How can I get diff to show only added and deleted lines? If diff can't do it, what tool can?

Question

How can I get diff to show only added and deleted lines? If diff can't do it, what tool can?

Answer 1

Another way to look at it:

Show lines that only exist in file a:

comm -23 a b

Show lines that only exist in file b:

comm -13 a b

Show lines that only exist in one line or the other:

comm -3 a b | sed 's/^\t//'

Answer 2

comm might do what you want. From its man page:

DESCRIPTION

Compare sorted files FILE1 and FILE2 line by line.

With no options, produce three-column output. Column one contains lines unique to FILE1, column two contains lines unique to FILE2, and column three contains lines common to both files.

These columns are suppressable with -1, -2 and -3 respectively.

Example:

[root@dev ~]# cat a
common
shared
unique

[root@dev ~]# cat b
common
individual
shared

[root@dev ~]# comm -3 a b
    individual
unique

And if you just want the unique lines and don't care which file they're in:

[root@dev ~]# comm -3 a b | sed 's/^\t//'
individual
unique

As the man page says, the files must be sorted beforehand.

Answer 3

That's what diff does by default... Maybe you need to add some flags to ignore whitespace?

diff -b -B

should ignore blank lines and different numbers of spaces.

Answer 4

No, diff doesn't actually show the differences between two files in the way one might think. It produces a sequence of editing commands for a tool like patch to use to change one file into another.

The difficulty for any attempt at doing what you're looking for is how to define what constitutes a line that has changed versus a deleted one followed by an added one. Also what to do when lines are added, deleted and changed adjacent to each other.