Code:
Difference, similarity, compare
1) diff, standard
2) cmp, standard
3) dwdiff, cdif (local), word differences
4) cmptree (local), dirdiff (GUI), compare directory trees
5) docdiff, compare by character, word, line, numerous output formats
6) meld (GUI), diff and merge files
7) numdiff, my-ndiff (local), snd (local), compare numerically
8) comm, standard, compare 2 sorted files, 3 column output
9) multi-comm (local), compare n files, need not be sorted
10) contrast (local), like comm, needs no sort, symmetric and
simple differences, intersection, union
11) diff.pl, perl work-alike for diff, https://metacpan.org/pod/distribution/PerlPowerTools/bin/diff
12) multi-diff (local), common lines, multiple files, re-write omitting common lines
13) psame (local), find similarity between 2 text files
14) fdupes, rdfind, duff, find duplicate files