Path: utzoo!utgpu!water!watmath!clyde!bellcore!faline!ulysses!allegra!princeton!udel!rochester!cornell!uw-beaver!teknowledge-vaxc!sri-unix!quintus!ok
From: ok@quintus.UUCP
Newsgroups: comp.unix.questions
Subject: Re: exit(-1), 0 is sometimes magic
Summary: but I tested it...
Keywords: exit, zero, shells
Message-ID: <571@cresswell.quintus.UUCP>
Date: 24 Jan 88 04:48:49 GMT
References: <502@cresswell.quintus.UUCP> <6935@brl-smoke.ARPA> <1179@wjvax.UUCP> <310@fig.bbn.com>
Organization: Quintus Computer Systems, Mountain View, CA
Lines: 55
Posted: Sat Jan 23 23:48:49 1988

In article <310@fig.bbn.com>, rsalz@bbn.com (Rich Salz) writes:
> Nope.  The use of && and || in csh is the opposite of that in /bin/sh.
> 	/bin/sh -c "/bin/test -d foo || mkdir foo"
> Means if the foo directory doesn't exist, make it.  That is, /bin/sh does
> the "conceptually right thing" in that it takes a||b to mean, do b if a
> failed.
> 
> The Cshell takes a||b in the mathematical sense; do b if a does an exit(0):
> 	/bin/csh -c "/bin/test -d foo || mkdir foo"
> will never make the directory, or mkdir will spit if it already exists.
> 
If you have a C-shell, do the following:
	cat >foo <<'EOF'
	#!/bin/csh
	(exit $1) && (echo true) || (echo false)
	'EOF'
	chmod +x foo
	foo 0
prints	true
	foo 1
prints	false
which indicates that the C-shell ***IS*** treating && and || the
way I said it does.  I tried Rich Salz's specific example, and
it ***DID*** make the directory.  This was under SunOS 3.2.
The C-shell manual explicitly says that "Pipelines that are separated by
&& or || form conditional sequences in which the execution of pipelines
on the right depends upon the success or failure, respectively, of the
pipeline on the left."  So it would be a very serious bug if Rich
Salz were right.

The C-shell, in addition to commands, has EXPRESSIONS, and
in expressions && and || do what he thinks.  But *ONLY* in expressions.
Unlike the Bourne shell, the C-shell doesn't have you use commands
in if and while.  You use *expressions* there.  The {command} form
converts a command to an expression, but mapping exit 0 to 1 and
exit non-zero to 0.  (It's in the manual.)

Suppose you want to create directory foo if it doesn't exist and
the current directory is writable.  Four ways:

sh :	test -d foo || ( test -w . && mkdir foo )

sh :	if test ! -d foo  && test -w . 
	    then
		mkdir foo
	    fi

csh:	test -d foo || ( test -w . && mkdir foo )

csh:	if ( ! { test -d foo } && { test -w . } ) mkdir foo
	       ^^^^^^^^^^^^^^^    ^^^^^^^^^^^^^ these are expressions!

Yes, it is confusing that the same operators are used with opposite
meanings.  Even so, it is wise to try one's examples before claiming
that they don't work...