# Trivial build helpers {#chap-trivial-builders} Nixpkgs provides a couple of functions that help with building derivations. The most important one, `stdenv.mkDerivation`, has already been documented above. The following functions wrap `stdenv.mkDerivation`, making it easier to use in certain cases. ## `runCommand` {#trivial-builder-runCommand} `runCommand :: String -> AttrSet -> String -> Derivation` `runCommand name drvAttrs buildCommand` returns a derivation that is built by running the specified shell commands. `name :: String` : The name that Nix will append to the store path in the same way that `stdenv.mkDerivation` uses its `name` attribute. `drvAttr :: AttrSet` : Attributes to pass to the underlying call to [`stdenv.mkDerivation`](#chap-stdenv). `buildCommand :: String` : Shell commands to run in the derivation builder. ::: {.note} You have to create a file or directory `$out` for Nix to be able to run the builder successfully. ::: ::: {.example #ex-runcommand-simple} # Invocation of `runCommand` ```nix (import {}).runCommand "my-example" {} '' echo My example command is running mkdir $out echo I can write data to the Nix store > $out/message echo I can also run basic commands like: echo ls ls echo whoami whoami echo date date '' ``` ::: ## `runCommandCC` {#trivial-builder-runCommandCC} This works just like `runCommand`. The only difference is that it also provides a C compiler in `buildCommand`'s environment. To minimize your dependencies, you should only use this if you are sure you will need a C compiler as part of running your command. ## `runCommandLocal` {#trivial-builder-runCommandLocal} Variant of `runCommand` that forces the derivation to be built locally, it is not substituted. This is intended for very cheap commands (<1s execution time). It saves on the network round-trip and can speed up a build. ::: {.note} This sets [`allowSubstitutes` to `false`](https://nixos.org/nix/manual/#adv-attr-allowSubstitutes), so only use `runCommandLocal` if you are certain the user will always have a builder for the `system` of the derivation. This should be true for most trivial use cases (e.g., just copying some files to a different location or adding symlinks) because there the `system` is usually the same as `builtins.currentSystem`. ::: ## `writeTextFile`, `writeText`, `writeTextDir`, `writeScript`, `writeScriptBin` {#trivial-builder-writeText} These functions write `text` to the Nix store. This is useful for creating scripts from Nix expressions. `writeTextFile` takes an attribute set and expects two arguments, `name` and `text`. `name` corresponds to the name used in the Nix store path. `text` will be the contents of the file. You can also set `executable` to true to make this file have the executable bit set. Many more commands wrap `writeTextFile` including `writeText`, `writeTextDir`, `writeScript`, and `writeScriptBin`. These are convenience functions over `writeTextFile`. Here are a few examples: ```nix # Writes my-file to /nix/store/ writeTextFile { name = "my-file"; text = '' Contents of File ''; } # See also the `writeText` helper function below. # Writes executable my-file to /nix/store//bin/my-file writeTextFile { name = "my-file"; text = '' Contents of File ''; executable = true; destination = "/bin/my-file"; } # Writes contents of file to /nix/store/ writeText "my-file" '' Contents of File ''; # Writes contents of file to /nix/store//share/my-file writeTextDir "share/my-file" '' Contents of File ''; # Writes my-file to /nix/store/ and makes executable writeScript "my-file" '' Contents of File ''; # Writes my-file to /nix/store//bin/my-file and makes executable. writeScriptBin "my-file" '' Contents of File ''; # Writes my-file to /nix/store/ and makes executable. writeShellScript "my-file" '' Contents of File ''; # Writes my-file to /nix/store//bin/my-file and makes executable. writeShellScriptBin "my-file" '' Contents of File ''; ``` ## `concatTextFile`, `concatText`, `concatScript` {#trivial-builder-concatText} These functions concatenate `files` to the Nix store in a single file. This is useful for configuration files structured in lines of text. `concatTextFile` takes an attribute