Can adb start my android device that shows up as "offline"? I can see my device in adb devices. It shows up as "offline".
When it is "ready":

$ adb shell
$ su
# reboot


This would reboot my device.
The exact problem is that my cellphone has been turning off on its own with no specific reason, so I was wondering if I could check the logcat to see the problem. I cannot start my device, I have to remove the battery and then start it, and when I do the log is empty...
But my questions here are:


If adb can detect my device why can't it reboot it?
Why can't I turn on my android device from adb when I can reboot it?
What does offline mean: is it turned on or not? (when it isn't usually it doesn't even show in adb devices)
I only encountered device shown via ADB as offline in two cases:


Device not authorized (ADB security features)
No proper udev rules setup


As the first case is most likely not your problem as these security features were not implemented in android when you asked this question. Your problem is most likely that you dont have proper udev rules.

You can fix this temporary bei starting adb on your computer as root:

adb kill-server; sudo $(which adb) start-server

or permanently by adding proper udev rules for your desired device vendor:

See https://wiki.cyanogenmod.org/w/UDEV for a collection of well-known vendors. if your vendor is not part of this you can add it by yourself. Plug the device via USB to your computer and check the output of: lsusb get the right vendorID of your device and add it to the udev rules file.

Also useful: http://developer.android.com/tools/device.html