GNU bug report logs - #33972
grep -vf <(<nothing>) filters all rows

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#33972: closed (grep -vf <(<nothing>) filters all rows)
Date: Thu, 02 Jan 2020 09:05:02 +0000

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Your message dated Thu, 2 Jan 2020 01:04:21 -0800
with message-id <a1d82b51-9882-7212-ec86-f961c4300bc2 <at> cs.ucla.edu>
and subject line grep -vf <(<nothing>) filters all rows
has caused the debbugs.gnu.org bug report #33972,
regarding grep -vf <(<nothing>) filters all rows
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


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From: luke <at> gavelfamily.com
To: bug-grep <at> gnu.org
Subject: grep -vf <(<nothing>) filters all rows
Date: Fri, 4 Jan 2019 00:04:28 -0500

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I have to pre-check for an empty set before passing it to grep:
�
SAVED_RECORDs=($(sqlite3 -list -separator '/' files.db "SELECT FileName FROM Files WHERE ModTime > '2018-12-31'")) #Returns an empty set
NEW_RECs=($(ls * | grep -vf <(echo ${SAVED_RECORDs[@]}
| tr ' ' '\n') #Returns an empty list because grep considers SAVED_RECORDs to be universal rather than null set
Workaround:


if [ ${#SAVED_RECORDs[@]} -gt 0 ]; then

� NEW_RECs=($(ls * | grep -vf <(echo ${SAVED_RECORDs[@]} | tr ' ' '\n') #Now returns the expected subtraction of SAVED_RECORDs from NEW_RECs

fi

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From: Paul Eggert <eggert <at> cs.ucla.edu>
To: luke <at> gavelfamily.com
Cc: 33972-done <at> debbugs.gnu.org
Subject: grep -vf <(<nothing>) filters all rows
Date: Thu, 2 Jan 2020 01:04:21 -0800

> I have to pre-check for an empty set before passing it to grep:
> ...
> Workaround:

You're right that the empty pattern matches every line, since every line has an
empty string in it somewhere. This is considered to be a feature of grep, not a
bug. As you have a workaround I'm closing the bug report.

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