set and expects two arguments, `name` and `files`. `name` corresponds to the name used in the Nix store path. `files` will be the files to be concatenated. You can also set `executable` to true to make this file have the executable bit set. `concatText` and`concatScript` are simple wrappers over `concatTextFile`. Here are a few examples: ```nix # Writes my-file to /nix/store/ concatTextFile { name = "my-file"; files = [ drv1 "${drv2}/path/to/file" ]; } # See also the `concatText` helper function below. # Writes executable my-file to /nix/store//bin/my-file concatTextFile { name = "my-file"; files = [ drv1 "${drv2}/path/to/file" ]; executable = true; destination = "/bin/my-file"; } # Writes contents of files to /nix/store/ concatText "my-file" [ file1 file2 ] # Writes contents of files to /nix/store/ concatScript "my-file" [ file1 file2 ] ``` ## `writeShellApplication` {#trivial-builder-writeShellApplication} This can be used to easily produce a shell script that has some dependencies (`runtimeInputs`). It automatically sets the `PATH` of the script to contain all of the listed inputs, sets some sanity shellopts (`errexit`, `nounset`, `pipefail`), and checks the resulting script with [`shellcheck`](https://github.com/koalaman/shellcheck). For example, look at the following code: ```nix writeShellApplication { name = "show-nixos-org"; runtimeInputs = [ curl w3m ]; text = '' curl -s 'https://nixos.org' | w3m -dump -T text/html ''; } ``` Unlike with normal `writeShellScriptBin`, there is no need to manually write out `${curl}/bin/curl`, setting the PATH was handled by `writeShellApplication`. Moreover, the script is being checked with `shellcheck` for more strict validation. ## `symlinkJoin` {#trivial-builder-symlinkJoin} This can be used to put many derivations into the same directory structure. It works by creating a new derivation and adding symlinks to each of the paths listed. It expects two arguments, `name`, and `paths`. `name` is the name used in the Nix store path for the created derivation. `paths` is a list of paths that will be symlinked. These paths can be to Nix store derivations or any other subdirectory contained within. Here is an example: ```nix # adds symlinks of hello and stack to current build and prints "links added" symlinkJoin { name = "myexample"; paths = [ pkgs.hello pkgs.stack ]; postBuild = "echo links added"; } ``` This creates a derivation with a directory structure like the following: ``` /nix/store/sglsr5g079a5235hy29da3mq3hv8sjmm-myexample |-- bin | |-- hello -> /nix/store/qy93dp4a3rqyn2mz63fbxjg228hffwyw-hello-2.10/bin/hello | `-- stack -> /nix/store/6lzdpxshx78281vy056lbk553ijsdr44-stack-2.1.3.1/bin/stack `-- share |-- bash-completion | `-- completions | `-- stack -> /nix/store/6lzdpxshx78281vy056lbk553ijsdr44-stack-2.1.3.1/share/bash-completion/completions/stack |-- fish | `-- vendor_completions.d | `-- stack.fish -> /nix/store/6lzdpxshx78281vy056lbk553ijsdr44-stack-2.1.3.1/share/fish/vendor_completions.d/stack.fish ... ``` ## `writeReferencesToFile` {#trivial-builder-writeReferencesToFile} Writes the closure of transitive dependencies to a file. This produces the equivalent of `nix-store -q --requisites`. For example, ```nix writeReferencesToFile (writeScriptBin "hi" ''${hello}/bin/hello'') ``` produces an output path `/nix/store/-runtime-deps` containing ```nix /nix/store/-hello-2.10 /nix/store/-hi /nix/store/-libidn2-2.3.0 /nix/store/-libunistring-0.9.10 /nix/store/-glibc-2.32-40 ``` You can see that this includes `hi`, the original input path, `hello`, which is a direct reference, but also the other paths that are indirectly required to run `hello`. ## `writeDirectReferencesToFile` {#trivial-builder-writeDirectReferencesToFile} Writes the set of references to the output file, that is, their immediate dependencies. This produces the equivalent of `nix-store -q --references`. For example, ```nix writeDirectReferencesToFile (writeScriptBin "hi" ''${hello}/bin/hello'') ``` produces an output path `/nix/store/-runtime-references` containing ```nix /nix/store/-hello-2.10 ``` but none of `hello`'s dependencies because those are not referenced directly by `hi`'s output.