entr

Using entr is the new way to do this (it's cross platform). Note entr doesn't use polling giving it a huge advantage over many of the alternatives.

Uses kqueue(2) or inotify(7) to avoid polling. entr was written to make rapid feedback and automated testing natural and completely ordinary.

On BSD it uses pledge(2)

You can install it with

apt-get install entr
dnf install entr

You can track a directory for new additions using

while $(true); do
  # echo ./my_watch_dir | entr -dnr echo "Running trigger..."
  echo ./my_watch_dir | entr -dnr ##MY COMMAND##
done;

Options explained (from the docs),

• -d Track the directories of regular files provided as input and exit if a new file is added. This option also enables directories to be specified explicitly. Files with names beginning with ‘.’ are ignored.
• -n Run in non-interactive mode. In this mode entr does not attempt to read from the TTY or change its properties.
• -r Reload a persistent child process. As with the standard mode of operation, a utility which terminates is not executed again until a file system or keyboard event is processed. SIGTERM is used to terminate the utility before it is restarted. A process group is created to prevent shell scripts from masking signals. entr waits for the utility to exit to ensure that resources such as sockets have been closed. Control of the TTY is not transferred the child process.