Optimistic locking for shell commands

Lock is a tool to make sure you run something only once at a time. For example:

$ lock foo sleep 60 &
[1] pidX
$ lock foo echo hi
Waiting for process pidX to release lock foo...
Waiting for process pidX to release lock foo...
Waiting for process pidX to release lock foo...
(60 times)
hi
[1]+  Done
$

Because both commands were run with the lock "foo", the second waited for the
first to return before running.