class main
    socket mtpd stream 600 system system
    user vpn
    group vpn net_admin inet net_raw
    disabled
    oneshot

service keystore /system/bin/keystore /data/misc/keystore
    class main
    user keystore
    group keystore drmrpc

service dumpstate /system/bin/dumpstate -s
    class main
    socket dumpstate stream 0660 shell log
    disabled
    oneshot

service mdnsd /system/bin/mdnsd
    class main
    user mdnsr
    group inet net_raw
    socket mdnsd stream 0660 mdnsr inet
    disabled
    oneshot

service pre-recovery /system/bin/uncrypt
    class main
    disabled
    oneshot

初略的查看了下上面的脚本，可能会发现在源码开头用import导入了其他的配置文件

import /init.environ.rc
import /init.usb.rc
import /init.${ro.hardware}.rc
import /init.${ro.zygote}.rc
import /init.trace.rc

这些文件都在手机的根目录

Paste_Image.png

对应到源码路径为system/core/rootdir和device/{vendor}/{hardware}/

对于以上语法详细介绍可以参考下面这个文件（/system/core/init/readme.txt）

readme.txt

Android Init Language
---------------------

The Android Init Language consists of four broad classes of statements,
which are Actions, Commands, Services, and Options.

All of these are line-oriented, consisting of tokens separated by
whitespace.  The c-style backslash escapes may be used to insert
whitespace into a token.  Double quotes may also be used to prevent
whitespace from breaking text into multiple tokens.  The backslash,
when it is the last character on a line, may be used for line-folding.

Lines which start with a # (leading whitespace allowed) are comments.

Actions and Services implicitly declare a new section.  All 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 or Service
is declared with the same name as an existing one, it is ignored as
an error.  (??? should we override instead)


Actions
-------
Actions are named sequences of commands.  Actions have a trigger which
is used to determine when the action should occur.  When an event
occurs which matches an action's trigger, that action is added to
the tail of a to-be-executed queue (unless it is already on the
queue).

Each action in the queue is dequeued in sequence and each command in
that action is executed in sequence.  Init handles other activities
(device creation/destruction, property setting, process restarting)
"between" the execution of the commands in activities.

Actions take the form of:

on <trigger>
   <command>
   <command>
   <command>


Services
--------
Services are programs which init launches and (optionally) restarts
when they exit.  Services take the form of:

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


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

critical
   This is a device-critical service. If it exits more than four times in
   four minutes, the device will reboot into recovery mode.

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

setenv <name> <value>
   Set the environment variable <name> to <value> in the launched process.

socket <name> <type> <perm> [ <user> [ <group> [ <context> ] ] ]
   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.
   Context 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 <securitycontext>
  Change to securitycontext 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.

Triggers
--------
   Triggers are strings which can be used to match certain kinds
   of events and used to cause an action to occur.

boot
   This is the first trigger that will occur when init starts
   (after /init.conf is loaded)

<name>=<value>
   Triggers of this form occur when the property <name> is set
   to the specific value <value>.

device-added-<path>
device-removed-<path>
   Triggers of these forms occur when a device node is added
   or removed.

service-exited-<name>
   Triggers of this form occur when the specified service exits.


Commands
--------

exec <path> [ <argument> ]*
   Fork and execute a program (<path>).  This will block until
   the program completes execution.  It is best to avoid exec
   as unlike the builtin commands, it runs the risk of getting
   init "stuck". (??? maybe there should be a timeout?)

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)

ifup <interface>
   Bring the network interface <interface> online.

import <filename>
   Parse an init config file, extending the current configuration.

hostname <name>
   Set the host name.

chdir <directory>
   Change working directory.

chmod <octal-mode> <path>
   Change file access permissions.

chown <owner> <group> <path>
   Change file owner and group.

chroot <directory>
  Change process root directory.

class_start <serviceclass>
   Start all services of the specified class if they are
   not already running.

class_stop <serviceclass>
   Stop all services of the specified class if they are
   currently running.

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


insmod <path>
   Install the module at <path>

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.

mount <type> <device> <dir> [ <mountoption> ]*
   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.
   <mountoption>s include "ro", "rw", "remount", "noatime", ...

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.
   Do NOT use this with paths leading to shell-writable or app-writable
   directories, e.g. /data/local/tmp, /data/data or any prefix thereof.

setcon <securitycontext>
   Set the current process security context to the specified string.
   This is typically only used from early-init to set the init context
   before any other process is started.

setenforce 0|1
   Set the SELinux system-wide enforcing status.
   0 is permissive (i.e. log but do not deny), 1 is enforcing.

setkey
   TBD

setprop <name> <value>
   Set system property <name> to <value>.

setrlimit <resource> <cur> <max>
   Set the rlimit for a resource.

setsebool <name> <value>
   Set SELinux boolean <name> to <value>.
   <value> may be 1|true|on or 0|false|off

start <service>
   Start a service running if it is not already running.

stop <service>
   Stop a service from running if it is currently running.

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.

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> <string>
   Open the file at <path> and write a string to it with write(2)
   without appending.


Properties
----------
Init updates some system properties to provide some insight into
what it's doing:

init.action 
   Equal to the name of the action currently being executed or "" if none

init.command
   Equal to the command being executed or "" if none.

init.svc.<name>
   State of a named service ("stopped", "running", "restarting")


Example init.conf
-----------------

# not complete -- just providing some examples of usage
#
on boot
   export PATH /sbin:/system/sbin:/system/bin
   export LD_LIBRARY_PATH /system/lib

   mkdir /dev
   mkdir /proc
   mkdir /sys

   mount tmpfs tmpfs /dev
   mkdir /dev/pts
   mkdir /dev/socket
   mount devpts devpts /dev/pts
   mount proc proc /proc
   mount sysfs sysfs /sys

   write /proc/cpu/alignment 4

   ifup lo

   hostname localhost
   domainname localhost

   mount yaffs2 mtd@system /system
   mount yaffs2 mtd@userdata /data

   import /system/etc/init.conf

   class_start default

service adbd /sbin/adbd
   user adb
   group adb

service usbd /system/bin/usbd -r
   user usbd
   group usbd
   socket usbd 666

service zygote /system/bin/app_process -Xzygote /system/bin --zygote
   socket zygote 666

service runtime /system/bin/runtime
   user system
   group system

on device-added-/dev/compass
   start akmd

on device-removed-/dev/compass
   stop akmd

service akmd /sbin/akmd
   disabled
   user akmd
   group akmd

Debugging notes
---------------
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
Andoird program logwrapper. This will redirect stdout/stderr into the
Android logging system (accessed via logcat).

For example
service akmd /system/bin/logwrapper /sbin/akmd

init.rc通常可以定义为以下几种结构：

1. Actioins

他是一组命令的集合，每个Actions都可以定义一个触发器，格式如下：

on   <trigger> 
      <command> 
      <command> 
      <command>

启动<command>类似于shell命令，对应一个函数，通常执行一条操作，比如启动akmd

2. Services

可以看到上面启动了zygote进程其他他就是app_process进程（system/bin/app_process，源码frameworks/base/cmds/app_process/app_main.cpp）

service zygote /system/bin/app_process -Xzygote /system/bin --zygote socket zygote 666

参考：
Android的init过程详解（一）

[初始化语言（init.rc）解析]