Path: utzoo!dciem!nrcaer!scs!spl1!laidbak!att!pacbell!ames!pasteur!ucbvax!hplabs!hpda!hpsemc!bd
From: bd@hpsemc.HP.COM (bob desinger)
Newsgroups: comp.sources.d
Subject: Re: Re: Improving SHAR
Message-ID: <1110003@hpsemc.HP.COM>
Date: 4 Jun 88 00:20:08 GMT
Article-I.D.: hpsemc.1110003
References: <2349@bgsuvax.UUCP>
Organization: HP Technology Access Center, Cupertino, CA
Lines: 49

William C. DenBesten (denbeste@bgsuvax.UUCP) writes:
> The solution ...
> is that we need a shar-archiver that includes a crc check in it,
> rather than a simple character count.
> There is not a standard unix utility that will do a crc.

How about the `sum' program?  The System V and Xenix versions offer
a -r option to produce BSD-compatible output, so you can be reasonably
portable (about 95%) with that.

The shar I use emits code to handle this, although I admit it has its
own sum code to compute the checksums.  (If I had written that part of
the code, I probably would have forked the sum program instead of
handcrafting the information myself.  But the code was written already
and it worked, so I left it in.)

The emitted shar looks like this.  Comments added for this article
appear after the "#".

	# Decide if we should use `sum -r' or just plain old `sum'
	if sum -r </dev/null >/dev/null 2>&1
	then	sumopt='-r'
	else	sumopt=''
	fi

	# The wrapped file goes here.
	# I sharred an empty file, so the numbers are 0 in this demo.

	# Now check the checksum.
	set `sum $sumopt <filename`
	if test $1 -ne 0
	then	echo ! filename checksum should be 0, but is $1
	fi

While we're on the subject, you can get more than just a simple
character count out of `wc'.  Here's my shar again:

	set `wc -lwc <filename`
	if test $1 -ne 0 -o $2 -ne 0 -o $3 -ne 0
	then	echo ! filename should have 0 lines, 0 words, and 0 characters
		echo ! but has $1 lines, $2 words, and $3 characters
	fi

This isn't as thorough as a real CRC, but on the other hand I've sent
out perhaps 100 megabytes of sources using only the wc-check and no one
has ever asked me to re-send because something was damaged in transit.
The medium is seems reliable.

-- bd