Differences Between Grep and Grep -f

grepregular expression

I have a file with the following 3 lines:

file.txt

aaa|
bbb|
ccc|

and another file with one line:

regex.txt

^aaa\|

$grep ^aaa\| file.txt yields:

aaa|

$grep -f regex.txt file.txt yields:

aaa|
bbb|
ccc|

Why are the results different for grep -f and grep?

$grep -V
grep (GNU grep) 3.1

$lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.1 LTS
Release:    18.04
Codename:   bionic

Best Answer

You're using grep with basic regular expressions. grep ^aaa\| file.txt is the same as typing grep "^aaa|" file.txt. While reading the file grep -f regex.txt file.txt is the same as grep "^aaa\|" file.txt., the escaped | means match ^aaa or "" which matches anything.

Related Solutions

Grep – How to Run Grep with Multiple AND Patterns

agrep can do it with this syntax:

agrep 'pattern1;pattern2'

With GNU grep, when built with PCRE support, you can do:

grep -P '^(?=.*pattern1)(?=.*pattern2)'

With ast grep:

grep -X '.*pattern1.*&.*pattern2.*'

(adding .*s as <x>&<y> matches strings that match both <x> and <y> exactly, a&b would never match as there's no such string that can be both a and b at the same time).

If the patterns don't overlap, you may also be able to do:

grep -e 'pattern1.*pattern2' -e 'pattern2.*pattern1'

The best portable way is probably with awk as already mentioned:

awk '/pattern1/ && /pattern2/'

With sed:

sed -e '/pattern1/!d' -e '/pattern2/!d'

Please beware that all those will have different regular expression syntax.

Grep – Why ‘Binary File Matches’ with Grep -I?

Looking at the grep manual, this seems to be because (bold mine):

If type is ‘without-match’, when grep discovers null input binary data it assumes that the rest of the file does not match; this is equivalent to the -I option.

However, grep also considers other data as indicating binary files:

Non-text bytes indicate binary data; these are either output bytes that are improperly encoded for the current locale (see Environment Variables), or null input bytes when the -z (--null-data) option is not given (see Other Options).

So the message won't be printed if:

the -I/--binary-files=without-match options are given
and the binary-ness is due to null bytes.

However, that's not the case with the example input. The example file is treated as binary because it doesn't fit the current locale (probably some UTF locale), not because it has null bytes. Otherwise:

% LC_ALL=C grep 8 test-file  
x���J�@`�y��
dIf��(��P������6u G7}S��8�0�&���'����9&�lL�Çl�t�y,蒩AU����F��cT�~
                                                                 �ʱ�k��]�Yx�$}�/����O�O�ù�4�����X��zd�Ȉ��W���,�2<�/�d�����+L
                                                                                                                            �Y�����7��PB

If I add a null byte to the file, grep fails successfully:

% printf '\0' >> test-file
% grep -I 8 test-file    
% echo $?
1

Best Answer

Related Solutions

Grep – How to Run Grep with Multiple AND Patterns

Grep – Why ‘Binary File Matches’ with Grep -I?

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