Newsgroups: comp.lang.tcl,comp.unix.shell
From: bet@ritz.mordor.com (Bennett Todd)
Subject: Re: "$@" versus ${1+"$@"}
References: <438pn6$abm@tbird2.pad.otc.com.au> <9526104.11082@mulga.cs.mu.OZ.AU> <448lkd$cg1@ukwsv3.ggr.co.uk>
Followup-To: comp.unix.shell
Date: Tue, 26 Sep 1995 14:35:45 GMT
Message-ID: <DFIoJL.934@ritz.mordor.com>

(This isn't really a TCL question; it's a Bourne Shell question; so I've
cross-posted, and set followups, to comp.unix.shell).

Once upon a time (or so the story goes) there was a Bourne Shell somewhere
which offered two choices for interpolating the whole command-line. The
simplest was $*, which just borfed in all the args, losing any quoting that
had protected internal whitespace. It also offered "$@", to protect
whitespace. Now the icko bit is how "$@" was implemented. In this early
shell, the two-character sequence $@ would interpolate as

	$1" "$2" "$3" "$4" ... $n

so that when you added the surrounding quotes, it finished quoting the whole
schmeer. Cute, cute, too cute.... Now consider what the correct usage

	"$@"

will expand to _if_there_are_no_args_:

	""

That's the empty string --- a single argument of length zero. That's _not_
the same as no args at all. So, someone came up with a clever application of
another Bourne Shell feature, conditional interpolation. The idiom

	${varname+value}

expands to ``value'' if ``varname'' is set, and nothing otherwise. Thus the
idiom under discussion

	${1+"$@"}

means exactly, precisely the same as a simple

	"$@"

without that ancient, extremely weird bug.

So now the question: what shells _had_ that bug? Are there any shells
shipped with any even vaguely recent OS that included it?

-- 
-Bennett
bet@mordor.com
http://www.mordor.com/bet/