Re: [gentoo-dev] RFC: esethome

[Mike Gilbert <floppym@gentoo.org>]

On Fri, Jun 15, 2012 at 10:05 AM, Peter Stuge <peter@stuge.se> wrote:
> Fabian Groffen wrote:
>> > > >>> +       if [[ ! -n $(egetent passwd "${euser}") ]] ; then
>> > > >>
>> > > >> "! -n" -> "-z"
>> > > >
>> > > > Does the $() argument ever need to be double quoted, or do all
>> > > > versions of bash actually have the string argument optional even
>> > > > though that's not what the man page reads?
>> > >
>> > > Ever?  Yes, but only if what is being returned can contain spaces
>> >
>> > Sorry, I should have mentioned that I had the case of the empty
>> > string in mind.
>>
>> Here for the same reason, the difference between [[ and [ is essential.
>
> It's not clear to me why?
>
> [] is shorthand for test. Both test and [[]] in my man bash read:
>
> --8<--
> Expressions are composed of the primaries described .. under
> CONDITIONAL EXPRESSIONS.
> -->8--
>
> There it says:
> --8<--
> Conditional expressions are used by the [[ compound command and
> the test and [ builtin commands
> -->8--
>
> and:
> --8<--
>       -z string
>              True if the length of string is zero.
>       string
>       -n string
>              True if the length of string is non-zero.
> -->8--
>
> ..which does not at all make it clear that the string is actually
> optional?
>
> Under Command Substitution it says:
> --8<--
> Embedded newlines are not deleted, but they may be removed during
> word splitting.
> ..
> If the substitution appears within double quotes, word splitting
> and pathname expansion are not performed on the results.
> -->8--
>
> ..confirming that there is some processing of the substitution.
>
>
> I also did the tests before asking the question. I'm not trying to
> say that the code doesn't work on my system. I'm asking if it will
> work the same on every version of bash, in spite of what seems to
> be a conflict between real world and documentation.
>
>
> //Peter

Word splitting does not occur within double brackets. This is
documented in the bash manual.

http://www.gnu.org/software/bash/manual/bashref.html#Conditional-Constructs

This causes empty output from a command substitution within double
brackets to be treated as a zero-length string.