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: luke <at> gavelfamily.com
Subject: bug#33972: closed (grep -vf <(<nothing>) filters all rows)
Date: Thu, 02 Jan 2020 09:05:02 +0000

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Your bug report

#33972: grep -vf <(<nothing>) filters all rows

which was filed against the grep package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 33972 <at> debbugs.gnu.org.

-- 
33972: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=33972
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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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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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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