Re: Stupid bash question

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On Wed, 2007-12-12 at 01:49 -0800, Khoa Ton wrote:
> Ralf Corsepius wrote:
> > On Wed, 2007-12-12 at 00:01 -0800, Dean S. Messing wrote:
> >> Ralf Corsepius wrote:
> >> : On Tue, 2007-12-11 at 21:29 -0800, Dean S. Messing wrote:
> > 
> >> : [1] http://cvs.savannah.gnu.org/viewvc/gnustandards/gnustandards/standards.texi
> >> : 
> >>
> >> Thanks, Ralf, for enlightening me.  I wrote "new coding standards",
> >> above, in response to Stepan Kasal's remark that "GNU Coding Standards
> >> now declare ...".  I suppose the latter is literally true even if the
> >> Standard was defined in 1992.
> >>
> >> Of course, with this new knowledge, I will feel as free as a bird to
> >> boldly ignore the Standard (in this respect) seeing how several other
> >> prominent linux executables (busybox, lvm, dump/restore, halt, to name
> >> a few) have been blithely ignoring it for more than a decade.  ½:-)
>  >
> > Well, of cause it's everybody's freedom to ignore the "insights" others
> > have accumulated over many years. But also consider, there are good
> > reasons why these recommendations exist and why some people consider
> > programs changing their behavior upon program name to be mal-designed.
> > 
> > Ralf
> 
> I would appreciate explanations or pointers to the reasons why some
> people consider programs changing their behavior upon program name to be
> mal-designed.
In short: It's unreliable.

A lengthier answer:


There are several aspects:

1. GNU programs tend to be installed under alternative names, often in
parallel to OS-vendor supplied programs, often into the same directories
or at least into the same PATHs as OS-vendor supplied programs. 

Think /usr/bin/make vs. /usr/bin/gmake vs. /usr/bin/gnu-make
vs. /opt/gnu/bin/make vs. E://com/linuxmake.exe vs. ~/check (because
some user copied over /usr/bin/<app> to ~/ because wants to experiment
with it).

Any heuristic to examine argv[] or argv[0] can't avoid to fail somewhere
or at least can't avoid to be somewhat unreliable/unsafe. It's
completely out of an implementors/coders control what a user might be
using as final program name.



2. Technical limitations.
Checking a program's name is not as trivial as it might seem.
- Some OSes mandate program suffixes. A well known example is "*.exe",
but there might be naming conventions on $WEIRDOS, coders/implementors
have never seen before.
- On some OSes, argv[0] is not available (E.g. some embedded OS don't
support it), on some OSes argv[0] carries an absolute path, on some OSes
only a program's basename.


Now consider that these limitations are pretty easy to overcome.
- GNU Standards recommend recommend using an option instead of
name-magic. Most programs already will have option processing, so the
price of adding them should be pretty small.

- An alternative to using options is would be to split the executable
and build several separate apps from them.

> For statically linked executables in disk space critical environments,
> busybox's use of this mechanism is rather elegant.
_statically_ linked in _space critical_ environments. ... That's a
special case, which would touch different questions.

Just take the GNU Standards as "recommendations, generations of smart
people have invested their brains in" - They are not a law. They help
circumventing some issues commonly met on "standard GNU platforms", but
they also have trade-offs and come at a price.

Ralf



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