[Android]init.rc的文法 Android Init Language

一. init.rc 用來初始化設置

Android 系統運作的第一個使用者空間的程式為 init, 該 code 在 system/core/init/init.c

該檔案中的main() 函數會讀取根目錄下的init.rc進行解析，進行一些簡單的初始化設定。

init.rc的code 在   system/core/rootdir/init.rc， 該腳本被直接安裝到目标系統的根檔案系統中，被init可執行程式解析

init執行code 如下：

init_parse_config_file("/init.rc");
           

其他的.rc 檔案也同樣的用處。檔案名格式大多為：init.XXX.rc

二. init.rc的文法

Android Init Language的文檔介紹在 system/core/init/readme.txt

以下将該文檔簡單的過一遍。

1. Android Init Language由四種類型的語句組成，分别是Actions, Commands, Services, Options.

The Android Init Language consists of four broad classes of statements,

which are Actions, Commands, Services, and Options.

2. 一行語句就是一條command，行可以用反斜杠續行。語句中的關鍵字由空格分開。

如果要表示空格，可用反斜杠。關鍵字中有空格，請用雙引号。

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-foldi

3. #是注釋。

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

4. 一個actions 或 services 會隐含聲明了一個新的段，所有commands 或 options 屬于最近的聲明的段。

将忽略在第一個段之前的 commands 或 options。

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.

5. 每一個actions 和 services 都會有獨一無二的名字。後面與前面發生重名的，那麼這個後面重名的将被當做錯誤一樣忽略掉。

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)

6. Actions， 由on 開始的語句塊。

Actions其實就是一組被命名的指令序列。actions 都有一個觸發條件，觸發條件決定了action何時執行。

當一個事件發生如果比對 action 的觸發條件，那麼這個 action 将會被添加到預備執行隊列的尾部（除非它已經在隊列當中）

每一個action中的指令将被順序執行。init 程序負責在其它activities（如：裝置建立/銷毀，屬性設定，程序重新開機）之間執行這些指令序列。

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.

action 的格式如下

on <trigger>

   <command>

   <command>

   <command>

7. Services

services 是一些由init 啟動 和 重新（如果有需要）啟動的程式，當然這些程式如果是存在的。

Services are programs which init launches and (optionally) restarts

when they exit.  

services 的格式如下：

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

   <option>

   <option>

   ...

8. Options

options 是service的修飾符，用來告訴init 怎樣及何時啟動service。

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

runs the service.

以下都是options

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.
    
           

9. Triggers

如果比對上triggers字串，就會去促發action.

   Triggers are strings which can be used to match certain kinds

   of events and used to cause an action to occur.

以下為trigger的字串

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.
           

10. 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 [email protected] 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.
           

11. 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")
           

12. 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 [email protected] /system
   mount yaffs2 [email protected] /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
           

13. Debugging notes

怎麼樣輸出log。

---------------

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

三. init.rc 添加code

mkdir /cache/recovery