bug#22945: Surprising behaviour (bug?) of zgrep in combination with the -f option and process substitutions

[Fulvio Scapin]

Hello.

There is a problem with zgrep whenever the -f option actually reads from
the output of a process substition in bash.
A willingly trivial example below.

$ mkdir /tmp/test

$ cd /tmp/test

$ cat > first
aaa

$ cat > second
bbb

$ cat > third
ccc

$ cat > fourth
ddd

$ tail *
==> first <==
aaa

==> fourth <==
ddd

==> second <==
bbb

==> third <==
ccc

$ gzip -9 *

$ ls
first.gz  fourth.gz  second.gz  third.gz

$ cat > patterns
aaa
bbb
ccc
ddd

$ tail patterns
aaa
bbb
ccc
ddd

$ zfgrep -f <( cat patterns ) first.gz fourth.gz second.gz third.gz
first.gz:aaa

$ zfgrep -f patterns first.gz fourth.gz second.gz third.gz
first.gz:aaa
fourth.gz:ddd
second.gz:bbb
third.gz:ccc



zfgrep -f <( cat patterns ) first.gz fourth.gz second.gz third.gz
translates in
zfgrep -f /dev/fd/XX first.gz fourth.gz second.gz third.gz
where XX is a number, 63 for instance .

The problem, from what I understand, arises since

zgrep -f patternfile a.gz b.gz c.gz

actually is a succession of

gzip -dc a.gz | grep -f patternfile
gzip -dc b.gz | grep -f patternfile
gzip -dc c.gz | grep -f patternfile


Since patternfile in this case is /dev/fd/XX, only the first invocation of
grep in the first pipeline actually reads a pattern list, while the second
and third invocation get nothing, giving no match for b.gz and c.gz as a
result.


>From /bin/zgrep (Version 1.6, Ubuntu 15.10) one can read

  (-f | --file)
   # The pattern is coming from a file rather than the command-line.
   # If the file is actually stdin then we need to do a little
   # magic, since we use stdin to pass the gzip output to grep.
   # Turn the -f option into an -e option by copying the file's
   # contents into OPTARG.
   case $optarg in
   (" '-'" | " '/dev/stdin'" | " '/dev/fd/0'")
     option=-e
     optarg=" '"$(sed "$escape") || exit 2;;
   esac
   have_pat=1;;

The workaround concerning stdin should (maybe) also apply to situations
such as the one in my example?

Thanks in advance.

Fulvio Scapin