Example Patch File

Here is an example patch file (aka "diff" or "patch"):

Index: tmp/hello.c
===================================================================
--- tmp.orig/hello.c        2009-04-22 09:59:28.707583382 -0600
+++ tmp/hello.c     2009-04-22 10:00:06.432872620 -0600
@@ -3,6 +3,6 @@
 int
 main ()
 {
-    printf ("Hello world\n");
+    printf ("Hello Doctor Who\n");
     return 0;
 }

The patch file indicates to perform the instructions shown in the following figure:

Syntax of a Patch File

Patch files are divided in two parts: (1) headers, and (2) hunks. The header forms the first few lines, and the hunk starts with an "@@" symbol. Here is how to interpret a diff file:

1. In the header, if the line starts with:

   • "---"

     This specifies the original file name (prior to being patched). Usually, this filename is ignored. The space between the filename and optional timestamp is supposed to be a tab character ("t").

   • "+++"

     This is the new file name that is going to be patched. The space between the filename and optional timestamp is supposed to be a tab character ("\t").

2. In the hunk (delimited by "@@"), if the line starts with:

   • " " (just a single space character)

     This is called a "context line". It doesn't result in any operation happening. But it does help you figure out how to locate other lines where operations do happen.

   • "-"

     This is a delete operation. The line indiciated by the "-" is to be removed.

   • "+"

     This is an insert operation. The line indiciated by the "+" is to be inserted.