Finding The Number Of Open File Descriptors Per Process On Linux And Unix

Hey There,

Today, we're going to take a look at a fairly simple process (no pun intended), but one that (perhaps) doesn't come up enough in our workaday environments that the answer comes to mind as obviously as it should. How does one find the number of open file descriptors being used by any given process?

The question is a bit of a trick, in and of itself, since some folks define "open file descriptors" as the number of files any given process has open at any given time. For our purposes, we'll be very strict, and make the (usually fairly large) distinction between "files open" and "open file descriptors."

Generally, the two easiest ways to find out how many "open files" a process has, at any given point in time, are to use the same utilities you'd use to find a process that's using a network port. On most Linux flavours, you can do this easily with lsof, and on most Unix flavours you can find it with a proc command, such as pfiles for Solaris.

This is where the difference in definitions makes a huge difference in outcome. Both pfiles and lsof report on information for "open files," rather than "open file descriptors," exclusively. So, if, for instance, we were running lsof on Linux against a simple shell process we might see output like this (all output dummied-up to a certain degree, to protect the innocent ;)

host # lsof -p 2034
CMD      PID USER   FD  TYPE DEVICE    SIZE  NODE NAME
process  2034  user1  cwd   DIR    3,5    4096 49430 /tmp/r (deleted)
process  2034  user1  rtd   DIR    3,7    1024     2 /
process  2034  user1  txt   REG    3,5  201840 49439 /tmp/r/process (deleted)
process  2034  user1  mem   REG    3,7  340771 40255 /lib/ld-2.1.3.so
process  2034  user1  mem   REG    3,7 4101836 40258 /lib/libc-2.1.3.so
process  2034  user1    0u  CHR  136,9         29484 /dev/pts/9
process  2034  user1    1u  CHR  136,9         29484 /dev/pts/9
process  2034  user1    2u  CHR  136,9         29484 /dev/pts/9
process  2034  user1    4r  CHR    5,0         29477 /dev/tty

However, if we check this same output by interrogating the /proc filesystem, we get much different results:

host # ls -l /proc/2034/fd/
total 0
lrwx------    1 user1   user1         64 Jul 30 15:16 0 -> /dev/pts/9
lrwx------    1 user1   user1         64 Jul 30 15:16 1 -> /dev/pts/9
lrwx------    1 user1   user1         64 Jul 30 15:16 2 -> /dev/pts/9
lrwx------    1 user1   user1         64 Jul 30 15:16 4 -> /dev/tty

So, we see that, although this one particular process has more than 4 "open files," it actually only has 4 "open file descriptors."

An easy way to iterate through each processes open file descriptors is to just run a simple shell loop, substituting your particular version of ps's arguments, like:

host # for x in `ps -ef| awk '{ print $2 }'`;do ls /proc/$x/fd;done

If you're only interested in the number of open file descriptors per process, you can shorten that output up even more:

host # for x in `ps -ef| awk '{ print $2 }'`;do ls /proc/$x/fd|wc -l;done

Here's to being able to over-answer that seemingly simple question in the future ;)

, Mike