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# Trivial build helpers {#chap-trivial-builders}
Nixpkgs provides a variety of wrapper functions that help build commonly useful derivations.
Like [`stdenv.mkDerivation`](#sec-using-stdenv), each of these build helpers creates a derivation, but the arguments passed are different (usually simpler) from those required by `stdenv.mkDerivation`.
## `runCommand` {#trivial-builder-runCommand}
`runCommand :: String -> AttrSet -> String -> Derivation`
The result of `runCommand name drvAttrs buildCommand` is a derivation that is built by running the specified shell commands.
By default `runCommand` runs in a stdenv with no compiler environment, whereas [`runCommandCC`](#trivial-builder-runCommandCC) uses the default stdenv, `pkgs.stdenv`.
`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 <nixpkgs> {}).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`.
:::
## Writing text files {#trivial-builder-text-writing}
Nixpkgs provides the following functions for producing derivations which write text files or executable scripts into the Nix store.
They are useful for creating files from Nix expression, and are all implemented as convenience wrappers around `writeTextFile`.
Each of these functions will cause a derivation to be produced.
When you coerce the result of each of these functions to a string with [string interpolation](https://nixos.org/manual/nix/stable/language/string-interpolation) or [`builtins.toString`](https://nixos.org/manual/nix/stable/language/builtins#builtins-toString), it will evaluate to the [store path](https://nixos.org/manual/nix/stable/store/store-path) of this derivation.
:::: {.note}
Some of these functions will put the resulting files within a directory inside the [derivation output](https://nixos.org/manual/nix/stable/language/derivations#attr-outputs).
If you need to refer to the resulting files somewhere else in a Nix expression, append their path to the derivation's store path.
For example, if the file destination is a directory:
```nix
{
my-file = writeTextFile {
name = "my-file";
text = ''
Contents of File
'';
destination = "/share/my-file";
};
}
```
Remember to append "/share/my-file" to the resulting store path when using it elsewhere:
```nix
writeShellScript "evaluate-my-file.sh" ''
cat ${my-file}/share/my-file
''
```
::::
### `makeDesktopItem` {#trivial-builder-makeDesktopItem}
Write an [XDG desktop file](https://specifications.freedesktop.org/desktop-entry-spec/1.4/) to the Nix store.
This function is usually used to add desktop items to a package through the `copyDesktopItems` hook.
`makeDesktopItem` adheres to version 1.4 of the specification.
#### Inputs {#trivial-builder-makeDesktopItem-inputs}
`makeDesktopItem` takes an attribute set that accepts most values from the [XDG specification](https://specifications.freedesktop.org/desktop-entry-spec/1.4/ar01s06.html).
All recognised keys from the specification are supported with the exception of the "Hidden" field. The keys are converted into camelCase format, but correspond 1:1 to their equivalent in the specification: `genericName`, `noDisplay`, `comment`, `icon`, `onlyShowIn`, `notShowIn`, `dbusActivatable`, `tryExec`, `exec`, `path`, `terminal`, `mimeTypes`, `categories`, `implements`, `keywords`, `startupNotify`, `startupWMClass`, `url`, `prefersNonDefaultGPU`.
The "Version" field is hardcoded to the version `makeDesktopItem` currently adheres to.
The following fields are either required, are of a different type than in the specification, carry specific default values, or are additional fields supported by `makeDesktopItem`:
`name` (String)
: The name of the desktop file in the Nix store.
`type` (String; _optional_)
: Default value: `"Application"`
`desktopName` (String)
: Corresponds to the "Name" field of the specification.
`actions` (List of Attribute set; _optional_)
: A list of attribute sets {name, exec?, icon?}
`extraConfig` (Attribute set; _optional_)
: Additional key/value pairs to be added verbatim to the desktop file. Attributes need to be prefixed with 'X-'.
#### Examples {#trivial-builder-makeDesktopItem-examples}
::: {.example #ex-makeDesktopItem}
# Usage 1 of `makeDesktopItem`
Write a desktop file `/nix/store/<store path>/my-program.desktop` to the Nix store.
