Re: [Help-bash] Awkward behavior of empty arrays

[Chet Ramey]

On 8/31/17 4:38 AM, Cristian Zoicas wrote:
> 
> Hello all
> 
> I want to create arrays and use them (easily). I am especially
> interested in empty arrays and I felt very uncomfortable with them
> since I found some  counterintuitive or undocumented behaviors.  I
> provide some examples below.
> 
> Example 1:
> 
>     1 set -u;
>     2 unset A;
>     3 declare -a A=;
>     4 echo "s1";
>     5 echo "size: address@hidden";
>     6 echo "s2";
>     7 for e in address@hidden; do echo "e: ${e}"; done
> 
>     Starting with  the line 3  I want to create  an empty array,  so I
>     declare A to have the array  attribute and then I (hope to) assign
>     null to it.  Some  people may say that I am  not right because the
>     manual says  "An array variable  is considered set if  a subscript
>     has been assigned  a value." They are somehow right  because I did
>     not assign any value to any  subscript, so the array should not be
>     initialized. However let's see what is going on.
> 
>     Even if I  did not assign any  subscript a value, the  line 5 says
>     that the size  of the array is  1 and it is  possible to reference
>     element ${A[0]} without  receiving any error due to  "set -u". Also
>     address@hidden expands to nothing.
> 
>     If the manual  is right, then the array should  not be defined. It
>     should not exist and I should  receive errors due to 'set -u', but
>     even  'declare -p  A' shows  that the  array exists  and it  has a
>     single element whose value is null.
> 
>     Is this behavior of assigning null to an array an to have an array
>     with a single null element the desired behavior?

Yes. The manual also says that assigning to an array without using a
subscript is equivalent to assigning to element 0 (or "0").

The other significant consequence of this is treating unsubscripted
references to an array as references to element 0 (or "0").

> 
> Example 2:
> 
>    1 set -u;
>    2 unset A;
>    3 declare -a A=();
>    4 echo "s1";
>    5 echo "size: address@hidden";
>    6 echo "s2";
>    7 for e in address@hidden; do echo "e: $e"; done
> 
>    Starting with  the line 3  I want to create  an empty array,  so I
>    declare A to have the array  attribute and then I (hope to) assign
>    an empty array to it.
> 
>    The line 5 behaves  well and says that the size of  the array is 0,
>    thus I imagine that the array  is empty. So according to the manual
>    address@hidden should expand to  nothing and  the for loop  in the  line 7
>    should not print anything. Instead the message 'bash: address@hidden: 
> unbound
>    variable' is printed. As if A is not set.

It isn't set because there aren't any subscripts with values.  Ultimately,
`declare -a A' and `declare -a A=()' are equivalent.

The real question is whether or not the size/length operator should error
out when there are no elements in the array. The current behavior could be
considered a bug.

> 
>    There is nothing in the manual that says that the () cannot be used
>    to create empty arrays. The syntax is also accepted
> 
>    It means that  if the size of the  array is 0 then the array cannot
>    be expanded. Otherwise  it can.   Such  behavior  forces  people to
>    write difficult to understand/maintain code.
> 
> Example 3:
> 
>    1 set -u;
>    2 unset A;
>    3 declare -a A=(1); unset A[0];
>    4 echo "s1";
>    5 echo "size: address@hidden";
>    6 echo "s2";
>    7 for e in address@hidden; do echo "e: $e"; done
> 
>    The behavior of this example is similar with the behavior of the
>    example 2 above.

There still aren't any subscripts with values.

Now, "making sense" is in the eye of the beholder.  I could see an
array with no elements being `set' in the sense that a string variable
whose value is null being set. I did not choose that alternative.

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