How would I reference a Linux man page in Harvard style?
This is my try, based on citation principles, Open University's guide to citing software in Harvard format, and Mendeley's guide to core Harvard style. 

In general, since the "man bash" page is a particular page, you may want to have the in-text citation specify that as the page/section. That will solve problems if you also want to reference the same manual about something else. Then you have just the more generic reference in the references section.

If this is the only manual citation:
In text: Free Software Foundation (2018)

In references: Free Software Foundation (2018) 'Bash manual page', Linux (Version X) [Computer program]. Available at [URL to get this version of Linux]. Copy of manual text available at http://man7.org/linux/man-pages/man1/bash.1.html (accessed DATE).

If referencing multiple parts of manual:

In text: Free Software Foundation (2018, bash manual page)

In references: Free Software Foundation (2018) Linux (Version X) [Computer program]. Available at [URL to get this version of Linux]. Copy of manual text available at https://www.kernel.org/doc/man-pages/ (accessed DATE).

Does the Free Software Foundation itself host a version of the manual online? That would be the ideal reference for the text. I realize that this response is kind of clunky, but it helps readers find what they need without having to download (your version of) Linux.