> 1. Don’t assume a terminal type. Look at `TERM` and use termcap/terminfo or a library built atop them for anything beyond line-oriented plain text output, or least assume a plain teletype unless you specifically recognize the user’s terminal type.

I agree, but these days I think you can mostly get away with assuming VT100-compatible behavior.

> 4. Use the standard `<sysexits.h>` exit codes. They exist for a reason and they make use of your tool within pipelines and programs more straightforward because they make it easier to trace why a failure occurred.

I just took a look at this header file. (It defines exit codes starting at 64.) I'm not sure I've ever seen a program that uses these codes. Many programs for UNIX-like systems just use exit(1) for generic errors, or maybe something to distinguish between data errors and usage errors such as unrecognized command-line options. I mostly use Linux; maybe it's more common on BSD-based systems (it appeared "somewhere after 4.3BSD"). For example curl defines nearly 100 distinct error codes; none of them are based on <sysexit.h>.

> 5. Include both in-binary `--help`/usage information and a man page. Often a user will just need a quick refresher on argument syntax, which is what the built-in help text is for; the man page should be a comprehensive reference with examples. It should never defer to a web page or GNU `info`—it’s fine if those exist too and are pointed out, but they should not be the primary user reference.

In practice, GNU `info` tends to be the primary reference for GNU programs. The man page is often missing a lot of information.