Android 在標准Linux系統上擴展了很多用戶。
在Android 4.1中，定義這些用戶的文件為
system/core/include/private/android_filesystem_config.h
在這裡也就直接粘貼過來吧
#define AID_ROOT 0 /* traditional unix root user */
#define AID_SYSTEM 1000 /* system server */
#define AID_RADIO 1001 /* telephony subsystem, RIL */
#define AID_BLUETOOTH 1002 /* bluetooth subsystem */
#define AID_GRAPHICS 1003 /* graphics devices */
#define AID_INPUT 1004 /* input devices */
#define AID_AUDIO 1005 /* audio devices */
#define AID_CAMERA 1006 /* camera devices */
#define AID_LOG 1007 /* log devices */
#define AID_COMPASS 1008 /* compass device */
#define AID_MOUNT 1009 /* mountd socket */
#define AID_WIFI 1010 /* wifi subsystem */
#define AID_ADB 1011 /* android debug bridge (adbd) */
#define AID_INSTALL 1012 /* group for installing packages */
#define AID_MEDIA 1013 /* mediaserver process */
#define AID_DHCP 1014 /* dhcp client */
#define AID_SDCARD_RW 1015 /* external storage write access */
#define AID_VPN 1016 /* vpn system */
#define AID_KEYSTORE 1017 /* keystore subsystem */
#define AID_USB 1018 /* USB devices */
#define AID_DRM 1019 /* DRM server */
#define AID_MDNSR 1020 /* MulticastDNSResponder (service discovery) */
#define AID_GPS 1021 /* GPS daemon */
#define AID_UNUSED1 1022 /* deprecated, DO NOT USE */
#define AID_MEDIA_RW 1023 /* internal media storage write access */
#define AID_MTP 1024 /* MTP USB driver access */
#define AID_UNUSED2 1025 /* deprecated, DO NOT USE */
#define AID_DRMRPC 1026 /* group for drm rpc */
#define AID_NFC 1027 /* nfc subsystem */
#define AID_SDCARD_R 1028 /* external storage read access */
#define AID_SHELL 2000 /* adb and debug shell user */
#define AID_CACHE 2001 /* cache access */
#define AID_DIAG 2002 /* access to diagnostic resources */
/* The 3000 series are intended for use as supplemental group id's only.
* They indicate special Android capabilities that the kernel is aware of. */
#define AID_NET_BT_ADMIN 3001 /* bluetooth: create any socket */
#define AID_NET_BT 3002 /* bluetooth: create sco, rfcomm or l2cap sockets */
#define AID_INET 3003 /* can create AF_INET and AF_INET6 sockets */
#define AID_NET_RAW 3004 /* can create raw INET sockets */
#define AID_NET_ADMIN 3005 /* can configure interfaces and routing tables. */
#define AID_NET_BW_STATS 3006 /* read bandwidth statistics */
#define AID_NET_BW_ACCT 3007 /* change bandwidth statistics accounting */
#define AID_QCOM_ONCRPC 3008 /* can read/write /dev/oncrpc files */
#if defined(MOTOROLA_UIDS)
#define AID_MOT_OSH 5000 /* OSH */
#define AID_MOT_ACCY 9000 /* access to accessory */
#define AID_MOT_PWRIC 9001 /* power IC */
#define AID_MOT_USB 9002 /* mot usb */
#define AID_MOT_DRM 9003 /* can access DRM resource. */
#define AID_MOT_TCMD 9004 /* mot_tcmd */
#define AID_MOT_SEC_RTC 9005 /* mot cpcap rtc */
#define AID_MOT_TOMBSTONE 9006
#define AID_MOT_TPAPI 9007 /* mot_tpapi */
#define AID_MOT_SECCLKD 9008 /* mot_secclkd */
#define AID_MOT_WHISPER 9009 /* Whisper Protocol access */
#define AID_MOT_CAIF 9010 /* can create CAIF sockets */
#define AID_MOT_DLNA 9011 /* DLNA native */
#endif // MOTOROLA_UIDS
#define AID_MISC 9998 /* access to misc storage */
#define AID_NOBODY 9999
#define AID_APP 10000 /* first app user */
#define AID_ISOLATED_START 99000 /* start of uids for fully isolated sandboxed processes */
#define AID_ISOLATED_END 99999 /* end of uids for fully isolated sandboxed processes */
#define AID_USER 100000 /* offset for uid ranges for each user */
普通應用程序使用的user ID 為AID_USER + app_id
比如u0_a46 1103 158 474424 30180 ffffffff 400afa70 S com.yixia.vitamio.media
它的user ID 就為10046
這裡需要討論的是，如何讓一個app具有system權限，這樣才可以對/data目錄下的文件進行修改操作。
方法很簡單，只需要在該app的AndroidManifest.xml中加一行代碼即可
在開頭處加上 android:sharedUserId="android.uid.system"
這樣之後，所編譯出來的app就是system用戶了
system 547 158 473676 39788 ffffffff 400afa70 S com.android.systemui
原來的一個普通app，現在變為system，就可以對系統文件進行操作了。
