Android ADB device offline

When I run the adb command to list devices, it show my connected device with "offline", and I cannot issue any command on it.

My solution is to run Task Manager (on Ubuntu) and kill the running adb, then re-run adb again, problem solved:)

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Thanks for Vladislav Ivanov comment. You can also enter the command:

adb kill-server

and then
adb start-server

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Related:
- Setup Android adb for Wifi debug