Bug#1068891: coreutils: is join -t '' just comm -12?

[Pádraig Brady]

On 12/04/2024 23:59, наб wrote:

Package: coreutils
Version: 9.1-1
Version: 9.4-3
Severity: normal

Dear Maintainer,

POSIX.1-202x/D3:
        −t char
                        Use character char as a separator, for both input and 
output. Every appearance of
                        char in a line shall be significant. When this option 
is specified, the collating
                        sequence shall be the same as sort without the −b 
option.
so obviously allowing -t '' is an extension.

Manual:
        -t CHAR
               use CHAR as input and output field separator

        Important: FILE1 and FILE2 must be sorted on the  join  fields.  E.g.,
        use  "sort  -k  1b,1"  if 'join' has no options, or use "join -t ''" if
        'sort' has no options.  Note, comparisons honor the rules specified by
        'LC_COLLATE'.   If  the  input  is  not sorted and some lines cannot be
        joined, a warning message will be given.

So given
        $ cat f1
        row1    f1      1
        urow1   f1      2
        $ cat f2
        row1    f2      1
        urow2   f2      2
which are stable against both sort and sort -k 1b,1
        $ join f?
        row1 f1 1 f2 1
        $ join f? -t '  '
        row1    f1      1       f2      1
is all as expected.

But
        $ join f? -t ''
returns empty. What would empty -t mean, anyway?
The empty string can either be found at every position
(clearly not the case here, otherwise this'd be joined on r and u)
or at no positions, so
        $ cat g1
        row1
        urow1
        $ cat g2
        row1
        urow2
        $ join g? -t ''
        row1
which is, well
        $ comm g? -12
        row1

Somehow I don't feel like this is a good recommendation?

Well sort with no options operates on the whole line.
So the corresponding join -t '' operates on the whole line.

cheers,
Pádraig