Get the unique lines of second file in result of comparing two files

I have two text files, and I want to read file1 line by line, searching for the same line in file2 and removing it from file2.

I have the pseudocode of:

for line in file1.txt
do sed search line and delete in file2.txt
done

3 Answers

You could accomplish this with grep.

Here is an example:

$ echo localhost > local_hosts
$ grep -v -f local_hosts /etc/hosts
127.0.1.1 ubuntu
# The following lines are desirable for IPv6 capable hosts
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

Generally you want to keep the lines in file2 that are not in file1 actually.

There are more possibilities of these,

comm <(sort file1) <(sort file2) -23

via join

join -v 1 <(sort file1) <(sort file2)

or via AWK which doesn't need to sort the files:

awk 'NR==FNR{lines[$0];next} !($0 in lines)' file2 file1

I found a way to do this through some more internet searches. And using just grep too without needing to sort the file.

grep -Fvxf file2 file1

This will display the new information on screen which presents a problem since I wanted to remove what was in file2 from file1 and have a new file1. Since the above code worked, I just had to add to it to get what I wanted.

grep -Fvxf file2 file1 > tempfile && mv tempfile file1

That solves my problems. Maybe not the best way but it does work.