Explain the deviousness of the Perl “preamble”

Question

The Perl manual describes a totally devious construct that will work under any of csh, sh, or Perl, such as the following:

eval \'(exit $?0)\' &         


        

Answer 1

From Tom Christiansen's collection Far More Than Everything You've Ever Wanted to Know About …:

Why we use eval 'exec perl $0 -S ${1+"$@"}'

Newsgroups: comp.lang.tcl,comp.unix.shell
From: bet@ritz.mordor.com (Bennett Todd)
Subject: Re: "$@" versus ${1+"$@"}
Followup-To: comp.unix.shell
Date: Tue, 26 Sep 1995 14:35:45 GMT
Message-ID:

(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/