Listing lines from just one file in DIFF

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

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'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