Date: Fri, 29 Jun 2001 02:32:51 -0500 From: Mike Meyer <mwm@mired.org> To: Dave Tweten <tweten@nas.nasa.gov> Cc: questions@freebsd.org Subject: Re: Bourne Shell Syntax Wierdness Message-ID: <15164.12067.829448.734806@guru.mired.org> In-Reply-To: <98860201@toto.iv>
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Dave Tweten <tweten@nas.nasa.gov> types:
> The sh man page says that the two operators "||" and "&&" have the
> same precedence. It also says that "a || b" means "execute b if a
> terminates abnormally" and "a && b" means "execute b if a terminates
> normally."
>
> So I don't understand why "true || true && echo Oops!" prints "Oops!"
The || and && operators are left associative, so that your example -
which is ambiguous given only the information you provided - is parsed
as "( true || true ) && echo Oops!". Since "( true || true )" always
terminates normally, it should print "echo Oops!".
> This is apparently not a bug in FreeBSD sh, because IRIX, IRIX64, and SunOS
> Bourne/Korn shells work the same way, but it certainly runs counter to my
> ability to read English and the contents of the man page.
But it matches the way most people read expressions. The world would
be a better place if we switched to an unambiguous system for
mathematical expressions :-).
> Incidently, "true || { true && echo Oops! }" prints nothing -- which
> I would have expected with or without the "{}".
That's what would happen if the operators were right associative
instead of left associative - which would collide with most peoples
expectations based on common mathematical usage.
<mike
--
Mike Meyer <mwm@mired.org> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
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