Patching

Often enough you will have to patch files manually, or share a bug fix with someone using diff.

Sharing a bug fix

Let's say you have those two files:

file

Code
Bug
Code

fixedfile

Code
Code
New feature

and "fixedfile" is the fixed/improved version of "file"

Use this command (make sure "fixedfile" goes after "file")

diff -u file fixedfile > fix

This will create a file called "fix" with this content

{

--- file	Mon Jan  3 19:14:28 2005
+++ fixedfile	Mon Jan  3 19:15:01 2005
@@ -1,4 +1,4 @@
 Something
-bug
 Something
+New feature

}

You can now publish, archive, or Email this patch.

In real life situations patches are often hundreds of time smaller than sending the whole fixed file so they are convenient.

Patch a file

You have this file

file

Code
Bug
Code

Which you want patched with "fix".

{

Hello friend, try this to get rid of the bug!

--- file	Mon Jan  3 19:14:28 2005
+++ fixedfile	Mon Jan  3 19:15:01 2005
@@ -1,4 +1,4 @@
 Something
-bug
 Something
+New feature

Blah, blah, and the new feature is cool!

}

All you have to do is put fix in the same directory as file, and type this:

patch <fix

You need not have the file "fix" perfectly cleaned up if it was posted; you can cut-and-paste an Email message and it will work!

Undo a patch

If you don't like the patch for some reason, you can type again

patch <fix

you will get a prompt asking

"Reversed (or previously applied) patch detected! Assume -R? [y]"

Type 'y' to undo all of the patch in "fix". If "fix" were a group of patches, you can type 'n' and type 'y' on the next prompt to reverse only one patch rather than the whole content of "fix".

You can also use

patch -R <fix

So you will reverse all patches in "fix" without prompts.

Useful to know

The patch command can supports multiple files

patch can still work if line number are off (usually due to skipping certain patches). However this can on occasion not do the right thing so backups are recommended. Consult the man page for more.