shell - executing bash loop while command is running

Question

I want to build a bash script that executes a command and in the meanwhile performs other stuff, with the possibility of killing the command if the script is killed. Say, executes a cp of a large file and in the meanwhile prints the elapsed time since copy started, but if the script is killed it kills also the copy. I don't want to use rsync, for 2 reasons: 1) is slow and 2) I want to learn how to do it, it could be useful. I tried this:

until cp SOURCE DEST
do
#evaluates time, stuff, commands, file dimensions, not important now
#and echoes something
done

but it doesn't execute the do - done block, as it is waiting that the copy ends. Could you please suggest something?

Answer 1

until is the opposite of while. It's nothing to do with doing stuff while another command runs. For that you need to run your task in the background with &.

cp SOURCE DEST &
pid=$!

# If this script is killed, kill the `cp'.
trap "kill $pid 2> /dev/null" EXIT

# While copy is running...
while kill -0 $pid 2> /dev/null; do
    # Do stuff
    ...
    sleep 1
done

# Disable the trap on a normal exit.
trap - EXIT

kill -0 checks if a process is running. Note that it doesn't actually signal the process and kill it, as the name might suggest. Not with signal 0, at least.