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91 lines
1.9 KiB
Markdown
91 lines
1.9 KiB
Markdown
---
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title: "Creating Patch Files"
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date: 2024-05-18T20:10:21-04:00
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draft: false
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tags: []
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math: false
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medium_enabled: false
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---
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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.
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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.
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Let's take the following C code as an example:
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```c
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#include <stdio.h>
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int main(void)
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{
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printf("Goodbye World\n");
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return 0;
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}
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```
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Instead, we want for it to print out the classic "Hello World". To do this, first let us save the original file.
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```bash
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cp hello-world.c hello-world.c.orig
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```
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Then let's edit `hello-world.c` to produce the desired result
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```c
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#include <stdio.h>
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int main(void)
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{
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printf("Hello World\n");
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return 0;
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}
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```
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Now we can create the patch file.
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```bash
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diff -Naur hello-world.c.orig hello-world.c > hello.patch
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```
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The options of `diff` are the following:
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| Flag | Description |
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| ---- | -------------------------------------------- |
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| `-N` | Treat absent files as empty |
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| `-a` | Treat all files as text |
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| `-u` | Output 3 lines of unified context |
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| `-r` | Recursively compare any subdirectories found |
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This will output the following patch file
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```diff
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--- hello-world.c.orig 2024-05-12 20:41:27.708297085 -0400
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+++ hello-world.c 2024-05-12 20:41:36.742348955 -0400
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@@ -2,7 +2,7 @@
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int main(void)
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{
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- printf("Goodbye World\n");
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+ printf("Hello World\n");
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return 0;
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}
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```
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In order to test out the patch, let us revert the hello-world.c file to it's original state.
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```bash
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cp hello-world.c.orig hello-world.c
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```
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Now apply the patch
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```bash
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patch < hello.patch
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```
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To undo the patch
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```bash
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patch -R hello-world.c < hello.patch
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```
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