Mix is a build tool that provides tasks for creating, compiling, and testing Elixir projects, managing its dependencies, and more.

Mix.Project

The foundation of Mix is a project. A project can be defined by using Mix.Project in a module, usually placed in a file named mix.exs:

defmodule MyApp.MixProject do
  use Mix.Project

  def project do
    [
      app: :my_app,
      version: "1.0.0"
    ]
  end
end

See the Mix.Project module for detailed documentation on Mix projects.

Once the project is defined, a number of default Mix tasks can be run directly from the command line:

• mix compile - compiles the current project
• mix test - runs tests for the given project
• mix run - runs a particular command inside the project

Each task has its own options and sometimes specific configuration to be defined in the project/0 function. You can use mix help to list all available tasks and mix help NAME to show help for a particular task.

The best way to get started with your first project is by calling mix new my_project from the command line.

Mix.Task

Tasks are what make Mix extensible.

Projects can extend Mix behaviour by adding their own tasks. For example, adding the task below inside your project will make it available to everyone that uses your project:

defmodule Mix.Tasks.Hello do
  use Mix.Task

  def run(_) do
    Mix.shell().info("Hello world")
  end
end

The task can now be invoked with mix hello.

See the Mix.Task behaviour for detailed documentation on Mix tasks.

Dependencies

Mix also manages your dependencies and integrates nicely with the Hex package manager.

In order to use dependencies, you need to add a :deps key to your project configuration. We often extract the list of dependencies into its own function:

defmodule MyApp.MixProject do
  use Mix.Project

  def project do
    [
      app: :my_app,
      version: "1.0.0",
      deps: deps()
    ]
  end

  defp deps do
    [
      {:ecto, "~> 2.0"},
      {:plug, github: "elixir-lang/plug"}
    ]
  end
end

You can run mix help deps to learn more about dependencies in Mix.

Environments

Mix supports different environments. Environments allow developers to prepare and organize their project specifically for different scenarios. By default, Mix provides three environments:

• :dev - the default environment
• :test - the environment mix test runs on
• :prod - the environment your dependencies run on

The environment can be changed via the command line by setting the MIX_ENV environment variable, for example:

$ MIX_ENV=prod mix run server.exs

You can also specify that certain dependencies are available only for certain environments:

{:some_test_dependency, "~> 1.0", only: :test}

When running Mix via the command line, you can configure the default environment or the preferred environment per task via the def cli function in your mix.exs. For example:

def cli do
  [
    default_env: :local,
    preferred_envs: [docs: :docs]
  ]
end

The environment can be read via Mix.env/0.

Targets

Besides environments, Mix supports targets. Targets are useful when a project needs to compile to different architectures and some of the dependencies are only available to some of them. By default, the target is :host but it can be set via the MIX_TARGET environment variable.

When running Mix via the command line, you can configure the default target or the preferred target per task via the def cli function in your mix.exs. For example:

def cli do
  [
    default_target: :local,
    preferred_targets: [docs: :docs]
  ]
end

The target can be read via Mix.target/0.

Configuration

Mix allows you configure the application environment of your application and of your dependencies. See the Application module to learn more about the application environment. On this section, we will focus on how to configure it at two distinct moments: build-time and runtime.

Avoiding the application environment

The application environment is discouraged for libraries. See Elixir's Library Guidelines for more information.

Build-time configuration

Whenever you invoke a mix command, Mix loads the configuration in config/config.exs, if said file exists. It is common for the config/config.exs file itself to import other configuration based on the current MIX_ENV, such as config/dev.exs, config/test.exs, and config/prod.exs, by calling Config.import_config/1:

import Config
import_config "#{config_env()}.exs"

We say config/config.exs and all imported files are build-time configuration as they are evaluated whenever you compile your code. In other words, if your configuration does something like:

import Config
config :my_app, :secret_key, System.fetch_env!("MY_APP_SECRET_KEY")

The :secret_key key under :my_app will be computed on the host machine before your code compiles. This can be an issue if the machine compiling your code does not have access to all environment variables used to run your code, as loading the config above will fail due to the missing environment variable. Furthermore, even if the environment variable is set, changing the environment variable will require a full recompilation of your application by calling mix compile --force (otherwise your project won't start). Luckily, Mix also provides runtime configuration, which is preferred in such cases and we will see next.

Runtime configuration

To enable runtime configuration in your release, all you need to do is to create a file named config/runtime.exs:

import Config
config :my_app, :secret_key, System.fetch_env!("MY_APP_SECRET_KEY")

This file is executed whenever your project runs. If you assemble a release with mix release, it also executes every time your release starts.

Aliases

Aliases are shortcuts or tasks specific to the current project.

