5

I know the file descriptor of a file opened by a process, but I don't know the process ID. How can I print file names from the Linux command prompt if I know the file descriptor of a file opened by a process?

tohuwawohu
  • 10,303
  • 1
  • 35
  • 63
techfun
  • 417
  • 3
  • 7
  • 10

2 Answers2

5

If you do not know the process ID, you will have to check all processes that have the same fd# open, since file descriptors are not globally unique. The smaller the fd#, the more processes will have it open (e.g. on my system, even if the fd# is around 30, I'd still need to guess between 15 processes, and if I was looking for fd# around 10, then the list would have ~170 processes).

The proc filesystem shows the file descriptors as symlinks under /proc/<pid>/fd.

# ls -l /proc/1/fd
lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/0 -> /dev/null
lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/1 -> /dev/null
lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/2 -> /dev/null
l-wx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/3 -> /dev/kmsg
lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/4 -> anon_inode:[eventpoll]
lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/5 -> anon_inode:[signalfd]
lr-x------ 1 root root 64 Feb 12 22:10 /proc/1/fd/6 -> /sys/fs/cgroup/systemd/
...etc...

For example, to look for fd #5 in all processes:

# ls -l /proc/*/fd/5
lrwx------ 1 root    root    64 Feb 12 22:10 /proc/1/fd/5 -> anon_inode:[signalfd]
lrwx------ 1 root    root    64 Feb 12 22:15 /proc/129/fd/5 -> socket:[6980]
lrwx------ 1 root    root    64 Feb 12 22:15 /proc/168/fd/5 -> socket:[7847]
lrwx------ 1 root    root    64 Feb 12 22:15 /proc/341/fd/5 -> anon_inode:[eventfd]
lr-x------ 1 root    root    64 Feb 12 22:15 /proc/342/fd/5 -> anon_inode:inotify
...etc...

The exact interface to resolve symlink targets is readlink():

# readlink /proc/427529/fd/7
/home/grawity/lib/dotfiles/vim/backup/%home%grawity%.bashrc.swp
u1686_grawity
  • 426,297
  • 64
  • 894
  • 966
1

From the lsof manpage:

To find the process that has /u/abe/foo open, use:

lsof /u/abe/foo

See also this tutorial on lsof ans these hints on lsof

Community
  • 1
tohuwawohu
  • 10,303
  • 1
  • 35
  • 63
  • Thanks for response. But Actually.I don't know in this case "/u/able/foo file". Only file descriptor is available – techfun Feb 12 '14 at 20:06
  • Could you please give an example? – tohuwawohu Feb 12 '14 at 20:06
  • I know file descriptor(integer number) of opened by a random process which is getting started on server. It stays open after process call. I need to know the file path. – techfun Feb 12 '14 at 20:08
  • 1
    Sorry, i'm completely confused. [This answer](http://superuser.com/a/716010/84724) tells you how to get all the file names including the path if you know the process. So either you know the file to check, then my answer applies, or you know the process, then the linked answer applies. I simply can't see what's missing :-( – tohuwawohu Feb 12 '14 at 20:13
  • I think there *is* some `lsof` incantation that would list a specific fd# across all processes, but I have no idea what it looks like. – u1686_grawity Feb 12 '14 at 20:19