join: make -t '' operate on the whole line

Previously passing an empty parameter to -t would
raise an error, but now it means to treat each line
as a single field for matching.  This matches the
default operation of `sort` which is usually used
in conjunction with join.

* src/join.c (main): Set the field delimiter to '\n' if
an empty parameter is passed to -t.
(usage): Mention the operation of -t ''.
* tests/misc/join: Add 2 new tests, for the existing -t '\0'
and the new -t '' functionality.
* doc/coreutils.texi (join invocation): Mention that
join -t '' always operates on the whole line, while
join -t '\0' usually does.
* NEWS: Mention the change in behavior.

1 files changed, 6 insertions, 1 deletions

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 2b3d32b94..e3e95f5b6 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -5462,6 +5462,8 @@ locales and options if the output of @command{sort} is fed to
 sort a file on its default join field, but if you select a non-default
 locale, join field, separator, or comparison options, then you should
 do so consistently between @command{join} and @command{sort}.
+If @samp{join -t ''} is specified then the whole line is considered which
+matches the default operation of sort.
 
 If the input has no unpairable lines, a @acronym{GNU} extension is
 available; the sort order can be any order that considers two fields
@@ -5572,7 +5574,10 @@ option---are subject to the specified @var{field-list}.
 Use character @var{char} as the input and output field separator.
 Treat as significant each occurrence of @var{char} in the input file.
 Use @samp{sort -t @var{char}}, without the @option{-b} option of
-@samp{sort}, to produce this ordering.
+@samp{sort}, to produce this ordering.  If @samp{join -t ''} is specified,
+the whole line is considered, matching the default operation of sort.
+If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
+character is used to delimit the fields.
 
 @item -v @var{file-number}
 Print a line for each unpairable line in file @var{file-number}