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

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Your message dated Thu, 2 Jan 2020 01:04:21 -0800
with message-id <address@hidden>
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.

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33972: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=33972
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--- Begin Message --- Subject: grep -vf <(<nothing>) filters all rows Date: Fri, 4 Jan 2019 00:04:28 -0500 User-agent: SquirrelMail/1.4.22

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

--- End Message ---

> --- Begin Message ---
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> Subject: 
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> grep -vf <(<nothing>) filters all rows
>
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> Date: 
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> Thu, 2 Jan 2020 01:04:21 -0800
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> User-agent: 
>
> Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2
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>
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> > 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.
>
>
> --- End Message ---