bash: silently kill background function process

kill $foo_pid
wait $foo_pid 2>/dev/null

BTW, I don't know about your massively cool progress bar, but have you seen Pipe Viewer (pv)? http://www.ivarch.com/programs/pv.shtml

Just came across this myself, and realised "disown" is what we are looking for.

foo &
foo_pid=$!
disown

boring_and_long_command
kill $foo_pid
sleep 10

The death message is being printed because the process is still in the shells list of watched "jobs". The disown command will remove the most recently spawned process from this list so that no debug message will be generated when it is killed, even with SIGKILL (-9).