watch + lsof == Instant Progress Bar

I don't know about you, but most of my direct usage of industrial Linux boils down to creating shell pipelines operating over gigabytes of data files. Occasionally I get a little too ambitious and end up staring at a terminal with no visual for an hour or two. As a lazy, impatient creature this offends my sensibilities!

These pipelines are ultimately pulling their input from disk, and complete when that input is exhausted. The kernel surely knows how far has been read into those files: we can just ask it.

One can poll the position of a file descriptor open for reading with lsof. That's great, but we'd really like a throw-away HUD we can occasionally check between SpongeBob episodes. Watch can help us with that; and by putting the two together we can make a doohick:

function watch_progress {
  local file=$1
  local size=`sudo du -b $file | awk '{print $1}'`
  local pid=${2:-`
    sudo lsof -F p $file | cut -c 2- | head -n 1
  `}

  local watcher=/tmp/watcher-$$
  cat <<EOF > $watcher
file=$file
size=$size
pid=$pid
EOF

  cat <<'EOF' >> $watcher
line=`sudo lsof -o -o 0 -p $pid | grep $file`
position=`echo $line | awk '{print $7}' | cut -c 3-`
progress=`echo "scale=2; 100 * $position / $size" | bc`
echo pid $pid reading $file: $progress% done
EOF

  chmod +x /tmp/watcher-$$
  watch /tmp/watcher-$$
}

Then

watch_progress input-file

..for an instant progress reading ^_^.