# Linux command: Diff/Patch

`diff` will calculate the difference of two files or folders and output with specific format. `patch` parse that specific format and apply difference to files or folders.

# Example for Files

Here we prepare two files: *file1.old.cpp* and *file1.new.cpp*

```cpp
// file1.old.cpp  
#include <iostream>

int main() {  
    return 0;  
}
```

```cpp
// file1.new.cpp  
#include <iostream>

int main() {  
    std::cout << "hello world" << std::endl;  
    return 0;  
}
```

Then the command can be:

```bash
diff -Naur file1.old.cpp file1.new.cpp > patch.diff
```

The patch.diff will be:

```diff
--- file1.old.cpp    2021-11-03 14:32:34.000000000 +0800  
+++ file1.new.cpp    2021-11-03 14:33:00.000000000 +0800  
@@ -1,5 +1,6 @@  
 #include <iostream>

int main() {  
**+    std::cout << "hello world" << std::endl;  
**    return 0;  
 }  
\\ No newline at end of file
```

## -Naur Parameters

*   N: Treat removed file as empty.   
    **This parameter allow patch command to remove or create files.**
*   a: Allow calculating difference of binary file.
*   r: Recursively visit the folder tree.
*   u: Specific format output of difference.

## -u: Specific format

Without this parameter, we’ll get the format below, and the patch command cannot parse it.

```bash
$ diff file1.old.cpp file1.new.cpp > patch.diff  
$ cat patch.diff  
3a4  
>     std::cout << "hello world" << std::endl;  
$ patch -p0 < patch.diff  
patch: **** Only garbage was found in the patch input.
```

# Example for Folder

```
old  
├── main.cpp (file1.old.cpp)  
└── legacy  
      └── 1.txt
```

```
new  
└── main.cpp (file1.new.cpp)
```

This example has a absent file: *old/legacy/1.txt* and diff treat *new/legacy/1.txt* as an empty file with timestamp = unix time 0.

```diff
diff -Naur old/legacy/1.txt new/legacy/1.txt  
--- old/legacy/1.txt    2021-11-03 14:47:46.000000000 +0800  
+++ new/legacy/1.txt    **1970-01-01 08:00:00.000000000 +0800**  
@@ -1 +0,0 @@  
\-123  
\\ No newline at end of file  
diff -Naur old/main.cpp new/main.cpp  
--- old/main.cpp    2021-11-08 11:55:32.000000000 +0800  
+++ new/main.cpp    2021-11-08 11:55:35.000000000 +0800  
@@ -1,5 +1,6 @@  
 #include <iostream>

int main() {  
+    std::cout << "hello world" << std::endl;  
    return 0;  
 }  
\\ No newline at end of file
```

# Patch

Patch old version to new version:

```bash
diff -Naur file1.old.cpp file1.new.cpp > patch.diff  
patch -p0 < patch.diff
```

“Patch” new version to old version:

```bash
patch -p0 < patch.diff -R
```

# Reference

*   [https://man7.org/linux/man-pages/man1/patch.1.html](https://man7.org/linux/man-pages/man1/patch.1.html)
*   [https://man7.org/linux/man-pages/man1/diff.1.html](https://man7.org/linux/man-pages/man1/diff.1.html)
*   [https://www.gnu.org/software/diffutils/manual/html\_node/Tips-for-Patch-Producers.html](https://www.gnu.org/software/diffutils/manual/html_node/Tips-for-Patch-Producers.html)