Re: GNU standards: What should happen with illegal option?

[Alfred M. Szmidt]

   The GNU standard for --version and --help says the program should
   write to standard output and exit with success. Cf.,
   
https://www.gnu.org/prep/standards/html_node/_002d_002dhelp.html#g_t_002d_002dhelp

Note that --version and --help are not error messages.

   What happens with an illegal option that the program rejects and
   exits?

Sometimes it makes sense for an illegal option to just cause a
warning, and sometimes it makes more sense to immediately exit the
program if an illegal option is seen.

   Does the message get written to standard output or standard
   error? 

Standard error in the general case, think of what the user would like
to see if the program never prints output (e.g. piping everything to
/dev/null) when something bad or unexpected is happening.

   Should the program exit with an error? (I'm fairly certain it
   should exit with an error, but I can't tell where the message should
   be printed).

It might continue, depending on the nature of the "error".  An error
in the general case is the immediate abortion of a program, but there
are other types of "errors" -- for example if the rm command cannot
delete a file, that is also an error but it would be quite annoying if
it aborted directly if it couldn't remove a file.

   If the information could be added to
   
https://www.gnu.org/prep/standards/html_node/Command_002dLine-Interfaces.html,
   then it would be greatly appreciated.

Would you like to suggest a updated text for the GCS?