From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jilles Tjoelker Subject: Re: Possibly wrong handling of $_? Date: Thu, 25 Dec 2014 15:32:26 +0100 Message-ID: <20141225143226.GA27266@stack.nl> References: <5499F8E5.205@gigawatt.nl> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from relay02.stack.nl ([131.155.140.104]:65191 "EHLO mx1.stack.nl" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752034AbaLYOc3 (ORCPT ); Thu, 25 Dec 2014 09:32:29 -0500 Content-Disposition: inline In-Reply-To: Sender: dash-owner@vger.kernel.org List-Id: dash@vger.kernel.org To: Vadim Zeitlin Cc: dash@vger.kernel.org, Harald van Dijk On Wed, Dec 24, 2014 at 12:33:32AM +0100, Vadim Zeitlin wrote: > On Wed, 24 Dec 2014 00:21:09 +0100 Harald van Dijk wrote: > HvD> On 23/12/2014 23:34, Vadim Zeitlin wrote: > HvD> > Hello, > HvD> > > HvD> > I'm not exactly sure if this is a bug because I didn't find any > HvD> > specification about how is this supposed to behave (to my surprise it > HvD> > turned out that $_ was not in POSIX), but please consider this: > HvD> > > HvD> > % zsh -c 'echo -n foo && echo $_' > HvD> > foofoo > HvD> > % bash -c 'echo -n foo && echo $_' > HvD> > foofoo > HvD> > % dash -c 'echo -n foo && echo $_' > HvD> > foo/usr/bin/dash > HvD> > HvD> This does come across as somewhat confusing, but $_ is really not a > HvD> special variable at all in dash. > Ah, this does explain it, thanks! Dash does implement $_, but only in interactive mode. Your selection of shells makes it appear as if dash is the odd one out, but in fact it is not. FreeBSD /bin/sh (also an Almquist shell derivative), mksh and ksh93 also only implement $_ in interactive mode. Details of how $_ differ as well, particularly in ksh93. It seems unwise to use this feature in scripts. > HvD> If dash did something special with $_, then I agree it would be nice if > HvD> it would be somewhat compatible with other shells. If dash simply does > HvD> not implement a feature, that feature is not required by any standard, > HvD> and that feature is not widely used, then I suspect there won't be a lot > HvD> of interest in implementing that feature. > Yes, I understand, somehow the idea that dash didn't implement it at all > just didn't occur to me, but clearly adding a non-standard new feature is > not nowhere near as important as fixing [what looked like] a bug. I'm not > sure about the "not widely part", but I don't have any non-anecdotal > evidence one way or the other. > HvD> Don't be put off by that, though. You are free, of course, if you feel > HvD> so, to attempt to convince people $_ is an important feature that all > HvD> shells should implement. If you have compelling use cases, if it solves > HvD> real problems, and if many other shells already implement it, you might > HvD> even get it standardised. I have never seen a need for it, but that's > HvD> just me speaking from personal experience, others may feel differently. > FWIW my initial problem started with using > builddir := $(shell mkdir -p some-long-make-expression && echo $_) > in a makefile, which seemed like a nice way to assign the value to the > variable only if the directory was successfully created. This can, of > course, be done in a myriad of other ways, but this one just seemed like > the most elegant to me. Whether this counts as a "need" or not is not for > me to say. > Personally I'd say the main argument for adding support for "$_" to dash > would be to avoid mysterious problems like the one I just had because dash > silently (i.e. without giving any errors) behaves differently from the > other shells, which in my case resulted in the makefile misbehaving only > under Debian (where dash is used as /bin/sh) but not under systems. Such a principle of least surprise is not a design goal for dash. -- Jilles Tjoelker