bug#21414: -F string with tailing newline always matches

[Paul Eggert]

Ian Brown - HNAS wrote:
> the pattern list used was the output of another command, it now needs to have 
> the terminating newline removed.

Typically the issue occurs with grep -f.  If the pattern list is generated via 
the shell, like this:

   grep "$(somecmd)" file

then the shell removes all trailing newlines from the output of SOMECMD, so it 
is not a problem in this case.  The shell even strips multiple trailing 
newlines, which in hindsight is probably a mistake but is standard behavior. 
So, for example:

   printf 'abc\n' | grep "$(printf 'xyz\n\n\n')"

does not output any matches, but:

   printf 'xyz\n\n\n' >pattern; printf 'abc\n' | grep -f pattern

outputs a match for abc, because the empty pattern matches every line.

> I didn't find any mention of this change of behaviour in the change logs

The change is listed in NEWS under grep-2.19 bug fixes, like this:

  grep no longer mishandles an empty pattern at the end of a pattern list.
  [bug introduced in grep-2.5]

This is due to commit 2d3832e1ff772dc1a374bfad5dcc1338350cc48b dated Fri Apr 11 
21:34:11 2014 +0900.  Here is the ChangeLog entry.

2014-04-11  Norihiro Tanaka  <address@hidden>

        grep: no match for the empty string included in multiple patterns
        * src/dfasearch.c (EGAcompile): Fix it.
        * src/kwsearch.c (Fcompile): Fix it.

This fixes Bug#17240, which essentially is the negation of your bug report, 
i.e., Bug#17240 asks for the standard grep behavior which we broke in grep 2.5. 
 You can see that bug report here:

http://bugs.gnu.org/17240