diff分析两个文件并打印不同的行。 本质上，它输出一组指令，用于如何更改一个文件使其与第二个文件相同。它实际上并没有改变文件; 然而，它可以为程序ed（或ex）生成一个脚本（带-e选项）。
diff如何工作
准备实验数据
[root@localhost ~]# cat file1.txt I need to buy apples. I need to run the laundry. I need to wash the dog. I need to get the car detailed. [root@localhost ~]# cat file2.txt I need to buy apples. I need to do the laundry. I need to wash the car. I need to get the dog detailed.
使用diff自动为我们显示两个文件之间哪些行不同：
[root@localhost ~]# diff file1.txt file2.txt 2,4c2,4 < I need to run the laundry. < I need to wash the dog. < I need to get the car detailed. --- > I need to do the laundry. > I need to wash the car. > I need to get the dog detailed.
我们来看看这个输出是什么意思。 需要记住的重要一点是，当diff向你描述这些差异时，他的目的是：告诉你如何改变第一个文件使其与第二个文件相匹配。
diff输出的第一行将包含：
对应于第一个文件的行号，
一个字母（a用于添加，c用于改变，d用于删除）
对应于第二个文件的行号。
“2,4c2,4”表示：“第一个文件中的第2行到第4行需要更改以匹配第二个文件中的第2行到第4行。” 然后它会告诉我们每个文件中的内容。前面有<的行是第一个文件的行;前面有>的行是第二个文件的行;
"---"仅仅是用于分割file1.txt和file2.txt 这两个文件
另一个例子
[root@localhost ~]# cat file1.txt I need to go to the store. I need to buy some apples. When I get home, I'll wash the dog. [root@localhost ~]# cat file2.txt I need to go to the store. I need to buy some apples. Oh yeah, I also need to buy grated cheese. When I get home, I'll wash the dog.
[root@localhost ~]# diff file1.txt file2.txt 2a3 > Oh yeah, I also need to buy grated cheese.
输出告诉我们“在第一个文件的第二行之后，需要添加一行：从第二个文件开始的第三行。” 然后它向我们展示了要在第一个文件添加的这一行是什么。
另一个例子
[root@localhost ~]# cat file1.txt I need to go to the store. I need to buy some apples. When I get home, I'll wash the dog. I promise. [root@localhost ~]# cat file2.txt I need to go to the store. I need to buy some apples. When I get home, I'll wash the dog.
[root@localhost ~]# diff file1.txt file2.txt 4d3 < I promise.
输出告诉我们“您需要删除第一个文件中的第4行，以便两行文件在第3行同步。” 然后它向我们显示需要删除的行的内容。
在上下文中查看差异输出
上面的例子显示了diff的默认输出。 它的目的是被计算机阅读，而不是人类阅读，因此对于人类目的而言，有时候可以看到更改的上下文。
GNU diff是大多数Linux用户将使用的版本，它提供了两种不同的方法来实现这一点：“上下文模式”和“统一模式”。
要在上下文模式下查看差异，请使用-c选项。 例如，假设file1.txt和file2.txt包含以下内容：
[root@localhost ~]# cat file1.txt apples oranges kiwis carrots [root@localhost ~]# cat file2.txt apples kiwis carrots grapefruits
[root@localhost ~]# diff -c file1.txt file2.txt *** file1.txt 2018-06-15 03:42:16.841974345 -0400 --- file2.txt 2018-06-15 03:42:45.919587320 -0400 *************** *** 1,4 **** apples - oranges kiwis carrots --- 1,4 ---- apples kiwis carrots + grapefruits
前两行：***标识源文件，---标识目标文件，然后又分别跟文件名，修改日期时间
"***************" 仅仅是分隔符方便阅读
***第一个文件行的范围****
第一个文件每行的内容。如果这行没变化，则以2个开头。如果这一行变化了，则一个标识字符+开头
标识字符如下：
---第二个文件行的范围----
Unified Mode
[root@localhost ~]# diff -u file1.txt file2.txt --- file1.txt 2018-06-15 03:42:16.841974345 -0400 +++ file2.txt 2018-06-15 03:42:45.919587320 -0400 @@ -1,4 +1,4 @@ apples -oranges kiwis carrots +grapefruits
这里差异显示我们一个文本，而不是两个单独的文本。
@@ -1,4 +1,4 @@
-1,4表示第一个文件1-4行，+1,4表示第二个文件1-4行
生成脚本
-e选项通过diff来输出一个脚本，该脚本可以被编辑程序ed或ex使用，该脚本包含一系列命令。 这些命令是c（change），a（add）和d（delete）的组合。
[root@localhost ~]# cat file1.txt Once upon a time, there was a girl named Persephone. She had black hair. She loved her mother more than anything. She liked to sit outside in the sunshine with her cat, Daisy. She dreamed of being a painter when she grew up. [root@localhost ~]# cat file2.txt Once upon a time, there was a girl named Persephone. She had red hair. She loved chocolate chip cookies more than anything. She liked to sit outside in the sunshine with her cat, Daisy. She would look up into the clouds and dream of being a world-famous baker.
[root@localhost ~]# diff -e file1.txt file2.txt 5,6c She would look up into the clouds and dream of being a world-famous baker. . 2,3c She had red hair. She loved chocolate chip cookies more than anything. .
也可以将差异内容重定向到文件，方便ed使用
[root@localhost ~]# diff -e file1.txt file2.txt > my-ed-script.txt [root@localhost ~]# cat my-ed-script.txt 5,6c She would look up into the clouds and dream of being a world-famous baker. . 2,3c She had red hair. She loved chocolate chip cookies more than anything. .
还差一步，需要在脚本中追加w，表示 编辑写入实际文件
[root@localhost ~]# echo "w" >> my-ed-script.txt
下面命令就可以修改原始文件
ed - file1.txt < my-ed-script.txt
-指示ed从标准输入读取，<将my-ed-script.txt重定向为标准输入。这条命令执行完后，源文件file1.txt就被修改了。
并排显示差异
[root@localhost ~]# diff -y file1.txt file2.txt Once upon a time, there was a girl named Persephone. Once upon a time, there was a girl named Persephone. She had black hair. | She had red hair. She loved her mother more than anything. | She loved chocolate chip cookies more than anything. She liked to sit outside in the sunshine with her cat, Daisy. She liked to sit outside in the sunshine with her cat, Daisy. She dreamed of being a painter when she grew up. | She would look up into the clouds and dream of being a world- <