Here are some of the more frequently used commands that I find useful.

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To search for a particular package by name or description:
From the command-line, use:


apt-cache search keyword

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The apt tool on Ubuntu 14.04 and above makes this very easy.


apt list --installed


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On older versions of  Ubuntu/Debian, try this instead:

dpkg --get-selections | grep -v deinstall


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Check available version of a package in Ubuntu repositories from command line:

apt-cache policy


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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.

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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