Nix Shell Locked

nix-shell-locked is a program which starts a new shell with some specified packages installed without installing them user or system wide. Packages are installed from a revision of nixpkgs taken from a flake.lock file whose path is configured in ~/.config/nix-shell-locked.toml. This is intended as a replacement for nix-shell in cases where flakes is used to manage either a NixOS system configuration or home-manager. The problem with nix-shell is that it uses the nixpkgs channel which can get out sync with the version of nixpkgs in a flake-managed system or home-manager config which can lead to runtime errors.

For Example

I use flakes to manage both my NixOS system configuration and home-manager. I want to try out the gameboy emulator "sameboy" but I don't want to commit to installing it system-wide or adding it to my home-manager config, so I install it in a transient shell with:

$ nix-shell -p sameboy

When I try to run it:

$ sameboy SameBoy v0.15.8 Couldn't find matching GLX visual

This looks like a version mismatch between a graphics library and sameboy. Graphics libraries are configured in the system-wide configuration which I manage with flakes which I update regularly. Since I use flakes for almost everything now I neglect to keep my channels up to date, so this version of sameboy is likely quite old. Ideally there would be a way to install sameboy in a transient shell where the version of sameboy comes from the same nixpkgs revision as the system-wide configuration, and there is.

My system config has a flake.lock file with a section:

```json

/path/to/config/repo/flake.lock

{ "nodes": { "nixpkgs": { "locked": { "lastModified": 1682268651, "narHash": "sha256-2eZriMhnD24Pmb8ideZWZDiXaAVe6LzJrHQiNPck+Lk=", "owner": "nixos", "repo": "nixpkgs", "rev": "e78d25df6f1036b3fa76750ed4603dd9d5fe90fc", "type": "github" },

```

Note the revision e78d25df6f1036b3fa76750ed4603dd9d5fe90fc.

We can make a transient shell with sameboy installed from this revision with:

$ nix shell nixpkgs/e78d25df6f1036b3fa76750ed4603dd9d5fe90fc#sameboy --command sameboy

This downloads and runs sameboy, taking a version that is compatible with the system-wide graphics library installation. This works but it's cumbersome. The nix-shell-locked command automates the above process. Make a config file ~/.config/nix-shell-locked.toml to tell nix-shell-locked where to look for the flake.lock file to use to obtain the current revision hash.

```toml

~/.config/nix-shell-locked.toml

flake_lockfile = "/path/to/config/repo/flake.lock" ```

Now you can start a transient shell with the correct version of sameboy by running: $ nix-shell-locked sameboy

Behind the scenes this is just running nix shell ... and all arguments after the first -- are passed along to nix shell, so you can do: $ nix-shell-locked sameboy -- --command sameboy --help SameBoy v0.15.8 Usage: sameboy [--fullscreen|-f] [--nogl] [--stop-debugger|-s] [rom]

You can pass multiple packages to nix-shell-locked to get a shell with all packages available: ``` $ nix-shell-locked ksh hello cowsay -- --command ksh -c "hello | cowsay -f tux"

< Hello, world! >

\ \ .--. |oo | |:/ | // \ \ (| | ) /'_ /`\ _)=(/ ```