What type of file has a number as the file extension?

Question

https://github.com/anatol/tup/blob/master/tup.1

It seems barely readable, what editor do I need for this?

Answer 1

That is a manual page. You can view it by downloading the file and doing this command in a terminal window (assuming you're on a UNIXy system like Linux or Mac OS X):

man path/to/tup.1

You can edit it with any text editor.

A little bit of extra info:

Man pages all end in the extensions .1 through .8 to refer to what section of the manual they belong to. Here is a list of the sections and their numbers*:

1. General commands
2. System calls
3. C library functions
4. Special files (usually devices, those found in /dev) and drivers
5. File formats and conventions
6. Games and screensavers
7. Miscellanea
8. System administration commands and daemons

*: List of sections from the Wikipedia page on man pages.

Answer 2

Technically, it's plain text, i.e. any text editor can edit it. As for what it's saying... It looks like some sort of scripting language, but not one I recognize. From line 365, Tup begins parsing this Tupfile, I'm assuming the file is using the syntax and language for Tup, but I'm not entirely sure what Tup is since I haven't looked at it yet.

As an aside, file extensions don't really mean anything. They're just a "rule of thumb" for determining file types. A lot of plain text files either have no extensions or "unconventional" extensions, like .old or .bak for backup copies of files, or .1 for file one.