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List:       busybox
Subject:    AW: BUG: grep -v -x -f does not work when pattern file is empty
From:       <dietmar.schindler () manroland-web ! com>
Date:       2018-04-05 9:57:17
Message-ID: 9bf2bf8dd0b34f949f2b563ac3e95aee () AUSMXMBX04 ! mrws ! biz
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> Von: Quentin Rameau
> Gesendet: Donnerstag, 5. April 2018 10:24
> 
> Hello Dietmar,
> 
> > > The standard specifies, for the -f flag:
> > > 
> > > "Patterns in pattern_file shall be terminated by a <newline>. A null
> > > pattern can be specified by an empty line in pattern_file."
> > 
> > The above does not rule an empty pattern_file (i. e. no patterns) out.
> > The actually relevant part from
> > http://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html
> > is:
> > 
> > -f  pattern_file
> > Read one or more patterns from the file named by the pathname
> > pattern_file. …
> > 
> > > So calling grep -f with an empty file is non-standard thus not a BUG
> > > per se.
> > 
> > True, but because of the wording "one or more patterns".
> 
> Well, it's a combination of both. You don't get an empty set of
> patterns without specifying[:] a pattern must be an empty line.

Hello, Quentin,

I beg to differ. Your subclause "a pattern must be an empty line" doesn't make sense, \
so I take you meant "a null pattern must be an empty line". But "a null pattern" is \
distinct from "an empty set of patterns"; a set of patterns that consists of one null \
pattern is not an empty set, but rather a set with cardinality one.

> But this is my mistake for keeping implied that yes, specifying an
> option means it mustn't be empty. ^^

It doesn't mean that as a matter of course. If the "one or more patterns" were \
omitted from the above, it would read "Read patterns from the file named by the \
pathname pattern_file." Then, zero patterns (i. e. an empty pattern_file) would be \
legal, and the sentences you cited would not apply at all, because they refer to \
                patterns, of which there aren't any in an empty file.
--
Regards,
Dietmar
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