```nix
{makeDesktopItem}:
makeDesktopItem {
name = "my-program";
desktopName = "My Program";
genericName = "Video Player";
noDisplay = false;
comment = "Cool video player";
icon = "/path/to/icon";
onlyShowIn = [ "KDE" ];
dbusActivatable = true;
tryExec = "my-program";
exec = "my-program --someflag";
path = "/some/working/path";
terminal = false;
actions.example = {
name = "New Window";
exec = "my-program --new-window";
icon = "/some/icon";
};
mimeTypes = [ "video/mp4" ];
categories = [ "Utility" ];
implements = [ "org.my-program" ];
keywords = [ "Video" "Player" ];
startupNotify = false;
startupWMClass = "MyProgram";
prefersNonDefaultGPU = false;
extraConfig.X-SomeExtension = "somevalue";
}
```
:::
::: {.example #ex2-makeDesktopItem}
# Usage 2 of `makeDesktopItem`
Override the `hello` package to add a desktop item.
```nix
{ copyDesktopItems
, hello
, makeDesktopItem }:
hello.overrideAttrs {
nativeBuildInputs = [ copyDesktopItems ];
desktopItems = [(makeDesktopItem {
name = "hello";
desktopName = "Hello";
exec = "hello";
})];
}
```
:::
### `writeTextFile` {#trivial-builder-writeTextFile}
Write a text file to the Nix store.
`writeTextFile` takes an attribute set with the following possible attributes:
`name` (String)
: Corresponds to the name used in the Nix store path identifier.
`text` (String)
: The contents of the file.
`executable` (Bool, _optional_)
: Make this file have the executable bit set.
Default: `false`
`destination` (String, _optional_)
: A subpath under the derivation's output path into which to put the file.
Subdirectories are created automatically when the derivation is realised.
By default, the store path itself will be a file containing the text contents.
Default: `""`
`checkPhase` (String, _optional_)
: Commands to run after generating the file.
Default: `""`
`meta` (Attribute set, _optional_)
: Additional metadata for the derivation.
Default: `{}`
`allowSubstitutes` (Bool, _optional_)
: Whether to allow substituting from a binary cache.
Passed through to [`allowSubsitutes`](https://nixos.org/manual/nix/stable/language/advanced-attributes#adv-attr-allowSubstitutes) of the underlying call to `builtins.derivation`.
It defaults to `false`, as running the derivation's simple `builder` executable locally is assumed to be faster than network operations.
Set it to true if the `checkPhase` step is expensive.
Default: `false`
`preferLocalBuild` (Bool, _optional_)
: Whether to prefer building locally, even if faster [remote build machines](https://nixos.org/manual/nix/stable/command-ref/conf-file#conf-substituters) are available.
Passed through to [`preferLocalBuild`](https://nixos.org/manual/nix/stable/language/advanced-attributes#adv-attr-preferLocalBuild) of the underlying call to `builtins.derivation`.
It defaults to `true` for the same reason `allowSubstitutes` defaults to `false`.
Default: `true`
`derivationArgs` (Attribute set, _optional_)
: Extra arguments to pass to the underlying call to `stdenv.mkDerivation`.
Default: `{}`
The resulting store path will include some variation of the name, and it will be a file unless `destination` is used, in which case it will be a directory.
::: {.example #ex-writeTextFile}
# Usage 1 of `writeTextFile`
Write `my-file` to `/nix/store/<store path>/some/subpath/my-cool-script`, making it executable.
Also run a check on the resulting file in a `checkPhase`, and supply values for the less-used options.
```nix
writeTextFile {
name = "my-cool-script";
text = ''
#!/bin/sh
echo "This is my cool script!"
'';
executable = true;
destination = "/some/subpath/my-cool-script";
checkPhase = ''
${pkgs.shellcheck}/bin/shellcheck $out/some/subpath/my-cool-script
'';
meta = {
license = pkgs.lib.licenses.cc0;
};
allowSubstitutes = true;
preferLocalBuild = false;
}
```
:::
::: {.example #ex2-writeTextFile}
# Usage 2 of `writeTextFile`
Write the string `Contents of File` to `/nix/store/<store path>`.
See also the [](#trivial-builder-writeText) helper function.
