Here are some of the more frequently used commands that I find useful.
To search for a particular package by name or description:
From the command-line, use:
apt-cache search keyword
The apt tool on Ubuntu 14.04 and above makes this very easy.
apt list --installed
On older versions of Ubuntu/Debian, try this instead:
dpkg --get-selections | grep -v deinstall
Check available version of a package in Ubuntu repositories from command line:
You may wonder if the given package is installed or not. It’s easy to find out too.
In the above output, you see two words namely Installed and Candidate.
Installed : This will tell you the version that you have currently installed in your Ubuntu system.
Candidate : This is actual version that will be installed from the Ubuntu repositories when you install the package using apt-get.
If your system is in state where apt-get is mostly unusable you could try using dpkg to remove the affected package, in this case try:
sudo dpkg --purge php5-memcache