New post

content/blog/patch-files.md

@@ -0,0 +1,91 @@
+---
+title: "Creating Patch Files"
+date: 2024-05-18T20:10:21-04:00
+draft: false 
+tags: []
+math: false
+medium_enabled: false
+---
+
+Distributing software can be difficult. Is the shared library called `x` on this distribution or is it called `y`? This may lead to a lot of distribution specific code.
+
+It might not make sense to commit these changes to the main software repository. An alternative is to create *patch files*. These are files that capture exactly how you want to update a given file.
+
+Let's take the following C code as an example:
+
+```c
+#include <stdio.h>
+
+int main(void)
+{
+  printf("Goodbye World\n");
+  return 0;
+}
+```
+
+Instead, we want for it to print out the classic "Hello World". To do this, first let us save the original file.
+
+```bash
+cp hello-world.c hello-world.c.orig
+```
+
+Then let's edit `hello-world.c` to produce the desired result
+
+```c
+#include <stdio.h>
+
+int main(void)
+{
+  printf("Hello World\n");
+  return 0;
+}
+```
+
+Now we can create the patch file.
+
+```bash
+diff -Naur hello-world.c.orig hello-world.c > hello.patch
+```
+
+The options of `diff` are the following:
+
+| Flag | Description                                  |
+| ---- | -------------------------------------------- |
+| `-N` | Treat absent files as empty                  |
+| `-a` | Treat all files as text                      |
+| `-u` | Output 3 lines of unified context            |
+| `-r` | Recursively compare any subdirectories found |
+
+This will output the following patch file
+
+```diff
+--- hello-world.c.orig  2024-05-12 20:41:27.708297085 -0400
++++ hello-world.c       2024-05-12 20:41:36.742348955 -0400
+@@ -2,7 +2,7 @@
+ 
+ int main(void)
+ {
+-  printf("Goodbye World\n");
++  printf("Hello World\n");
+   return 0;
+ }
+```
+
+In order to test out the patch, let us revert the hello-world.c file to it's original state.
+
+```bash
+cp hello-world.c.orig hello-world.c
+```
+
+Now apply the patch
+
+```bash
+patch < hello.patch
+```
+
+To undo the patch
+
+```bash
+patch -R hello-world.c < hello.patch
+```
+