Re: man page confusion about array with no members: var=()

[Chet Ramey]

On 7/13/16 4:36 PM, idallen@idallen-fibe.dyndns.org wrote:

> Bash Version: 4.3
> Patch Level: 46
> Release Status: release
> 
> Description:
>       The BASH man pags says "When there are no array members,
>       ${name[@]} expands to nothing." and then later "An array variable
>       is considered set if a subscript has been assigned a value.".
> 
>       The first sentence tells me that ${name[@]} is a valid use of a
>       variable that exists but has no members, but the second sentence
>       implies that if the array has no members it is considered "unset"
>       and thus ${name[@]} would not be a valid use of the name.

Thanks for the report.  The closest analog of the behavior you describe
is the use of $@ and $* when there are no positional parameters.  Posix
decided a while back that these would not constitute referencing an unset
variable, and I think it's reasonable to make ${var[@]} and ${var[*]}
behave similarly.

In bash-4.4 these uses will not generate an `unset variable' error.

> Repeat-By:
>       #!/bin/bash -u
> 
>       # make BASH complain about unset variables
>       set -o nounset
> 
>       echo 'ONE: set var= and try $var and ${var[@]} - both work without 
> error'
>       unset var
>       var=
>       ( echo "ONE: [$var]" )
>       ( echo "ONE: [${var[@]}]" )

`var' isn't unset, and it's not an array.  The man page says explicitly
that the null string is a valid value.

> 
>       echo 'TWO: set var=() and try again - both fail with "unbound variable"'
>       unset var
>       var=()
>       ( echo "TWO: [$var]" )
>       ( echo "TWO: [${var[@]}]" )

This is technically unset, but will no longer generate an error.


-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://cnswww.cns.cwru.edu/~chet/