Introduction to the usage of diff and patch commands for creating and applying file patches in unix systems.

1. Patching files in Unix
with diff and patch commands
Alberto Bustamante Reyes
http://alb3rtobr.github.io

2. • Compares two files and gives the necessary steps to
be applied to the first one to become the same as the
second one.
diff command
I need to buy:
Apples
Cheese
Bread
Butter
Oranges
You need to buy:
Red Apples
Cheese
Butter
Cookies
Oranges
a.txt b.txt

3. diff example
$-> diff a.txt c.txt
1,2c1,2
< I need to buy:
< Apples
---
> You need to buy:
> Red Apples
4d3
< Bread
5a5
> Cookies
1
2
3
4
5
6
1
2
3
4
5
6
I need to buy:
Apples
Cheese
Bread
Butter
Oranges
You need to buy:
Red Apples
Cheese
Butter
Cookies
Oranges

4. diff example
$-> diff a.txt c.txt
1,2c1,2
< I need to buy:
< Apples
---
> You need to buy:
> Red Apples
4d3
< Bread
5a5
> Cookies
1
2
3
4
5
6
1
2
3
4
5
6
I need to buy:
Apples
Cheese
Bread
Butter
Oranges
You need to buy:
Red Apples
Cheese
Butter
Cookies
Oranges
List of differences

5. diff example
$-> diff a.txt c.txt
1,2c1,2
< I need to buy:
< Apples
---
> You need to buy:
> Red Apples
4d3
< Bread
5a5
> Cookies
1
2
3
4
5
6
1
2
3
4
5
6
I need to buy:
Apples
Cheese
Bread
Butter
Oranges
You need to buy:
Red Apples
Cheese
Butter
Cookies
Oranges
Change line 1 to 2 of a.txt by
line 1 to 2 of b.txt

6. diff example
$-> diff a.txt c.txt
1,2c1,2
< I need to buy:
< Apples
---
> You need to buy:
> Red Apples
4d3
< Bread
5a5
> Cookies
1
2
3
4
5
6
1
2
3
4
5
6
I need to buy:
Apples
Cheese
Bread
Butter
Oranges
You need to buy:
Red Apples
Cheese
Butter
Cookies
Oranges
Delete from line 4 to 3 (not
included)

7. diff example
$-> diff a.txt c.txt
1,2c1,2
< I need to buy:
< Apples
---
> You need to buy:
> Red Apples
4d3
< Bread
5a5
> Cookies
1
2
3
4
5
6
1
2
3
4
5
6
I need to buy:
Apples
Cheese
Bread
Butter
Oranges
You need to buy:
Red Apples
Cheese
Butter
Cookies
Oranges
Add line 5

8. diff example (context)
$ -> diff -c a.txt b.txt
*** a.txt Thu Oct 23 12:39:32 2014
--- b.txt Thu Oct 23 12:39:47 2014
***************
*** 1,6 ****
! I need to buy:
! Apples
Cheese
- Bread
Butter
Oranges
--- 1,6 ----
! You need to buy:
! Red Apples
Cheese
Butter
+ Cookies
Oranges

9. diff example (context)
$ -> diff -c a.txt b.txt
*** a.txt Thu Oct 23 12:39:32 2014
--- b.txt Thu Oct 23 12:39:47 2014
***************
*** 1,6 ****
! I need to buy:
! Apples
Cheese
- Bread
Butter
Oranges
--- 1,6 ----
! You need to buy:
! Red Apples
Cheese
Butter
+ Cookies
Oranges
List of differences in the context
of both files content

10. diff example (context)
$ -> diff -c a.txt b.txt
*** a.txt Thu Oct 23 12:39:32 2014
--- b.txt Thu Oct 23 12:39:47 2014
***************
*** 1,6 ****
! I need to buy:
! Apples
Cheese
- Bread
Butter
Oranges
--- 1,6 ----
! You need to buy:
! Red Apples
Cheese
Butter
+ Cookies
Oranges
Change
Change

11. diff example (context)
$ -> diff -c a.txt b.txt
*** a.txt Thu Oct 23 12:39:32 2014
--- b.txt Thu Oct 23 12:39:47 2014
***************
*** 1,6 ****
! I need to buy:
! Apples
Cheese
- Bread
Butter
Oranges
--- 1,6 ----
! You need to buy:
! Red Apples
Cheese
Butter
+ Cookies
Oranges
Delete

12. diff example (context)
$ -> diff -c a.txt b.txt
*** a.txt Thu Oct 23 12:39:32 2014
--- b.txt Thu Oct 23 12:39:47 2014
***************
*** 1,6 ****
! I need to buy:
! Apples
Cheese
- Bread
Butter
Oranges
--- 1,6 ----
! You need to buy:
! Red Apples
Cheese
Butter
+ Cookies
Oranges
Add

13. diff example (unified)
$ -> diff -u a.txt b.txt
--- a.txt 2014-10-23 15:54:50.588923000 +0200
+++ b.txt 2014-10-23 15:54:59.549414000 +0200
@@ -1,6 +1,6 @@
-I need to buy:
-Apples
+You need to buy:
+Red Apples
Cheese
-Bread
Butter
+Cookies
Oranges

14. diff example (unified)
$ -> diff -u a.txt b.txt
--- a.txt 2014-10-23 15:54:50.588923000 +0200
+++ b.txt 2014-10-23 15:54:59.549414000 +0200
@@ -1,6 +1,6 @@
-I need to buy:
-Apples
+You need to buy:
+Red Apples
Cheese
-Bread
Butter
+Cookies
Oranges
List of differences in a unified
way

15. • Uses the diff command output
• Apply a patch
• Revert a patch
• Very common error message:
Patch command
$ -> patch < a-to-b.patch
can't find file to patch at input line 1
Perhaps you used the wrong -p or --strip option?
File to patch:
$ -> diff -u a.txt b.txt > a-to-b.patch
$ -> patch < a-to-b.patch
patching file a.txt
$ -> patch –R < a-to-b.patch

16. • Common problem:
▫ User A creates a patch for /myapp/example/files/a.txt
▫ The header of the patch will contain the path of the file:
$ -> cat changes-in-a.patch
--- /myapp/example/files/a.txt.backup 2014-10-23 15:54:50.588923000 +0200
+++ /myapp/example/files/a.txt 2014-10-23 15:54:59.549414000 +0200
▫ User A sends the patch to User B.
▫ User B file is stored in a different path, i.e, /myapp/files/a.txt
▫ User B gets an error because patch command will try to apply
the changes in the path stated in the patch file header.
Strip (-p) option

17. › Indicates how many slashes are strip from the file names
found in the patch
› If a patch was generated for /myapp/example/files/a.txt the
patch command will try to apply it to the same path.
› Using strip option:
– p0 -> /myapp/example/files/a.txt
– p1 -> myapp/example/files/a.txt
– p2 -> example/files/a.txt
Strip (-p) option
$ -> patch –p<number> < file.patch

18. Thanks for watching!