How to Patch Files [commands diff and patch]

cp gmail-contacts-00001.vcf GmailContacts.vcf ./patches/

cd patches/

1. Clearly mark OLD and NEW files:
rename files to *-old and *-new
	$ mv gmail-contacts-00001.vcf gmail-contacts-old.vcf && mv GmailContacts.vcf gmail-contacts-new.vcf

2. Compare files:
create the patch file using diff command
	$ diff -u gmail-contacts-old.vcf gmail-contacts-new.vcf >gmail-contacts.patch

3. 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.
	$ patch --dry-run < gmail-contacts.patch

3. Update OLD file:
create backup file of gmail-contacts-old.vcf before patching i.e. creates gmail-contacts-old.vcf.orig before applying patch to the older filer
	$ patch -b < gmail-contacts.patch

You can also use -V to decide the backup filename format as shown below. Now you will have a file name “gmail-contacts-old.vcf.~1~”.
			$ patch -b -V numbered  > gmail-contacts.patch

4. 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 -R < gmail-contacts.patch