Creating and using patch files from diff

1. Create a Patch File using diff

To understand this, let us create a small C program named hello.c

#include <stdio.h> int main() {printf("Hello World\n");}

Now, copy the hello.c to hello_new.c

$ cp hello.c hello_new.c

Edit the hello_new.c as shown below to make some small changes:

#include <stdio.h>int main(int argc, char *argv[]) {printf("Hello World\n");return 0;}

Finally, create the patch file using diff command as shown below:

$ diff -u hello.c hello_new.c > hello.patch

The above command will create a patch file named “hello.patch”.

--- hello.c	2014-10-07 18:17:49.000000000 +0530+++ hello_new.c	2014-10-07 18:17:54.000000000 +0530@@ -1,5 +1,6 @@ #include <stdio.h> -int main() {+int main(int argc, char *argv[]) { 	printf("Hello World\n");+	return 0; }

2. Apply Patch File using Patch Command

The “patch” command takes a patch file as input and apply the differences to one or more original file(s), producing patched versions.

patch -p[num] < patchfilepatch [options] originalfile patchfile

Use the patch command as shown below to apply the hello.patch to the original hello.c source code.

$ patch < hello.patchpatching file hello.c

The hello.patch file contains the name of the file to be patched. Once the file is patched, both hello.c and hello_new.c will have the content.

3. Create a Patch From a Source Tree

The above example was so simple that it works only with one file. We will see how to create and apply patch for a complete source tree by taking “openvpn” source code as example.

I’ve downloaded 2 version of openvpn, openvpn-2.3.2 and openvpn-2.3.4.

tar -xvzf openvpn-2.3.2.tar.gztar -xvzf openvpn-2.3.4.tar.gz

Now we will create the patch using the following command.

diff -Naur /usr/src/openvpn-2.3.2 /usr/src/openvpn-2.3.4 > openvpn.patch

The above command will operate recursively and find the differences, and place those differences in the patch file.

4. Apply Patch File to a Source Code Tree

The following patch commands can be used to apply the patch to source tree.

# patch -p3 < /root/openvpn.patchpatching file openvpn-2.3.2/aclocal.m4patching file openvpn-2.3.2/build/Makefile.inpatching file openvpn-2.3.2/build/msvc/Makefile.in...

Please note that we are executing the command from /usr/src/. The patch file contains all the filenames in absolute path format( from root ). So when we execute from /usr/src, without the “-p” option, it will not work properly.

-p3 tells the patch command to skip 3 leading slashes from the filenames present in the patch file. In our case, the filename in patch file is “/usr/src/openvpn-2.3.2/aclocal.m4”, since you have given “-p3”, 3 leading slashes, i.e. until /usr/src/ is ignored.

5. Take a Backup before Applying the Patch using -b

You can take a backup of the original file before applying the patch command using the -b option as shown below.

$ patch -b < hello.patchpatching file hello.c

Now you will have a file name “hello.c.orig”, which is the backup of the original hello.c.

You can also use -V to decide the backup filename format as shown below. Now you will have a file name “hello.c.~1~”.

$ patch -b -V numbered < hello.patchpatching file hello.c

6. Validate the Patch without Applying (Dry-run Patch File)

You can dry run the patch command to see if you are getting any errors, without patching the file using –dry-run option as shown below.

$ patch --dry-run < hello.patchpatching file hello.c

You can see that hello.c is not modified at all.

7. Reverse a Patch that is Already Applied (Undo a Patch)

You can use the -R option to reverse a patch which is applied already.

$ patch < hello.patchpatching file hello.c$ ls -l hello.c-rw-r--r-- 1 lakshmanan users  94 2014-10-07 20:05 hello.c$ patch -R < hello.patchpatching file hello.c$ ls -l hello.c-rw-r--r-- 1 lakshmanan users  62 2014-10-07 20:04 hello.c

You can notice from the filesize, that the patch, which is applied already is reversed when we used the -R option.