Re: Possibly wrong handling of $_?

[Harald van Dijk <harald@gigawatt.nl>]

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