init.rc文件的分析
init.rc文件有两份，一个对应于normal mode，一个是recovery mode.分别对应如下文件：
- normal mode: ./system/core/rootdir/init.rc
- recovery mode: ./bootable/recovery/etc/init.rc
init.rc文件的语法&作用，定义在./system/core/init/readme.txt。为阅读方便，拷贝如下：
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
Services
--------
Services are programs which init launches and (optionally) restarts
when they exit. Services take the form of:
service [ ]*
从顶层来讲，可以认为init.rc包括两种：action & service. 其中action的语法如下：
on
即当某个触发条件(trigger)发生的时候，这个action就执行一系列的command.
service的语法是：
service [ ]*
每个服务有个名字(name)，以及该服务对应的程序（pathname, 二进制可执行程序或者shell脚本等）；此外，可以给这个程序传入参数(argument).
service包括一系列的option，用于规定service的执行规则等。
init进程在解析init.rc的时候，会主动去启动这些服务；如：
service flash_recovery /system/etc/install-recovery.sh
class main
oneshot
或者在在action中引用它，如：
# adbd is controlled via property triggers in init..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
或
service console /system/bin/sh
class core
console
disabled
user shell
group log
seclabel u:r:shell:s0
on property:ro.debuggable=1
start console
init.rc中可以使用的command，定义在./system/core/init/keyword.h中，如下：
#ifndef KEYWORD
int do_chroot(int nargs, char **args);
int do_chdir(int nargs, char **args);
int do_class_start(int nargs, char **args);
int do_class_stop(int nargs, char **args);
int do_class_reset(int nargs, char **args);
int do_domainname(int nargs, char **args);
int do_exec(int nargs, char **args);
int do_export(int nargs, char **args);
int do_hostname(int nargs, char **args);
int do_ifup(int nargs, char **args);
int do_insmod(int nargs, char **args);
int do_mkdir(int nargs, char **args);
int do_mount_all(int nargs, char **args);
int do_mount(int nargs, char **args);
int do_powerctl(int nargs, char **args);
int do_restart(int nargs, char **args);
int do_restorecon(int nargs, char **args);
int do_restorecon_recursive(int nargs, char **args);
int do_rm(int nargs, char **args);
int do_rmdir(int nargs, char **args);
int do_setcon(int nargs, char **args);
int do_setenforce(int nargs, char **args);
int do_setkey(int nargs, char **args);
int do_setprop(int nargs, char **args);
int do_setrlimit(int nargs, char **args);
int do_setsebool(int nargs, char **args);
int do_start(int nargs, char **args);
int do_stop(int nargs, char **args);
int do_swapon_all(int nargs, char **args);
int do_trigger(int nargs, char **args);
int do_symlink(int nargs, char **args);
int do_sysclktz(int nargs, char **args);
int do_write(int nargs, char **args);
int do_copy(int nargs, char **args);
int do_chown(int nargs, char **args);
int do_chmod(int nargs, char **args);
int do_loglevel(int nargs, char **args);
int do_load_persist_props(int nargs, char **args);
int do_wait(int nargs, char **args);
#define __MAKE_KEYWORD_ENUM__
#define KEYWORD(symbol, flags, nargs, func) K_##symbol,
enum {
K_UNKNOWN,
#endif
KEYWORD(capability, OPTION, 0, 0)
KEYWORD(chdir, COMMAND, 1, do_chdir)
KEYWORD(chroot, COMMAND, 1, do_chroot)
KEYWORD(class, OPTION, 0, 0)
