#!/bin/bash

pipe=/tmp/ffmpeg

trap "rm -f $pipe" EXIT

# creating the FIFO
[[ -p $pipe ]] || mkfifo $pipe

while true; do
    # can't just use "while read line" if we
    # want this script to continue running.
    read line < $pipe

    # now implementing a bit of security,
    # feel free to improve it.
    # we ensure that the command is a ffmpeg one.
    [[ $line =~ ^ffmpeg ]] && bash <<< "$line"
done

echo "ffmpeg -version" > /tmp/ffmpeg

if [[ -p /tmp/ffmpeg ]]; then
    echo "ffmpeg -version" > /tmp/ffmpeg
else
    echo >&2 "ffmpeg FIFO isn't open :/"
fi