reference to : http://blog.csdn.net/wangkaiblog/article/details/46050587

本来以为存放在／systen／bin／下的monkey、input、am等adb shell命令都是二进制可执行程，偶尔cat input下，发现不是二进制文件，内容如下：

# Script to start "input" on the device, which has a very rudimentary
# shell.
#
base=/system
export CLASSPATH=$base/framework/input.jar
exec app_process $base/bin com.android.commands.input.Input $*

这段脚本的作用就是通过app_process工具启动放在／system／farmework／下的input.jar 文件。我查看了下framework文件夹里面的确有input.jar文件。

What is this app_process

Although not that popular “app_process” command is used heavily in Android, for example the well know “am” script is using it. Everybody knows how to start an application from adb shell:
am start -n yourpackagename/.activityname
However “am” is just a shell script and what stays in the base of the am script is our app_process binary:

root@android:/ # cat /system/bin/am
cat /system/bin/am
# Script to start "am" on the device, which has a very rudimentary
# shell.
base=/system
export CLASSPATH=$base/framework/am.jar
exec app_process $base/bin com.android.commands.am.Am "$@"
root@android:/ #

So app_process is the binary used to start java code. All the apps, instrumentations or installations are tarted using this.

Lets see the official usage output:

root@android:/ # app_process
Usage: app_process [java-options] cmd-dir start-class-name [options]
Not quite complete as usual :)