diff的三种模式
文件内容：
test1.txt
$ cat test1.txt
a
a
b
c
d
e

test2.txt
$ cat test2.txt
a
a
p
g
c
d
e

文件对比：
$ diff test1.txt test2.txt
3c3,4
< b
---
> p
> g

$ diff -c test1.txt test2.txt

*** test1.txt 2012-09-09 19:08:28.878588706 -0700
--- test2.txt 2012-09-09 19:09:07.802589127 -0700
***************
*** 1,6 ****
a
a
! b
c
d
e
--- 1,7 ----
a
a
! p
! g
c
d
e

$ diff -u test1.txt test2.txt
--- test1.txt 2012-09-09 19:08:28.878588706 -0700
+++ test2.txt 2012-09-09 19:09:07.802589127 -0700
@@ -1,6 +1,7 @@
a
a
-b
+p
+g
c
d
e