Bug in parameter expansion with :- ?

Bug in parameter expansion with :- ?

[Ronald Fischer]

This expands correctly to a '+':

      ${unset_variable:-+}

This however doesn't expand to a '-', but to the empty string:

    ${unset_variable:--}

I didn't find anything in the man page which would explain this
behaviour.

A note aside: Bash expands in the latter case to a dash (-). 

My zsh version:

zsh 5.1.1 (x86_64-unknown-cygwin)


Ronald

Re: Bug in parameter expansion with :- ?

[Peter Stephenson]

On Tue, 10 Jan 2017 16:35:00 +0100
Ronald Fischer <ynnor@mm.st> wrote:

> This expands correctly to a '+':
> 
>       ${unset_variable:-+}
> 
> This however doesn't expand to a '-', but to the empty string:
> 
>     ${unset_variable:--}

You may be getting into confusions with options to builtins rather than
the expansion.

% print -r -- ${unset_variable:--} 
-

pws

Re: Bug in parameter expansion with :- ?

[Ronald Fischer]

On Tue, Jan 10, 2017, at 16:43, Peter Stephenson wrote:
> On Tue, 10 Jan 2017 16:35:00 +0100
> Ronald Fischer <ynnor@mm.st> wrote:
> 
> > This expands correctly to a '+':
> > 
> >       ${unset_variable:-+}
> > 
> > This however doesn't expand to a '-', but to the empty string:
> > 
> >     ${unset_variable:--}
> 
> You may be getting into confusions with options to builtins rather than
> the expansion.
> 
> % print -r -- ${unset_variable:--} 
> -

Yes I do! You are right - not a bug!

Ronald

Re: Bug in parameter expansion with :- ?

[Nikolay Aleksandrovich Pavlov (ZyX)]

10.01.2017, 18:56, "Ronald Fischer" <ynnor@mm.st>:
> On Tue, Jan 10, 2017, at 16:43, Peter Stephenson wrote:
>>  On Tue, 10 Jan 2017 16:35:00 +0100
>>  Ronald Fischer <ynnor@mm.st> wrote:
>>
>>  > This expands correctly to a '+':
>>  >
>>  > ${unset_variable:-+}
>>  >
>>  > This however doesn't expand to a '-', but to the empty string:
>>  >
>>  > ${unset_variable:--}
>>
>>  You may be getting into confusions with options to builtins rather than
>>  the expansion.
>>
>>  % print -r -- ${unset_variable:--}
>>  -
>
> Yes I do! You are right - not a bug!
>
> Ronald

For checking what argument is literally I would suggest `printf '<%s>' $arg` instead: this will show trailing spaces due to `<>` around `%s`, let you distinguish between variable accidentally becoming an array and determine which strings are in one array item exactly (`printf '<%s>' 1 2` results in `<1><2>`, harder to miss then a space). And this also does not look like you are fighting with zsh (`-r` and `--` are there to prevent some misinterpretations).