Linux – How to tell what files a process has open

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I have a daemon process and I want to know what files it has open (and ideally what its CWD is). Is there any shell command that can tell me that?

Best Answer

I do love lsof, but I think it's overkill for a simple question like this. The /proc filesystem contains everything you want to know. Perhaps an example would be best:

# ps ax|grep tail
 7196 pts/4    S+     0:00 tail -f /var/log/messages
 8773 pts/0    R+     0:00 grep tail
# ls -l /proc/7196/cwd
lrwxrwxrwx 1 insyte insyte 0 2009-07-29 19:05 /proc/7196/cwd -> /home/insyte
# ls -l /proc/7196/fd
total 0
lrwx------ 1 insyte insyte 64 2009-07-29 19:05 0 -> /dev/pts/4
lrwx------ 1 insyte insyte 64 2009-07-29 19:05 1 -> /dev/pts/4
lrwx------ 1 insyte insyte 64 2009-07-29 19:02 2 -> /dev/pts/4
lr-x------ 1 insyte insyte 64 2009-07-29 19:05 3 -> /var/log/messages

So as you can see, the /proc/$PID directory contains a symlink called "cwd" that links the the CWD of the process. The same is true for the open filedescriptors listed in /proc/$PID/fd.

The /proc/$PID hierarchy contains a wealth of information about all running processes. Worth poking around in!

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