Git Pocket Guide

Chapter 11. Understanding Patches

A “patch” is a compact representation of the differences between two files, intended for use with line-oriented text files. It describes how to turn one file into another, and is asymmetric: the patch from file1 to file2 is not the same as the patch for the other direction (it would say to delete and add opposite lines, as we will see). The patch format uses context as well as line numbers to locate differing file regions, so that a patch can often be applied to a somewhat earlier or later version of the first file than the one from which it was derived, as long as the applying program can still locate the context of the change.

The terms “patch” and “diff” are often used interchangeably, although there is a distinction, at least historically. A diff only need show the differences between two files, and can be quite minimal in doing so. A patch is an extension of a diff, augmented with further information such as context lines and filenames, which allow it to be applied more widely. These days, the Unix diff program can produce patches of various kinds.

Here’s a simple patch, generated by git diff:

diff --git a/foo.c b/foo.c
index 30cfd169..8de130c2 100644
--- a/foo.c
+++ b/foo.c
@@ -1,5 +1,5 @@
 #include <string.h>

 int check (char *string) {
-    return !strcmp(string, "ok");
+    return (string != NULL) && !strcmp(string, "ok");
 }

Breaking this into sections:

diff --git a/foo.c b/foo.c

This is the Git diff header; diff --git isn’t a literal command, but ...