diff is a *nix command that takes two files and compares them, reporting on the differences between them. For example:
$ diff foo.bar foo.bar
$
When the files are identical, no output is generated (as in this case, comparing a file to itself).
The problem with diff is that its output may not be immediately obvious to the ordinary user. For example, consider the two files:
foo.bar
this is a simple 3-line file
this is the third line
and
foo.baz
this is a simple 3-line file
this is the third line
AND SURPRISE! a fourth line
Run diff like this:
$ diff foo.baz foo.bar
1c1
< foo.baz
---
> foo.bar
4d3
< AND SURPRISE! a fourth line
The output is very clear--if you are a computer. diff is particularly useful for using in scripts that look at two files and then pass along the differences to some other program, which can then do something with that information automatically.
As a person, it's also fairly clear once I can get my head wrapped around how it works: the output tells me what I have to do to make the first file identical to the second file. The output starts with a line number(s) in the first file, a letter ("a" for add, "c" for change, and "d" for delete) that tells me what to do with that line in the first file, and line number(s) from the second file.
Following the codes that specify line numbers and action, there will be one (or more) lines underneath, with either the "<" or ">" symbols at the left. ">" means "put this in" and "<" means "take this out".
The first 4 lines of the diff output, 1c1 tells me that I need to change the first line in foo.baz from “foo.baz” (delete the first line) to “foo.bar” (add that as the first line).
The last two lines tell me that I need to delete the fourth line, and then the files will be synchronized from the third line onward.
Comparing the two files the “other” way gives this result:
$ diff foo.bar foo.baz
1c1
< foo.bar
---
> foo.baz
3a4
> AND SURPRISE! a fourth line
Again, the first line needs to be changed, but this time instead of deleting a line, a line needs to be added to make the two files identical.
It started to make much more sense when I started to play around with files that were almost the same and just experimented with them.