Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 6255 (apt) - open (11: Resource temporarily unavailable) code example

Example: why upgrade ubuntu then frequently shows this message "It iwating for cache lock: Could not get lock /var/lib/dpkg/lock.frontend.

It iwating for cache lock: Could not get lock /var/lib/dpkg/lock.frontend.