Diff not working as I expect

Question

I have 2 file, a.txt and b.txt and I want to compare them.

a.txt contains:

abc
jkl < jkl
mno > mno
pqr <> pqr

b.txt contains:

abc
jkl < jkl
mno > mno
pqr <> pqrs
stu

I'm using this script:

$ diff a.txt b.txt | grep "> " | cut -c3- > c.txt

Which results in c.txt:

pqr <> pqr
pqr <> pqrs
stu

Why is pqr <> pqr being included in the results? How can I resolve this?

Answer 1

As @StephaneChazelas noted, you forgot to anchor your regular expression, so the "> " will match the > in the line pqr <> pqr that is only in a.txt.

But it looks like you want a filter to only show the lines that are new or changed in b.txt. If you are using GNU diff, it offers you some options to achieve this directly:

$ diff --old-line-format '' --unchanged-line-format '' --new-line-format '%L' a.txt b.txt 
pqr <> pqrs
stu

The --old-line-format '' removes all lines that are not in b.txt, the --unchanged-line-format '' removes all lines common to both files and the --new-line-format '%L' shows just the line contents for all new or changed lines in b.txt. The leading > is suppressed.

Answer 2

As @StephaneChazelas stated in the comments, this would appear to have nothing to do with SSH. Running everything locally I get the same results as you, mainly:

$ diff a.txt b.txt | grep "> " | cut -c3-
pqr <> pqr
pqr <> pqrs
stu

The diff command is identifying lines that are different and the line pqr <> pqrs is unique among the 2 files. You can use a side by side diff to see which lines are lining up if it helps.

$ diff -y a.txt b.txt
abc                                                             abc
jkl < jkl                                                       jkl < jkl
mno > mno                                                       mno > mno
pqr <> pqr                                                    | pqr <> pqrs
                                                              > stu

Lines that are similar but not identical are marked with a |. Lines that are unique are marked with a > if only in file #2, or a < if only present in file #1. Identical lines do not show any such marking.

Answer 3

Maybe you have extra white spaces.

try

diff -w -B file1.txt file2.txt

-w Ignore all white space.

-B Ignore changes whose lines are all blank.