Again

Here's the manual page, which explains what it does and provides examples of its usage.

again has at least two uses. First, it can be useful for monitoring system activity by repeatedly calling a command that lists, for example, file sizes or disk usage. Second, it can be used as a cheap-and-dirty wrapper for writing curses applications. For example, again has been used to write a simple yet highly functional keyboard-based file manager. The advantage of this approach is that the application gets clean, fast screen updates without requiring the developer to know the details of curses. While this is arguably inefficient, the additional overhead is generally marginal on today's systems.

Here also are the source, and -- for FreeBSD -- the port. The Makefile with the source should be fairly self-explanatory.

again was developed on FreeBSD, but may (should?) work on other Unix-like operating systems such as Linux.