I would like to get (GNU)DIFF to printout only lines that are different in one file. So given

    ==> diffa.txt <==
    line2 - in a only
    line4 changed

    ==> diffb.txt <==
    line4 changed in b
    line6 in b only

i would like diff --someoption diffa.txt diffb.txt to produce

    line2 - in a only

    line4 changed

The following looks as though it should be helpful but it is a bit cryptic :

          Similar, but format GTYPE input groups with GFMT.

          Similar, but format all input lines with LFMT.

          Similar, but format LTYPE input lines with LFMT.

   LTYPE is `old', `new', or `unchanged'.
          GTYPE is LTYPE or `changed'.

          GFMT may contain:

   %<     lines from FILE1

   %>     lines from FILE2
  • 2
    The man page is definitely a bit terse on these flags! Good question. Oct 18, 2013 at 0:01
  • 2
    I would like to point out that ' info diff ' will give complete examples, there is a wealth of information in info. Jul 6, 2016 at 17:24

3 Answers 3


Not sure diff alone can do it but you can always use the power of other GNU utilities to help you.

diff -u diffa.txt diffb.txt | grep '^-[^-]' | sed 's/^-//'

It does the diff, then selects only the lines that begins with '-' - those are changed and have values from diffa.txt file, then sed just remove those '-' signs.

Edit: After few experiments with diff, looks like the below command produces what you want:

diff --changed-group-format='%<' --unchanged-group-format='' diffa.txt diffb.txt
  • For a one off the pipe method is fine. I would prefer ` perl -ne"print if s/^-//" ` but that is taste. I think I see what --changed-group-format='%<' are doing for me now....
    – justintime
    Apr 27, 2010 at 14:08
  • Thanks. :D I needed something like this to list all my packages between two computers and I used sdiff but this looks a bit better.
    – Rob
    Sep 21, 2011 at 14:34

More simple method is to use comm linux utility (It needs sorted file for input). It writes to standard output:

  • lines that are unique for diffa.txt

  • lines that are unique for diffb.txt

  • lines that are common

and you can suppress each one of them by parameter 1,2 or 3 accordingly. So in youre case it will look like this :

comm -23 diffa.txt diffb.txt

It suppresses lines that are unique for diffb.txt, lines that are common and prints out lines that are unique only for diffa.txt

Source from:https://www.tutorialspoint.com/unix_commands/comm.htm

  • I found this a lot easier than diff which isn't doing what I expect. And I believe it's preinstalled both on BSD (i.e. Mac OS X) as well as Ubuntu so no package manager nightmares. May 3, 2017 at 6:19
  • If diffa.txt and diffb.txt are not already sorted, we can do the following: comm -23 <(sort diffa.txt) <(sort diffb.txt)
    – Jea
    Jul 22, 2022 at 13:08

I'd like to mention that comm expects sorted input files and thus reports different results than diff.

diff --changed-group-format='%<' --unchanged-group-format='' diffa.txt diffb.txt

is universal. Kudos to @vava

  • This is a very useful remark.
    – user93692
    Mar 25, 2020 at 17:43

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