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The Flox Catalog tracks nixpkgs, which means there can be a short delay between an upstream release and its availability in the catalog. See What is the Base Catalog? for details on how the catalog tracks nixpkgs and how often it updates. If you need a newer version right away, you can override the existing build recipe to point at the new release, then build and publish it so the updated version is available everywhere. This tutorial walks through the full workflow using Nix expression builds.

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Scenario

Imagine the Flox Catalog currently provides hello version 2.12.1, but you need version 2.12.2. Rather than waiting for the catalog to catch up, you’ll override the build recipe to use the newer release.

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Create an environment

Let’s start by creating a fresh environment for our override: 
$ mkdir hello-override && cd hello-override
$ flox init
⚡︎ Created environment 'hello-override' (aarch64-darwin)

Next:
  $ flox search <package>    <- Search for a package
  $ flox install <package>   <- Install a package into an environment
  $ flox activate            <- Enter the environment
  $ flox edit                <- Add environment variables and shell hooks
  $ flox push                <- Use the environment from other machines or
                                share it with someone on FloxHub

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Write the override

Create a Nix expression that takes the existing hello build recipe and overrides its version and src attributes:
The attributes you need to override depend on how the package is defined in nixpkgs. Different packages may use different attribute names or build patterns. Check the nixpkgs source for the package you’re modifying.
mkdir -p .flox/pkgs/hello
.flox/pkgs/hello/default.nix
{ hello, fetchurl }:

hello.overrideAttrs (finalAttrs: _oldAttrs: {
  version = "2.12.2";
  src = fetchurl {
    url = "mirror://gnu/hello/hello-${finalAttrs.version}.tar.gz";
    hash = "";
  };
})
The hash is set to an empty string because we don’t know the correct value yet. We’ll let the build tell us in the next step.

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Set up Git

Nix expression builds require that all files in .flox/pkgs/ are tracked by Git. Let’s initialize a repository and add our files: 
git init && git add .

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Get the correct hash

Run flox build and it will fail with the expected hash: 
$ flox build
warning: found empty hash, assuming 'sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA='
Building hello-2.12.2 in Nix expression mode
...
error: hash mismatch in fixed-output derivation:
         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
            got:    sha256-WpqZbcKSzCTc9BHO6H6S9qrluNE72caBm0x6nc4IGKs=
Copy the hash from the got: line and paste it into your expression:
.flox/pkgs/hello/default.nix
{ hello, fetchurl }:

hello.overrideAttrs (finalAttrs: _oldAttrs: {
  version = "2.12.2";
  src = fetchurl {
    url = "mirror://gnu/hello/hello-${finalAttrs.version}.tar.gz";
    hash = "sha256-WpqZbcKSzCTc9BHO6H6S9qrluNE72caBm0x6nc4IGKs=";
  };
})

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Build the package

Now run flox build again: 
$ flox build
Building hello-2.12.2 in Nix expression mode
...
Completed build of hello-2.12.2 in Nix expression mode

✨ Build completed successfully. Output created: ./result-hello
Verify it works: 
$ ./result-hello/bin/hello
Hello, world!

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Publish the package

The flox publish command requires a remote and all tracked files committed and pushed. Let’s set that up and publish:
The following example uses $PWD as the Git remote for simplicity. In practice, you should use a proper remote repository (for example on GitHub) so that others can reproduce your build.
$ git remote add origin "$PWD"
$ git add .
$ git commit -m "Add hello override"
$ git push origin main
$ flox publish hello
Building hello-2.12.2 in Nix expression mode
Completed build of hello-2.12.2 in Nix expression mode

✔ Package published successfully.

Use 'flox install myuser/hello' to install it.
The flox publish command performs a clean build from a temporary checkout to ensure the package is fully reproducible. See the publishing concept page for more details.

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Install from another environment

Once published, the overridden package is available in any Flox environment. Let’s create a new environment and install it there: 
$ mkdir ~/myproject && cd ~/myproject
$ flox init
$ flox install myuser/hello
✔ 'myuser/hello' installed to environment 'myproject'
Verify you have the overridden version: 
$ flox activate -- hello --version
hello (GNU Hello) 2.12.2

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Next steps

This tutorial covered the simplest override — bumping a version number. The Nix expression builds concept page covers additional patterns: For a full walkthrough of the build and publish workflow, including manifest builds, see the Building and publishing packages tutorial.