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From: guy@rlgvax.UUCP (Guy Harris)
Newsgroups: net.unix-wizards
Subject: Re: /bin/sh feature?
Message-ID: <2087@rlgvax.UUCP>
Date: Mon, 9-Jul-84 21:55:31 EDT
Article-I.D.: rlgvax.2087
Posted: Mon Jul  9 21:55:31 1984
Date-Received: Thu, 12-Jul-84 04:12:42 EDT
References: <274@uvm-cs.UUCP> <49@pixutl.UUCP>
Organization: CCI Office Systems Group, Reston, VA
Lines: 38

> The problem is that sh parses comment lines starting with a ":" (don't
> ask me why...) and waits for the closing "'".

The reason is simple - a line starting with a ":" isn't a comment in the
sense that most programming languages use the word "comment".  ":" is a
built-in command in the shell that does nothing.  Think of it as equivalent
to "echo >/dev/null 2>&1".  It returns an "exit status" of 0 (true).  As
such, it's useful when doing "while true" (since it's built-in, it's faster
than "true" which is a shell file) and useful when you want something like

	if ! cmp -s $1 $2
	then
		diff $1 $2
	fi

which compares two files and, if unequal, "diff"s them.  Unfortunately, you
can't negate the exit status of a "list" in an "if", so you have to do
something like

	if cmp -s $1 $2
	then
		:
	else
		diff $1 $2
	fi

(Yes, I know you could run the "cmp" and then do a

	if test $? -ne 0

but that's *UG*ly.)

Fortunately, every Bourne shell except the V7 shell supports *real* comments,
done with "#".  Everything following the "#" on the line is ignored, even by
the shell's lexical analyzer.

	Guy Harris
	{seismo,ihnp4,allegra}!rlgvax!guy