Dump what an arbitrary linux process writes to some fd

snoopfd

#!/bin/sh -e
target=${1?usage: $0 pid [fd]}
fd=${2-1}
fifo=fifo.$$
log=log.$$

mkfifo $fifo
echo writing to $log
tee $log <$fifo >/proc/$target/fd/$fd &

gdb -p $target --batch-silent -ex "call (int) open(\"$fifo\", 1)" -ex "call (int) dup2(\$1, $fd)" -ex "call (int) close(\$1)"

rm $fifo
tail -f $log