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

textutil

Used to manipulate text files of various formats.
More information: <https://ss64.com/osx/textutil.html>.
Trick copyright: tl;dr; <https://github.com/tldr-pages/tldr>

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

click the source code to copy