While administrating a box, you may wanted to find out what a processes is doing and find out how many file descriptors (fd) are being used. You will surprised to find out that process does open all sort of files:
=> Actual log file
=> /dev files
=> UNIX Sockets
=> Network sockets
=> Library files /lib /lib64
=> Executables and [...]

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Linux: Find out how many file descriptors are being used