```nix
writeTextFile {
name = "my-file";
text = ''
Contents of File
'';
}
```
:::
::: {.example #ex3-writeTextFile}
# Usage 3 of `writeTextFile`
Write an executable script `my-script` to `/nix/store/<store path>/bin/my-script`.
See also the [](#trivial-builder-writeScriptBin) helper function.
```nix
writeTextFile {
name = "my-script";
text = ''
echo "hi"
'';
executable = true;
destination = "/bin/my-script";
}
```
:::
### `writeText` {#trivial-builder-writeText}
Write a text file to the Nix store
`writeText` takes the following arguments:
a string.
`name` (String)
: The name used in the Nix store path.
`text` (String)
: The contents of the file.
The store path will include the name, and it will be a file.
::: {.example #ex-writeText}
# Usage of `writeText`
Write the string `Contents of File` to `/nix/store/<store path>`:
```nix
writeText "my-file"
''
Contents of File
''
```
:::
This is equivalent to:
```nix
writeTextFile {
name = "my-file";
text = ''
Contents of File
'';
}
```
### `writeTextDir` {#trivial-builder-writeTextDir}
Write a text file within a subdirectory of the Nix store.
`writeTextDir` takes the following arguments:
`path` (String)
: The destination within the Nix store path under which to create the file.
`text` (String)
: The contents of the file.
The store path will be a directory.
::: {.example #ex-writeTextDir}
# Usage of `writeTextDir`
Write the string `Contents of File` to `/nix/store/<store path>/share/my-file`:
```nix
writeTextDir "share/my-file"
''
Contents of File
''
```
:::
This is equivalent to:
```nix
writeTextFile {
name = "my-file";
text = ''
Contents of File
'';
destination = "share/my-file";
}
```
### `writeScript` {#trivial-builder-writeScript}
Write an executable script file to the Nix store.
`writeScript` takes the following arguments:
`name` (String)
: The name used in the Nix store path.
`text` (String)
: The contents of the file.
The created file is marked as executable.
The store path will include the name, and it will be a file.
::: {.example #ex-writeScript}
# Usage of `writeScript`
Write the string `Contents of File` to `/nix/store/<store path>` and make the file executable.
```nix
writeScript "my-file"
''
Contents of File
''
```
:::
This is equivalent to:
```nix
writeTextFile {
name = "my-file";
text = ''
Contents of File
'';
executable = true;
}
```
### `writeScriptBin` {#trivial-builder-writeScriptBin}
Write a script within a `bin` subirectory of a directory in the Nix store.
This is for consistency with the convention of software packages placing executables under `bin`.
`writeScriptBin` takes the following arguments:
`name` (String)
: The name used in the Nix store path and within the file created under the store path.
`text` (String)
: The contents of the file.
The created file is marked as executable.
The file's contents will be put into `/nix/store/<store path>/bin/<name>`.
The store path will include the the name, and it will be a directory.
::: {.example #ex-writeScriptBin}
# Usage of `writeScriptBin`
```nix
writeScriptBin "my-script"
''
echo "hi"
''
```
:::
This is equivalent to:
```nix
writeTextFile {
name = "my-script";
text = ''
echo "hi"
'';
executable = true;
destination = "bin/my-script";
}
```
### `writeShellScript` {#trivial-builder-writeShellScript}
Write a Bash script to the store.
`writeShellScript` takes the following arguments:
`name` (String)
: The name used in the Nix store path.
`text` (String)
: The contents of the file.
The created file is marked as executable.
The store path will include the name, and it will be a file.
This function is almost exactly like [](#trivial-builder-writeScript), except that it prepends to the file a [shebang](https://en.wikipedia.org/wiki/Shebang_%28Unix%29) line that points to the version of Bash used in Nixpkgs.
<!-- this cannot be changed in practice, so there is no point pretending it's somehow generic -->
::: {.example #ex-writeShellScript}
# Usage of `writeShellScript`
```nix
writeShellScript "my-script"
''
echo "hi"
''
```
:::
This is equivalent to:
```nix
writeTextFile {
name = "my-script";
text = ''
#! ${pkgs.runtimeShell}
echo "hi"
'';
executable = true;
}
```
### `writeShellScriptBin` {#trivial-builder-writeShellScriptBin}
Write a Bash script to a "bin&
|