$ sudo apt-get upgrade
Message returned :
The following packages have been kept back:
linux-generic-pae linux-headers-generic-pae linux-image-generic-pae
If the dependencies have changed or require another package to be removed, it will be listed as “kept-back”.
$ sudo apt-get update && apt-get upgrade && apt-get dist-upgrade ; to be safe
$ sudo apt-get dist-upgrade ; is the generally recommended solution
Will install all pending upgrades and can remove packages.
There are notes suggesting that it can be Dangerous on a stable system. Unlikely if package list is in order.
$ sudo apt-get update ; just refreshes the list of available packages
$ sudo apt-get upgrade ; will upgrade any installed packages to latest version
$ sudo apt-get dist-upgrade ; same as upgrade but uses a smart conflict resolution
$ sudo apt-get do-release-upgrade ; is for stepping between major releases