Tài liệu tham khảo WikiPedia: Biff //edwardbetts.com/find_link?q=Biff http://www.mcsr.olemiss.edu/unixhelp/didyou/biff.h... http://www.unixguide.net/unix/faq/1.3.shtml http://www.catb.org/jargon/html/B/biff.html http://www.freebsd.org/cgi/man.cgi?query=biff&sekt...