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

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

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

[Message part 1 (text/plain, inline)]

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

[Message part 2 (text/html, inline)]

Message #10 received at 33972-done <at> debbugs.gnu.org (full text, mbox):

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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