flyinweb's blog

 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 other programs etc

The best way to find out this information is use lsof command or explore /proc/PID directory for each running processes (including kernel processes), containing information about that process.

Step # 1 Find out program PID

Let us find out PID for mysqld process
# ps aux | grep mysqld
OR
# pidof mysqld
Output:

28290

Step # 2 List file opened by pid 28290

Use lsof command or /proc/PID file system to display fd lists:
# lsof -p 28290
OR
# cd /proc/28290/fd
# ls -l | less
You can count open file, enter:
# ls -l | wc -l

More about /proc/PID & procfs

/proc (or procfs) is a pseudo-file system that it is dynamically generated after each reboot. It is used to access kernel information. procfs is also used by Solaris, BSD, AIX and other UNIX like operating systems.

So now you know how many file descriptors are being used by a process. You will find more interesting stuff in /proc/PID directory. For example:

• /proc/PID/cmdline : process arguments
• /proc/PID/cwd : process current working directory (symlink)
• /proc/PID/exe : path to actual process executable file (symlink)
• /proc/PID/environ : environment used by process
• /proc/PID/root : the root path as seen by the process. For most processes this will be a link to / unless the process is running in a chroot jail.
• /proc/PID/status : basic information about a process including its run state and memory usage.
• /proc/PID/task : hard links to any tasks that have been started by this (the parent) process.

As you see, /proc is an essentials file system for admin work. Just browser through our previous article to get more information about /proc: