root nexus7 under arch

download clockwork_recovery.img from http://robertpitt.me/2012/07/rooting-the-nexus-7-via-linux/( perhaps it can be downloaded in its official website, 

download superuser.zip from its official page. The arm edition.

download android-tools which include adb and fastboot.

Make sure the udev rules are for nexus7.

Step:

　　DeveloperOption is invisable for android 4.2, so in about tablet, click 7 times on the device number, the option will appear. Enable usb debug.

　　adb reboot boatloader

　　sudo fastboot oem unlock

　　fastboot flash recovery /path/to/clockwork_recovery.img

　　fastboot erase cache

　　adb reboot recovery

choose to install zip from sdcard, find the supersu.zip, install it, than thoose the "yes, unable recovery flash"

OK.