Error: device offline

The emulator is running. But when I write adb shell in my shell I get:

error:device offline

What is the reason for this? How do I get rid of it?

How do I authorize a device on adb?

Open a command window and enter "adb devices". Watch the device's screen for any Authorization message and allow the connection.

How do I restart using adb?

This is exactly what you think it is — a way to reboot your device from the command line. Running it is simple: just type adb reboot and enter. Before you say "I can just push the button!" you have to understand that these commands can be scripted, and your device can reboot in the middle of a script if you need it to.

If "adb devices" lists your device but as "offline" chances are your path leads to an old version of adb (eg 1.0.29) which has problems with latest Android 4 devices. Make sure "adb version" returns 1.0.31 or greater. Starting with Android 4.2.2, you must confirm on your device that it is being attached to a trusted computer. It will work with adb version 1.0.31 and above.