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bug#23090: true and false not POSIX

From: Stephane Chazelas
Subject: bug#23090: true and false not POSIX
Date: Tue, 22 Mar 2016 15:08:28 +0000
User-agent: Mutt/1.5.21 (2010-09-15)

2016-03-22 15:39:52 +0100, Ruediger Meier:
> On Tuesday 22 March 2016, Stephane Chazelas wrote:
> > 2016-03-22 13:43:30 +0100, Ruediger Meier:
> Ok, but it must not use STDOUT and it must return 0.
> true --help may predict the future of the stars as long as it exits 0 
> and does _not_ print anything. 

No, it is only required to exit with 0 and print nothing if
called as specified, that is without arguments. If passed any
argument, the behaviour is not specified.

Just like

dirname --help

is unspecified.

dirname -- foo/bar

is specified as utilities (other than the special builtins that
are not required to follow syntax guidelines) that take operands
must recognise (and ignore) a leading "--" argument.

true --

is not as true doesn't take operands. In practice though, you
could do "true -- --arbitrary args being ignored". That seems to
work with all the "true" implementations I've tried, but POSIX
would still leave that unspecified.

> > A conforming application should not pass arguments to "true".
> Why not?

Because then the behaviour would be unspecified.

> > There are a number of other utilities that don't accept options
> > in their specification (like "exec", "dirname",...) and still
> > implementations (including certified ones) do support options.
> You may add options as long as you don't violate the specification.
> BTW coreutils "echo --help" is also wrong. Here POSIX explicitly 
> says: "Implementations shall not support any options".

Yes, the behaviour of echo when passed a "--help" argument is
clearly specified.

> BTW I know about POSIXLY_CORRECT env. I just ask this: Is it worth to 
> violate parts of POSIX just for minor cosmetical reasons?
> I mean echo -n/-e may be an improvement though non-posix. But 
> echo --version is a violation just for cosmetics, true --version is 
> even worse.

AFAICT, echo --version is a violation, true --version is not.

> > sh -c ''
> > awk 'BEGIN{exit}'
> > printf ''
> These are no binaries but shell commandlines. You can't pass it to other 
> programs, like gdb, setarch or whatever.

POSIX systems are required to have sh, awk and printf commands
in the filesystem.

There shouldn't be any problem passing them to gdb/setarch/env
or whatever.

replace the non-portable/non-standard:

execlp("true", "true", "--help", "and", "other", "arbitrary", "args", 0);


execlp("awk", "awk", "BEGIN{exit}", "--help", "sh", "and", "other", 
"arbitrary", "args", 0);

for instance

(btw, sh -c '' may not be that good a choice because if sh is
yash, you'd get errors if arguments contain invalid characters).


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