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From: ignatz@wam.umd.edu (Mark J. Sienkiewicz)
Newsgroups: comp.os.os9
Subject: Re: SILENT ERROR exits SHell script?
Keywords: OSK shell bug
Message-ID: <1991Jun5.184640.4481@wam.umd.edu>
Date: 5 Jun 91 18:46:40 GMT
References: <1991Jun4.230458.25390@cbnewsd.att.com>
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In article <1991Jun4.230458.25390@cbnewsd.att.com> knudsen@cbnewsd.att.com (michael.j.knudsen) writes:
>If I run the DIFF utility in a shell script,
>it runs fine but immediately exits the script upon completion.
>No error message at all -- it just acts as if the script had
>reached normal EOF.

>I know that non-zero error returns will exit a shell script,
>but they always print at least the error number.
>How is it that DIFF can break out of the script with no error message?

Microware's shell exits whenever a process returns a non-zero exit code.
The exit code you are getting is 1 (or 000:001 in microware terms).  This
error seems to get special treatment by not being printed, but only
if you ar executing a script.  Is this weird or what?

>I don't know where this DIFF came from (Microware, or PD, or what),
>but it has another bug that may be related.  If you type just
>"diff" with no arguments, it responds with
>	diff: use: diff file1 file2
>ERROR 000:002  Keyboard Abort

Here are the standard exit codes for diff:
	0 no differences
	1 the two files are different
	2 some error occured

The diff you have is probably either GNU diff or maybe stolen/licensed
from Unix.  This means you should just ignore the goofy error messages
and get on with whatever you are doing.

I would also ask "Why do you have diff in a shell script?"  Microwares
shell doesn't have any way to branch on the result (at least not in
the OS9 I have).  Normally you would write

	if diff filea fileb > /dev/null
	then
		do-something-because-they-are-the-same
	else
		do-something-because-they-are-different
	fi

>I understand this DIFF is buggy and should be replaced with a
>better version.  But just for my education, how can it *silently*
>force the Shell to exit the script?

If you have GNU diff, it is in pretty good shape except that it sometimes
needs a lot of memory.   (of course, OS9 can't swap processes out)