You could also use locate to look for commands. Why do people use locate if find does the job? Because locate is much faster than find since it just searches through database(s) of indexed locations to find your file/regex.

Examples:

locate some-file.avi searches through database(s) of almost every file on the disk for a file called "some-file.avi".

locate -i "some-file.avi" will ignore the case of the file you are searching for.

locate -i "*.txt" will display a list of locations of all the files with **.txt* extension on your system.

man locate for more info on the file.

You might need to run updatedb first to ensure the index database is up to date, otherwise, 'locate' might not return what you are looking for.

Start by clicking the "Home Folder" icon in the launcher.

In the window that opens, click "Search".

Type what you want to search for in the box, then press enter.

Under the dropdown for location, choose your hard drive, then click reload.

The results will then be displayed. Hope that helps!

If you're looking for a string inside a file, you can use grep. Here's a sample command:

grep -r -i "some string" /home/yourusername

This will find "some string" in /home/yourusername directory. The search will ignore case (-i) and recurse directories (-r). You can use / as the directory to search in the whole directory but that might not be very efficient.

On Ubuntu, i know that everyone wants to be dogmatic about using command line all the time, and I have in the past been that way, but I love the Gnome "Search for files..." tool. I think its awesome.

For a Desktop setup, Install "gnome-search-tool"
sudo apt-get install gnome-search-tool

Don't install this in a server, it will also install the ubuntu desktop package.
Thanks to @Rinzwind for pointing that out.

Filters inlude:

1. Choose the folder you want to search. In this case \
2. Contains the text - this is the file name.
3. Date Modified - less or more than in number of days.
4. File size - at least or at most a specified size. You can also search empty files.
5. File owner - user, group or unrecognized owner.
6. Regular expressions - matches regular expressions.
7. You can choose to show hidden files, follow symbolic links or exclude other filesystems.

If you don't want to remember find (which is a very powerful) parameters you could install from official repos:

• kfind - file search utility
• gnome-search-tool - GNOME tool to search files
• catfish - File searching tool which is configurable via the command line

The three of them are great, but I found kfind the best.

From Ubuntu Community help wiki you might find useful:

• dpkg -L|--listfiles ... List files `owned' by package(s).
• dpkg -S|--search ... Find package(s) owning file(s).

If need to find nested in some dirs:

find / -type f -wholename "*foo/bar/filename"