Possibly wrong handling of $_?

Possibly wrong handling of $_?

[Vadim Zeitlin]

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 Hello,

 I'm not exactly sure if this is a bug because I didn't find any
specification about how is this supposed to behave (to my surprise it
turned out that $_ was not in POSIX), but please consider this:

	% zsh -c 'echo -n foo && echo $_'
	foofoo
	% bash -c 'echo -n foo && echo $_'
	foofoo
	% dash -c 'echo -n foo && echo $_'
	foo/usr/bin/dash

I've tested several different versions of zsh (4 and 5) and bash (3 and 4)
and dash 0.5.5 and 0.5.7 from Debian and they all produced the results as
above.

 Shouldn't dash follow the other shells here?
VZ

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Re: Possibly wrong handling of $_?

[Harald van Dijk]

On 23/12/2014 23:34, Vadim Zeitlin wrote:
>   Hello,
>
>   I'm not exactly sure if this is a bug because I didn't find any
> specification about how is this supposed to behave (to my surprise it
> turned out that $_ was not in POSIX), but please consider this:
>
> 	% zsh -c 'echo -n foo && echo $_'
> 	foofoo
> 	% bash -c 'echo -n foo && echo $_'
> 	foofoo
> 	% dash -c 'echo -n foo && echo $_'
> 	foo/usr/bin/dash

This does come across as somewhat confusing, but $_ is really not a 
special variable at all in dash. The shell you're using to launch dash 
does make it a special variable. That shell puts _ in the environment, 
which dash then picks up, and ignores, other than making it available as $_.

You can see what your usual shell is doing by testing with other 
commands: just run

   env | grep '^_='

and you'll probably see

   _=/usr/bin/env

It works the same way when starting dash.

> I've tested several different versions of zsh (4 and 5) and bash (3 and 4)
> and dash 0.5.5 and 0.5.7 from Debian and they all produced the results as
> above.
>
>   Shouldn't dash follow the other shells here?
> VZ

If dash did something special with $_, then I agree it would be nice if 
it would be somewhat compatible with other shells. If dash simply does 
not implement a feature, that feature is not required by any standard, 
and that feature is not widely used, then I suspect there won't be a lot 
of interest in implementing that feature.

Don't be put off by that, though. You are free, of course, if you feel 
so, to attempt to convince people $_ is an important feature that all 
shells should implement. If you have compelling use cases, if it solves 
real problems, and if many other shells already implement it, you might 
even get it standardised. I have never seen a need for it, but that's 
just me speaking from personal experience, others may feel differently.

Cheers,
Harald van Dijk

Re[2]: Possibly wrong handling of $_?

[Vadim Zeitlin]

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On Wed, 24 Dec 2014 00:21:09 +0100 Harald van Dijk <harald@gigawatt.nl> 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!

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.

 Anyhow, thanks again for your explanation!
VZ

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Re: Possibly wrong handling of $_?

[Jilles Tjoelker]

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 <harald@gigawatt.nl> 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

Re: Possibly wrong handling of $_?

[Harald van Dijk]

On 25/12/2014 15:32, Jilles Tjoelker wrote:
> 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 <harald@gigawatt.nl> 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.

Ah, apologies. In interactive mode, it indeed implements it, but 
differently from bash. In dash, only commands that specify a command 
name cause $_ to be set. For example,

   dash -ic 'if a=b; then c=d; else e=f; fi; echo $_'

still prints

   /usr/bin/dash

(or wherever dash is installed), because if statements, and variable 
assignments, have no effect on $_. In bash, this prints a blank line, 
because the prior value of $_ has not survived those variable assignments.

Cheers,
Harald van Dijk