Flow control для запущенного скрипта

cat $file | while pv -L $(<speedlimit.txt); do :; done

       -R PID, --remote PID
              If  PID  is  an instance of pv that is already running, -R PID will
              cause that instance to  act  as  though  it  had  been  given  this
              instance's  command  line  instead.   For example, if pv -L 123K is
              running with process ID 9876, then running pv -R 9876 -L 321K  will
              cause  it  to start using a rate limit of 321KiB instead of 123KiB.
              Note that some options cannot be changed while running, such as -c,
              -l, -f, -D, -E, and -S.