156 lines
5.4 KiB
Markdown
156 lines
5.4 KiB
Markdown
---
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title: "Chapter 2: Your first shell"
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date: 2023-09-15
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weight: 2
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---
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## Creating a shell with flake.nix
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First, create an empty git repository where your shells will live:
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```txt
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hut% mkdir jungle-examples
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hut% cd jungle-examples
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hut% git init
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Initialized empty Git repository in /home/Computational/rarias/jungle-examples/.git/
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```
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And then, place a file named `flake.nix` on the repo with this content:
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```nix
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{
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inputs.jungle.url = "jungle";
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nixConfig.bash-prompt = "\[nix-develop\]$ ";
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outputs = { self, jungle }:
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let
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pkgs = jungle.outputs.packages.x86_64-linux;
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in {
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devShells.x86_64-linux.default = pkgs.mkShell rec {
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pname = "my-shell";
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buildInputs = with pkgs.bsc; [
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ovni osumb # other packages here...
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];
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};
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};
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}
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```
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This file defines a how to create a shell in the Nix language with the
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`pkgs.mkShell` function using the packages listed in `buildInputs`. It also
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requests the packages to be taken from the *jungle* input, which corresponds to
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the set of packages that [we defined earlier](../ch1#where-packages-come-from),
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tuned for the cluster. We will describe it in more detail later.
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The tool `nix develop` tries to find a flake.nix in the current directory and
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enter the shell described by `devShells.x86_64-linux.default` (or the
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corresponding architecture).
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Now, **it is important that all the files of the repository are committed in
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git**, as nix will only read what is in the index of git. If we try to enter the
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shell with the `nix develop` command, it will complain and fail:
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```txt
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hut% nix develop
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warning: Git tree '/home/Computational/rarias/jungle-examples' is dirty
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error: getting status of '/nix/store/0ccnxa25whszw7mgbgyzdm4nqc0zwnm8-source/flake.nix': No such file or directory
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```
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The first warning states that the git directory has modified files not added to
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the index. Then the error occurs because the flake.nix is not in the index of
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git, so `nix develop` doesn't see it. So let's add it to a commit and try again:
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```txt
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hut% git add flake.nix
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hut% git commit flake.nix -m 'First shell'
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[master (root-commit) eb8a4ac] First shell
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1 file changed, 13 insertions(+)
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create mode 100644 flake.nix
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hut% nix develop
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warning: creating lock file '/home/Computational/rarias/jungle-examples/flake.lock'
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warning: Git tree '/home/Computational/rarias/jungle-examples' is dirty
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[nix-develop]$
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```
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In the `flake.nix` we have set the shell prompt to `[nix-develop]` so we can
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easily spot that we are inside a `nix develop` shell. To exit:
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```txt
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[nix-develop]$ exit
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hut%
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```
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## Using the flake.lock file
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Now we see the `creating lock file` message and the git tree becomes dirty
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again (however, we enter the shell successfully).
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This `flake.lock` file that has been created collects the current state of the
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jungle packages in a file, so future invocations will use the same versions. We
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can see more details with `nix flake metadata`:
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```txt
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hut% nix flake metadata
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warning: Git tree '/home/Computational/rarias/jungle-examples' is dirty
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Resolved URL: git+file:///home/Computational/rarias/jungle-examples
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Locked URL: git+file:///home/Computational/rarias/jungle-examples
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Path: /nix/store/bckxqjkkv52hy4pzgb96r7fchhmvmql8-source
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Revision: eb8a4ac544a74e3995d859c751e9ff4339de6509-dirty
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Last modified: 2023-09-15 13:06:12
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Inputs:
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└───jungle: path:/nix/store/3wv6q0f3pkgw840nnkn4jsp9xi650dyj-source?lastModified=1694772033&narHash=sha256-7a09O0Jb8WncxeB32ywmQEMqJdEFLrOG/XVT9bdII6I%3D&rev=653d411b9e46076a7878be9574ed6b3bd627cff1&revCount=195
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├───agenix: github:ryantm/agenix/d8c973fd228949736dedf61b7f8cc1ece3236792
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│ ├───darwin: github:lnl7/nix-darwin/87b9d090ad39b25b2400029c64825fc2a8868943
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│ │ └───nixpkgs follows input 'jungle/agenix/nixpkgs'
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│ ├───home-manager: github:nix-community/home-manager/32d3e39c491e2f91152c84f8ad8b003420eab0a1
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│ │ └───nixpkgs follows input 'jungle/agenix/nixpkgs'
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│ └───nixpkgs follows input 'jungle/nixpkgs'
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├───bscpkgs: git+https://pm.bsc.es/gitlab/rarias/bscpkgs.git?ref=refs/heads/master&rev=3a4062ac04be6263c64a481420d8e768c2521b80
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│ └───nixpkgs follows input 'jungle/nixpkgs'
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└───nixpkgs: github:NixOS/nixpkgs/e56990880811a451abd32515698c712788be5720
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```
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Now, as long as we keep these two files `flake.nix` and `flake.lock`, we can
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reproduce the same shell in the future, so let's add the lock file into git too.
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```txt
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hut% git commit -m 'Add flake.lock file'
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[master d3725ec] Add flake.lock file
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1 file changed, 135 insertions(+)
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create mode 100644 flake.lock
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hut% git status
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On branch master
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nothing to commit, working tree clean
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```
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## Using the shell with nix develop
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Now, the invocations of `nix develop` won't complain that the git tree is dirty
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anymore and will enter the shell:
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```txt
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hut% nix develop
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[nix-develop]$
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```
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And the requested packages are now available:
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```txt
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[nix-develop]$ which ovniemu
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/nix/store/0yzas8007x9djlpbb0pckcr1vhd0mcfy-ovni-1.3.0/bin/ovniemu
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```
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The packages of the shell are listed in the `$buildInputs` variable, in case you
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need to examine them:
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```txt
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[nix-develop]$ printf '%s\n' $buildInputs
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/nix/store/0yzas8007x9djlpbb0pckcr1vhd0mcfy-ovni-1.3.0
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/nix/store/lnjirzllhjn2fadlqzrz7a547iawl8jc-osu-micro-benchmarks-7.1-1
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[nix-develop]$ exit
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hut%
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```
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In the [next chapter](../ch3) we will see how to add more packages and also how to modify
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them.
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