Everybody knows about diff.
I read about sdiff today for side by side diff.
Here is the one I found useful in some cases but I forget it so I am saving it here:
cmp – compare two files
cmp [-l | -s] file1 file2 [skip1 [skip2]]
The cmp utility compares two files of any type and writes the results to
the standard output. By default, cmp is silent if the files are the
same; if they differ, the byte and line number at which the first differ-
ence occurred is reported.
May be I will add some examples to show the difference but later.