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Re: Next steps for /bin/sh

On Tue, 01 Mar 2016 01:02:30 +0700
Robert Elz <kre%munnari.OZ.AU@localhost> wrote:

> But in real life, when we aren't writing stuff like "false && false"
> or "false && true", when someone writes a command like
> 	$x_option && command implementing -x
> they aren't asserting that "${x_option}" is supposed to always be
> true, and that the script should abort if that is not the case,
> what they are doing is making a one line version of
> 	if $x_option
> 	then
> 		command implementing -x
> 	fi

Robert, thank you for your detailed and illuminating replies.  

Only after reading the above did I finally understand why   X && Y  is
special under -e.  I use it myself -- doesn't everyone? -- and never
thought it should cause the script to terminate.  

You gave two other unfortunate examples: the effect of a subshell, and
what to do with the build_and_run example.  I doubt the wisdom of
following Posix here because of "how useless it is to attempt to rely
upon -e".  If I may suggest, do you think we could craft a simpler,
better rule instead?  

In general I expect scripts under -e to behave as though in a
Makefile (with -j1 option).  IMO that provides (or should provide) the
best guidance: evaluate each statement's status *as if* in a subshell.  

For example, your "unexpected" result is completely ordinary under make:

	$ make
	if false; true; then echo fail; fi

Similarly, we want, 

> 	build_and_run command-that-should-not-fail ||
> 		mail -s help kre </dev/null

to evaluate as  

	(build_and_run command-that-should-not-fail) ||
		mail -s help kre </dev/null

(except of course for side effects of build_and_run).

Afaict the && "longcircuit" is the only desirable exception.  Is
it? If so, would a good policy be

1.  Evaluate exit status of each pipeline *as if* in a subshell
    (like make), except
2.  For X && Y constructions, evaluate as  if "X; then Y; fi". 

You're doing the work; I don't want to muddy the waters.  It just seems
to me that when Posix is providing lousy guidance, innovation is
better then emulation.  


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