I just learn the command "diff"
now I have 2 files named text1 and text2
and the contents are like this ：
text1:
a
b
c
================
text2:
c
d
e
================
now I use the command :

diff -u text1 text2 > text 3
and the result will be ：
--- text1
+++ text2
-a
-b
+d
+e
c
=================
I want to keep the same contents in 2 files
how should I do
please help me
thank you.

You mean you want to apply the text3 file so text1 and text2 are the same?

It helps you think about it if you alter your filenames slightly:

diff file1 file2 > file12.diff

file12.diff contains instructions- to turn file1 into file2, it is called a "patch". (A whole lot of stuff probably just snapped into perspective?)

patch file1 file12.diff

The result will still be called file1 - you can use patch and diff with version numbers to help with this.

I think I didn't explain my question clearly...

if i have a file named text3 and the contents are as below:
================================================================
+a
+b
c
-d
-e
================================================================

all i want to do is to delete the line being with "+" and "-"

then the file text3 will have the content:
==================================================================
c
==================================================================
Is there any linux command can help me delete the line in the content?

I have one more question here, how can I use "sort" to make the text3 contents like this:
=====================================================================
c
+a
+b
-d
-e
=====================================================================
I just want to pick up the lines without starting symbol "+" and "-"

Try this:

fgrep -xf file1 file2

Yeah, I just need this command
thank you and thank Simon Bridge for helping solve the problem.

Ah - so you wanted to find what files text1 and text2 had in common?

Yeah, I just wanted to find the same components in the 2 files.

However, You let me learn more about linux command, I am appreciated.