KEYWORD(class_start, COMMAND, 1, do_class_start)
KEYWORD(class_stop, COMMAND, 1, do_class_stop)
KEYWORD(class_reset, COMMAND, 1, do_class_reset)
KEYWORD(console, OPTION, 0, 0)
KEYWORD(critical, OPTION, 0, 0)
KEYWORD(disabled, OPTION, 0, 0)
KEYWORD(domainname, COMMAND, 1, do_domainname)
KEYWORD(exec, COMMAND, 1, do_exec)
KEYWORD(export, COMMAND, 2, do_export)
KEYWORD(group, OPTION, 0, 0)
KEYWORD(hostname, COMMAND, 1, do_hostname)
KEYWORD(ifup, COMMAND, 1, do_ifup)
KEYWORD(insmod, COMMAND, 1, do_insmod)
KEYWORD(import, SECTION, 1, 0)
KEYWORD(keycodes, OPTION, 0, 0)
KEYWORD(mkdir, COMMAND, 1, do_mkdir)
KEYWORD(mount_all, COMMAND, 1, do_mount_all)
KEYWORD(mount, COMMAND, 3, do_mount)
KEYWORD(on, SECTION, 0, 0)
KEYWORD(oneshot, OPTION, 0, 0)
KEYWORD(onrestart, OPTION, 0, 0)
KEYWORD(powerctl, COMMAND, 1, do_powerctl)
KEYWORD(restart, COMMAND, 1, do_restart)
KEYWORD(restorecon, COMMAND, 1, do_restorecon)
KEYWORD(restorecon_recursive, COMMAND, 1, do_restorecon_recursive)
KEYWORD(rm, COMMAND, 1, do_rm)
KEYWORD(rmdir, COMMAND, 1, do_rmdir)
KEYWORD(seclabel, OPTION, 0, 0)
KEYWORD(service, SECTION, 0, 0)
KEYWORD(setcon, COMMAND, 1, do_setcon)
KEYWORD(setenforce, COMMAND, 1, do_setenforce)
KEYWORD(setenv, OPTION, 2, 0)
KEYWORD(setkey, COMMAND, 0, do_setkey)
KEYWORD(setprop, COMMAND, 2, do_setprop)
KEYWORD(setrlimit, COMMAND, 3, do_setrlimit)
KEYWORD(setsebool, COMMAND, 2, do_setsebool)
KEYWORD(socket, OPTION, 0, 0)
KEYWORD(start, COMMAND, 1, do_start)
KEYWORD(stop, COMMAND, 1, do_stop)
KEYWORD(swapon_all, COMMAND, 1, do_swapon_all)
KEYWORD(trigger, COMMAND, 1, do_trigger)
KEYWORD(symlink, COMMAND, 1, do_symlink)
KEYWORD(sysclktz, COMMAND, 1, do_sysclktz)
KEYWORD(user, OPTION, 0, 0)
KEYWORD(wait, COMMAND, 1, do_wait)
KEYWORD(write, COMMAND, 2, do_write)
KEYWORD(copy, COMMAND, 2, do_copy)
KEYWORD(chown, COMMAND, 2, do_chown)
KEYWORD(chmod, COMMAND, 2, do_chmod)
KEYWORD(loglevel, COMMAND, 1, do_loglevel)
KEYWORD(load_persist_props, COMMAND, 0, do_load_persist_props)
KEYWORD(ioprio, OPTION, 0, 0)
#ifdef __MAKE_KEYWORD_ENUM__
KEYWORD_COUNT,
};
#undef __MAKE_KEYWORD_ENUM__
#undef KEYWORD
#endif
而以上各个命令对应的函数也可以在init目录下面找到，比如mkdir命令对应的函数名称是do_mkdir：
KEYWORD(mkdir, COMMAND, 1, do_mkdir)其原型是：
int do_mkdir(int nargs, char **args);具体实现在./system/core/init/buildins.c中：
int do_mkdir(int nargs, char **args)
{
mode_t mode = 0755;
int ret;
/* mkdir [mode] [owner] [group] */
if (nargs >= 3) {
mode = strtoul(args[2], 0, 8);
}
ret = make_dir(args[1], mode);
/* chmod in case the directory already exists */
if (ret == -1 && errno == EEXIST) {
ret = _chmod(args[1], mode);
}
if (ret == -1) {
return -errno;
}
if (nargs >= 4) {
uid_t uid = decode_uid(args[3]);
gid_t gid = -1;
if (nargs == 5) {
gid = decode_uid(args[4]);
}
if (_chown(args[1], uid, gid) < 0) {
return -errno;
}
/* chown may have cleared S_ISUID and S_ISGID, chmod again */
if (mode & (S_ISUID | S_ISGID)) {
ret = _chmod(args[1], mode);
if (ret == -1) {
return -errno;
}
}
}
return 0;
}