Android启动流程分析(六) init.rc的语法

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在init.rc的解析之前，我们要先了解init.rc里面到底写了什么，它的语法是怎么样的。

其实，非常建议去读一下android关于init.rc的说明文件：system/core/init/readme.txt

这个里面，已经讲的非常的清楚。但是这里，还是简单的总结一下。

init.rc首先是位于system/core/rootdir/init.rc.

我们截取两个小段来看一下：

import /init.environ.rcimport /init.usb.rcimport /init.${ro.hardware}.rcimport /init.${ro.zygote}.rcimport /init.trace.rc..........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...........## Daemon processes to be run by init.##service ueventd /sbin/ueventd    class core    critical    seclabel u:r:ueventd:s0service 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:s0service healthd /sbin/healthd    class core    critical    seclabel u:r:healthd:s0

从这一小段的代码中，可以分析init.rc的主要语法。

首先，init.rc中有三个关键字，分别为import, on, service.

on代表的是action，这个表明它下面是一组需要执行的command

其格式如下：

 on <trigger>
          <command>

          <command>

service代表的是服务，是由init进程启动，或者重新启动的程序

其格式如下：

    service <name> <pathname> [ <argument> ]*
          <option>
          <option>
command是在满足trigger后，需要执行的命令

option是service的修饰符，表示的是何时并且如何启动service的程序。

另外，其他的option和command的下属关键字，可以参考readme.txt.

这里仅仅贴上做为参考：

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 bywhitespace.  The c-style backslash escapes may be used to insertwhitespace into a token.  Double quotes may also be used to preventwhitespace 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 commandsor options belong to the section most recently declared.  Commandsor options before the first section are ignored.Actions and Services have unique names.  If a second Action or Serviceis declared with the same name as an existing one, it is ignored asan error.  (??? should we override instead)Actions-------Actions are named sequences of commands.  Actions have a trigger whichis used to determine when the action should occur.  When an eventoccurs which matches an action's trigger, that action is added tothe tail of a to-be-executed queue (unless it is already on thequeue).Each action in the queue is dequeued in sequence and each command inthat 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) restartswhen they exit.  Services take the form of:service <name> <pathname> [ <argument> ]*   <option>   <option>   ...Options-------Options are modifiers to services.  They affect how and when initruns 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_hardwareinsmod <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   TBDsetprop <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|offstart <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 intowhat it's doing:init.action    Equal to the name of the action currently being executed or "" if noneinit.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 defaultservice adbd /sbin/adbd   user adb   group adbservice usbd /system/bin/usbd -r   user usbd   group usbd   socket usbd 666service zygote /system/bin/app_process -Xzygote /system/bin --zygote   socket zygote 666service runtime /system/bin/runtime   user system   group systemon device-added-/dev/compass   start akmdon device-removed-/dev/compass   stop akmdservice akmd /sbin/akmd   disabled   user akmd   group akmdDebugging 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 theAndoird program logwrapper. This will redirect stdout/stderr into theAndroid logging system (accessed via logcat).For exampleservice akmd /system/bin/logwrapper /sbin/akmd

在有了这些基础知识后，我们再去接着分析init.rc的parse过程。