what 's difference between android system user and linux root user

摘自： http://stackoverflow.com/questions/16933108/what-s-difference-between-android-system-user-and-linux-root-user
Q： When I connect my phone to computer,then I use "adb shell" command to communicate with my phone and input ps command to output processes info that currently run on my phone,I find two special users,one is root,the other is system,as far as I know,android
based on linux,so root user is the bigest authority user,but I confuse with system user,maybe I can consider it as a normal user in linux system,But it is special in Android,it has a lot of permissions related to android,so Can someone tell me the real different
between root user and system user,and why android need to add a system user?thanks:)

A：

s far as Linux is concerned, the

system

user is just a regular user (UID 1000). Android services however give it special permissions and you can get access to pretty much anything. Most services have code like this somewhere:

private static final void enforceSystemOrRoot(String message) {
final int uid = Binder.getCallingUid();
if (uid != Process.SYSTEM_UID && uid != 0) {
throw new SecurityException(message);
}
}

What that does is deny access to anyone who is not root or

system

. The

shell

user (UID 2000) (what you get when doing

adb shell

) is another user with a lot of power (member of many groups). Cf.

system

:

$ su 1000
$ id
uid=1000(system) gid=1000(system)
groups=1003(graphics),1004(input),1007(log),1009(mount),
1011(adb),1015(sdcard_rw),1028(sdcard_r),3001(net_bt_admin),
3002(net_bt),3003(inet),3006(net_bw_stats)

shell

:

$ adb shell
shell@android:/ $ id
uid=2000(shell) gid=2000(shell)
groups=1003(graphics),1004(input),1007(log),1009(mount),
1011(adb),1015(sdcard_rw),1028(sdcard_r),3001(net_bt_admin),
3002(net_bt),3003(inet),3006(net_bw_stats)

Android uses a separate user for every app, and system services also have their dedicated users (

media

,

radio

,

wifi

, etc.). Very few thins run as root (mostly native daemons).

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system user can not access other user's files. If you do:ls -l /data/data/com.google.android.gm/shared_prefs/ You will got 'permission denied' If you are root user, you can do anything.You can access all the file system, wipe all the data. You can also reboot the phone, etc.