mjhjmtu

Saying that I have two files: a.txt and b.txt.

The content of a.txt:

hello world

The content of b.txt:

hello world
something else

Of course I can use vimdiff to check their difference, I can make sure that a.txt is a subset of b.txt, which means that b.txt must contain all of lines existing in a.txt (just like the example above).

My question is how to record lines which exists in b.txt but doesn't exist in a.txt into a file?

comm -1 -3 a.txt b.txt > c.txt

The -1 excludes lines that are only in a.txt, and the -3 excludes lines that are in both. Thus only the lines exclusively in b.txt are output (see man comm or comm --help for details). The output is redirected to c.txt

If you want the difference between the two files, use diff rather than comm. e.g.

diff -u a.txt b.txt > c.txt

If you dont care for subset, you can use just

diff a.txt b.txt|grep ">"|cut -c 3- > foo.txt

.

$ cat a.txt
hello world
$ cat b.txt
hello world
something else
$ diff a.txt b.txt|grep ">"|cut -c 3- > foo.txt
$ cat foo.txt
something else