Here's a list of 10 commands which may come handy when using the command line in Linux:
Search for all files modified in the last N days containing a specific text in their name
find DIR -mtime -N -name "*TEXT*"
Among the commands: Find removed (deleted) files still in use via /proc, On-the-fly unrar movie in .rar archive and play it, does also work on part archives, List programs with open ports and connections, Manually pause/unpause an application (process), etc.
htop - Undoubtedly the most famous of the top-like tools. It implements some extra options to the "top" command, like colors, ability to scroll horizontally and vertically, and a better interaction with the processes listed.
nethogs - network information, but instead of displaying the traffic by protocol, nethogs shows the bandwidth usage by process. Very interesting.
1. Show/purge unused linux kernel images and modules:
dpkg -l 'linux-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d'
2. Rip audio from a video file:
A user a the Ubuntu Forums has compiled a list of interesting and useful terminal commands that can be used in Ubuntu: