A file descriptor (FD) is an abstract indicator for a file accessing. In Unix-like systems, file descriptors can refer to many different objects besides files, such as pipes, unix domain sockets, and internet sockets.
lsof (list open files) is an open source command to report a list of open files and the processes that opened them. To find all file descriptors used by the process with pid, we may issuing the command:
lsof -p pid
To find all internet sockets used by the process with pid, we may issue:
lsof -i -n -P | grep pid
where, -i specifies listing IP sockets only, -n no translation of hostnames, and -P no translation of port names.
What if lsof is not available on your system?
If your system implements the procfs (proc filesystem, /proc), all file descriptors used by the process with pid can be found in the directory /proc/pid/fd. Therefore, on linux systems, you may issue:
ls -l /proc/pid/fd
to get your job done. However, other approach is needed for FreeBSD systems, since procfs is being gradually phased out on FreeBSD. Both fstat (-- identify active files) and procstat (-- get detailed process information) allow us to achieve our goal. You may issue:
fstat -p pid
or,
procstat -f pid
where, pid is the process id of your interest.
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