How to know if a background job is finished?

If you want to fire off a background job, do some other stuff, then stop and wait for the background job to finish, you can do

nohup do_something &
pid=$!
...more stuff...
wait $pid

Alternatively, you can test for the job having exited like this:

nohup do_something &
pid=$!
...more stuff...
ps -p $pid > /dev/null
[ $? == 1 ] && echo "it's gone, buddy!"