In the Mix.Task section, we have defined a task that would be available to everyone using our project as a dependency. What if we wanted the task to only be available for our project? Just define an alias:

defmodule MyApp.MixProject do
  use Mix.Project

  def project do
    [
      app: :my_app,
      version: "1.0.0",
      aliases: aliases()
    ]
  end

  defp aliases do
    [
      c: "compile",
      hello: &hello/1,
      paid_task: &paid_task/1
    ]
  end

  defp hello(_) do
    Mix.shell().info("Hello world")
  end

  defp paid_task(_) do
    Mix.Task.run("paid.task", [
      "first_arg",
      "second_arg",
      "--license-key",
      System.fetch_env!("SOME_LICENSE_KEY")
    ])
  end
end

In the example above, we have defined three aliases. One is mix c which is a shortcut for mix compile. Another is named mix hello and the third is named mix paid_task, which executes the code inside a custom function to invoke the paid.task task with several arguments, including one pulled from an environment variable.

Aliases may also be lists, specifying multiple tasks to be run consecutively:

[all: [&hello/1, "deps.get --only #{Mix.env()}", "compile"]]

In the example above, we have defined an alias named mix all, that prints "Hello world", then fetches dependencies specific to the current environment, and compiles the project.

Aliases can also be used to augment existing tasks. Let's suppose you want to augment mix clean to clean another directory Mix does not know about:

[clean: ["clean", &clean_extra/1]]

Where &clean_extra/1 would be a function in your mix.exs with extra cleanup logic.

If the alias is overriding an existing task, the arguments given to the alias will be forwarded to the original task in order to preserve semantics. Otherwise arguments given to the alias are appended to the arguments of the last task in the list.

Another use case of aliases is to run Elixir scripts and shell commands, for example:

# priv/hello1.exs
IO.puts("Hello One")

# priv/hello2.exs
IO.puts("Hello Two")

# priv/world.sh
#!/bin/sh
echo "world!"

# mix.exs
defp aliases do
  [
    some_alias: ["hex.info", "run priv/hello1.exs", "cmd priv/world.sh"]
  ]
end

In the example above we have created the alias some_alias that will run the task mix hex.info, then mix run to run an Elixir script, then mix cmd to execute a command line shell script. This shows how powerful aliases mixed with Mix tasks can be.

One commit pitfall of aliases comes when trying to invoke the same task multiple times. Mix tasks are designed to run only once. This prevents the same task from being executed multiple times. For example, if there are several tasks depending on mix compile, the code will be compiled only once.

Similary, mix format can only be invoked once. So if you have an alias that attempts to invoke mix format multiple times, it won't work unless it is explicitly reenabled using Mix.Task.reenable/1:

another_alias: [
  "format --check-formatted priv/hello1.exs",
  "cmd priv/world.sh",
  fn _ -> Mix.Task.reenable("format") end,
  "format --check-formatted priv/hello2.exs"
]

Some tasks are automatically reenabled though, as they are expected to be invoked multiple times, such as: mix cmd, mix do, mix xref, etc.

Finally, aliases defined in the current project do not affect its dependencies and aliases defined in dependencies are not accessible from the current project, with the exception of umbrella projects. Umbrella projects will run the aliases of its children when the umbrella project itself does not define said alias and there is no task with said name.

Environment variables

Several environment variables can be used to modify Mix's behaviour.

Mix responds to the following variables:

• MIX_ARCHIVES - specifies the directory into which the archives should be installed (default: ~/.mix/archives)

• MIX_BUILD_PATH - sets the project Mix.Project.build_path/0 config. This option must always point to a subdirectory inside a temporary directory. For instance, never "/tmp" or "_build" but "_build/PROD" or "/tmp/PROD", as required by Mix. This environment variable is used mostly by external build tools. For your CI servers, you likely want to use MIX_BUILD_ROOT below.

• MIX_BUILD_ROOT - sets the root directory where build artifacts should be written to. For example, "_build". If MIX_BUILD_PATH is set, this option is ignored.

• MIX_DEBUG - outputs debug information about each task before running it

• MIX_DEPS_PATH - sets the project Mix.Project.deps_path/0 config for the current project (default: deps)

• MIX_ENV - specifies which environment should be used. See Environments

• MIX_EXS - changes the full path to the mix.exs file

• MIX_HOME - path to Mix's home directory, stores configuration files and scripts used by Mix (default: ~/.mix)

• MIX_INSTALL_DIR - (since v1.12.0) specifies directory where Mix.install/2 keeps install cache

• MIX_INSTALL_FORCE - (since v1.13.0) runs Mix.install/2 with empty install cache

• MIX_PATH - appends extra code paths

• MIX_QUIET - does not print information messages to the terminal

• MIX_REBAR3 - path to rebar3 command that overrides the one Mix installs (default: ~/.mix/rebar3)

• MIX_TARGET - specifies which target should be used. See Targets

• MIX_XDG - asks Mix to follow the XDG Directory Specification for its home directory and configuration files. This behaviour needs to be opt-in due to backwards compatibility. MIX_HOME has higher preference than MIX_XDG. If none of the variables are set, the default directory ~/.mix will be used

Environment variables that are not meant to hold a value (and act basically as flags) should be set to either 1 or true, for example:

$ MIX_DEBUG=1 mix compile