Edsger Wybe Dijkstra (EWD) was one of the most influential members of
computing science’s founding generation and he honored by ACM Turing
Award. URL: http://www.cs.utexas.edu/users/EWD/

One Day One Command
===================

lsof — LiSt Open Files.

Summary:

lsof lists all open files belonging to all active processes. An open
file may be a regular file, a Dir, a special file, an executing text
reference, a library, a stream or a network file (Internet socket, NFS
file or UNIX domain socket).          

Examples:

$ lsof — List all open files.

$ lsof -i — List all open Internet files.

$ lsof -U — List all open Unix Domain files.

$ lsof -u user1 — List all files opened by User1.

$ lsof -p 1234 — List all files opened by process with PID of 1234.

$ lsof /dev/hda2 — List all open files on device /dev/hda2.

$ lsof -s — List all open files with size.

$ lsof -r 5 — Repeat the command for every 5 Sec.

Read: man lsof (More examples are available)