Question

I am having trouble searching for lines with . in complex (chained) grep commands.
Is this my error or a bug?

grep '\.' filename | grep 1    # works correctly (finds lines with '.' and '1')
grep 1 filename | grep '\.'    # fails: ignores 2nd grep, returns lines with or without '.'

It is not the pipe that causes this; it is really something about grep.
This frankly seems implausible, yet it is true:

cat filename | grep 1 | grep '\.'   # works correctly (and provides an easy work-around for the bug)

Adding more protection does not help:

grep 1 filename | grep '\\.'      # fails

If there is a something behind or in front of the wildcard, it works:

grep 1 filename | grep 'a\.'     #  works
grep 1 filename | grep '\.a'     #  works

But not if the something is a range:

grep 1 filename | grep '[0-9]\.'    # fails
grep 1 filename | grep '\.[0-9]'    # fails

I can’t believe this is really a bug – I must be missing something, right?

Thanks!

uname -a
Linux conception 2.6.32-33-generic #70-Ubuntu SMP Thu Jul 7 21:09:46 UTC 2011 i686 GNU/Linux

Answer 1

First of all, “chained” grep command is something like “mauvais ton”. Most of times grep | grep can be replaced with one grep with complicated regular expression.

Grepping lines with . and 1 in it.

$> cat ./text | grep -P "(\..*1|1.*\.)"
line with 1 and . in it

-P means perl regex
"(\..*1|1.*\.)" is (., some symbols and 1) or (1, some symbols and .)

So, if you not pretty sure about wildcards usage, regular expressions could be your useful, safe, and powerful tool.

$> cat ./text 
line without anything
another line
line with.ok
and one another
line with 1
some text

line with 1 and . in it
empty line
last line

First examples grep '\.' filename | grep 1, grep 1 filename | grep '\.', cat filename | grep 1 | grep '\.' returnes right answer

line with 1 and . in it

Second example grep 1 filename | grep '\\.' failed because you’re grepping \ with any symbol after it.