From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Mon,  5 Oct 2009 16:54:23 +1100
From: Sam Watkins <sam@nipl.net>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
Message-ID: <20091005055423.GA14691@nipl.net>
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Subject: Re: [9fans] mishandling empty lists - let's fix it
Topicbox-Message-UUID: 7e332230-ead5-11e9-9d60-3106f5b1d025

sqweek:
> It seems to me the obvious way to gain consistency is to do the list parsing
> in one place only:

hi sqweek,

Thanks for the thoughtful response.  You are right, it could be fixed with
another tool like xargs.  I wrote a similar tool "modify" which I use to modify
files in place with standard tools, like `modify nl : a b c` to number lines in
three files.  Given that nearly all the tools do already handle lists of
arguments (wrongly), I saw a need to correct them.

Plan 9 tools that accept options already do accept `--' to mark the end of
options and the start of proper arguments, it is a necessary feature.

I will show what sort of change would be needed for grep.  I have not looked at
the source for plan 9 grep, so this is just an example.

If the existing code was:

	char *pattern;
	int use_stdin, prefix_filename;
	...
		if strcmp(argv[i], "--") == 0
			...
	...
	pattern = argv[0]; ++argv; --argc;
	use_stdin = !argc;
	prefix_filename = argc > 1;

The changed code would be:

	char *pattern;
	int dwim, use_stdin, prefix_filename;
	dwim = 1;
	...
		if strcmp(argv[i], "--") == 0
			dwim = 0
			...
	...
	pattern = argv[0]; ++argv; --argc;
	use_stdin = dwim && !argc;
	prefix_filename = !dwim || argc > 1;

The difference from current logic is very slight.  It's not rocket science.

Then something like:

  grep foo -- `{find ...}

would work correctly and consistently.

  grep foo -- *

will still not work because rc's globbing also fears zero and returns the
pattern instead of an empty list if there are no matches.

Sam