List the Open Files of a Hung Process with lsof on Linux

A process hangs or leaks descriptors and you want to see every file, socket and pipe it holds.

echo data > in.txt
sleep 30 < in.txt > out.txt &
pid=$!
sleep 0.5
lsof -p "$pid" -a -d 0-9
for f in /proc/$pid/fd/*; do echo "fd ${f##*/} -> $(readlink "$f")"; done
kill $pid

COMMAND  PID USER   FD   TYPE DEVICE SIZE/OFF     NODE NAME
sleep   2740 root    0r   REG  0,185        5  3027775 /tmp/run/S1888-1/w/in.txt
sleep   2740 root    1w   REG  0,185        0  3027776 /tmp/run/S1888-1/w/out.txt
sleep   2740 root    2w  FIFO   0,16      0t0 14100374 pipe
fd 0 -> /tmp/run/S1888-1/w/in.txt
fd 1 -> /tmp/run/S1888-1/w/out.txt
fd 2 -> pipe:[14100374]

lsof -p PID prints one line per open file with its descriptor, and -a -d 0-9 combined the PID with a descriptor range so that only the low numbers show. The run shows 0r reading in.txt, 1w writing out.txt and descriptor 2 on a pipe. The same facts come from /proc/PID/fd, where each entry is a link to the target, which the loop prints with readlink; the pipe is named by its inode. See proc_pid_fd.

lsof can take a long time on a busy system, and without root it shows only your own processes. -a makes the options AND together, and without it they OR, which gives much more output than expected. A deleted file shows (deleted) after its name while some process still holds it open.