textutil

Used to manipulate text files of various formats. More information: https://ss64.com/osx/textutil.html.

Display information about foo.rtf:

textutil -info {{path/to/foo.rtf}}

Convert foo.rtf into foo.html:

textutil -convert {{html}} {{path/to/foo.rtf}}

Convert rich text to normal text:

textutil {{path/to/foo.rtf}} -convert {{txt}}

Convert foo.txt into foo.rtf, using Times 10 for the font:

textutil -convert {{rtf}} -font {{Times}} -fontsize {{10}} {{path/to/foo.txt}}

Load all RTF files in the current directory, concatenates their contents, and writes the result out as index.html with the HTML title set to "Several Files":

textutil -cat {{html}} -title "Several Files" -output {{path/to/index.html}} *.rtf