YARA is an extremely flexible identification and classification engine written by Victor Manuel Alvarez of Hipasec Sistemas. It runs on Windows, Linux and Mac OS X, and can be used through its command-line interface or from your own Python scripts with the yara-python extension.
YARA rules are easy to write and understand. They have a syntax that resembles a C struct declaration. However creating thousands of rules takes a lot of time and effort. That’s why it makes more sense to use ClamAV signatures. Usually ClamAV signatures can be found under /usr/local/share/clamav or /usr/lib/clamav on Linux systems. This is where you will find the main.cld and daily.cld. Alternately, they may have .cvd extensions, main.cld file contains the primary base of signatures and daily.cld contains incremental daily updates.
These tools are well explained in the Malware Analyst's Cookbook and DVD book.