Could not see "persist.service.adb.enable" value from default.prop of Android device? I read some guide to get the persist.service.adb.enable value, and it said that it belongs to default.prop file on device. Its content should be:

#
# ADDITIONAL_DEFAULT_PROPERTIES
#
ro.secure=1
ro.debuggable=0
persist.service.adb.enable=0


But when i connect device to PC (Linux), try to pull out this file from device (both emulator and Samsung Galaxy Nexus), the content of default.prop file is only: 

#
# ADDITIONAL_DEFAULT_PROPERTIES
#
ro.secure=1
ro.debuggable=0
persist.sys.usb.config=mass_storage


So I could not see persist.service.adb.enable. Could anyone tell me why, and how can i got it?
According to the official android documentation, another way to get this value is to use the getprop command. (Link takes you to search results for getprop.)

I have not tested this.

I'm assuming you want to also set this value, not just get it? You can use the command line on the phone to do so.  (Disclaimer: I've only tried this in Donut, Froyo, and Gingerbread.)

setprop persist.service.adb.enable 1

This is the USB debugging setting; you can also just turn on USB debugging.