How to modify source code on Gentoo

The cool thing about Gentoo is that you can modify a package’s source code as you see fit and Portage will still manage that package for you. This is done using patches. Modifying the sources directly won’t work, as emerge will re-fetch the package.

For this example, I’ll modify sys-process/btop-1.1.2.

Preparing and modifying the sources

  1. Fetch the sources with emerge --fetchonly btop

  2. Extract them to a/ and b/ (don’t use any other names) in the current directory:

    tar xf /var/cache/distfiles/btop-1.1.2.tar.gz -C a
tar xf /var/cache/distfiles/btop-1.1.2.tar.gz -C b
  3. Modify the sources in b/ as you see fit. Don’t touch a/.

Generating a patch

A patch is just the diff -ru between the original package and the modified version. The patch may be called anything, but must have a .patch or .diff extension

Generate the patch:

mkdir -p                /etc/portage/patches/sys-process/btop-1.1.2
touch                   /etc/portage/patches/sys-process/btop-1.1.2/lowerdelay.patch
diff -ru a b | sudo tee /etc/portage/patches/sys-process/btop-1.1.2/lowerdelay.patch >/dev/null

Applying the patch

Re-emerge the package: emerge -1 btop. You should see something like:

 * Messages for package sys-process/btop-1.1.2:

 * User patches applied.

Additional reading