[SOLVED] Why does grep return a match when I used -w and not without it?

Issue

I ran the following command:

grep -f fileA.txt -v fileB.txt

Let's say in fileB there is a string like this: astringlikethis12345

Because I wanted to see what is in fileB but not in fileA, but it returns no hits. but if i do the same command with -w:

grep -f fileA.txt -v -w fileB.txt

it returns the string I was looking for, why is this? -w is to match the whole string, but even without it, it should still return the hit, right? Whether partial or full.

Thank you

Solution

check in fileA for the word you want to exclude in searching of fileA. There is might be a space or tab next to that word .

It is working fine as expected on my system

from grep help :

-w, --word-regexp force PATTERN to match only whole words

-v, --invert-match select non-matching lines

[root@project1-master ~]# cat filea

this

[root@project1-master ~]# cat fileawithspacenexttothis

this

[root@project1-master ~]# cat fileb

this

that

these

thisthis

this this that

1. "grep -f filea -v fileb" is equivalent to "grep -v this fileb" , so it returns that & these

[root@project1-master ~]# grep -f filea -v fileb that

these

2."grep -f filea -v -w fileb" is equivalent to grep -v -w this fileb , so it will display 'thisthis' as well as there is word boundary for this in 'thisthis'

[root@project1-master ~]# grep -f filea -v -w fileb

that

these

thisthis

3. grep -f fileawithspacenexttothis -v fileb is equivalent to "grep -v 'this ' fileb"

[root@project1-master ~]# grep -f fileawithspacenexttothis -v fileb

that

these

thisthis

4.grep -f fileawithspacenexttothis -v -w fileb is equivalent to "grep -v -w 'this ' fileb" , it will display "thisthis" & "this this that" as there is no word boundary in them for "this "

[root@project1-master ~]# grep -f fileawithspacenexttothis -v -w fileb

that

these

thisthis

this this that

May be it will be usefull if you provide sample content of filea & fileb and grep output.

Answered By - confused genius