SHELL

waiting for cache lock, Could not get lock /var/lib/dpkg/lock-frontend.

sudo lsof /var/lib/dpkg/lock
or 
sudo lsof /var/lib/apt/lists/lock
or
sudo lsof /var/lib/dpkg/lock-frontend

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.

Ubuntu Waiting for cache lock: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 15551 (apt)

Ubuntu: Waiting for cache lock: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 15551 (apt)