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Environ is a Clojure library for managing environment settings from a number of different sources. It works well for applications following the 12 Factor App pattern.

Currently, Environ supports four sources, resolved in the following order:

  1. A .lein-env file in the project directory
  2. A .boot-env file on the classpath
  3. Environment variables
  4. Java system properties

The first two sources are set by the lein-environ and boot-environ plugins respectively, and should not be edited manually.

The .lein-env file is populated with the content of the :env key in the Leiningen project map. Keywords with a project namespace are looked up in the project map. For example:

{:env {:app-version :project/version}}

This looks up the :version key in the Leiningen project map. You can view the full project map by using lein-pprint.

The .boot-env file is populated by the environ.boot/environ Boot task.


Include the following dependency in your project.clj file:

:dependencies [[environ "1.1.0"]]

If you want to be able to draw settings from the Leiningen project map, you'll also need the following plugin:

:plugins [[lein-environ "1.1.0"]]

If you are using the Boot toolchain, you may want to read and write settings from build pipelines. In build.boot, add the dependency:

:dependencies '[[boot-environ "1.1.0"]]

Then require the environ boot task.

(require '[environ.boot :refer [environ]])


Let's say you have an application that requires a database connection. Often you'll need three different databases, one for development, one for testing, and one for production.

Lets pull the database connection details from the key :database-url on the environ.core/env map.

(require '[environ.core :refer [env]])

(def database-url
  (env :database-url))

The value of this key can be set in several different ways. The most common way during development is to use a local profiles.clj file in your project directory. This file contained a map that is merged with the standard project.clj file, but can be kept out of version control and reserved for local development options.

{:dev  {:env {:database-url "jdbc:postgres://localhost/dev"}}
 :test {:env {:database-url "jdbc:postgres://localhost/test"}}}

In this case we add a database URL for the dev and test environments. This means that if you run lein repl, the dev database will be used, and if you run lein test, the test database will be used.

In the case of Boot, you have the full flexibility of tasks and build pipelines, meaning that all the following are valid:

$ boot environ -e database-url=jdbc:postgres://localhost/dev repl
(environ :env {:database-url "jdbc:postgres://localhost/dev"})

The latter form can be included in custom pipelines and `task-options!'.

The task also creates or updates a .boot-env file in the fileset. This is useful for tasks that create their own pods like boot-test, which won't see changes in the environ vars.

When you deploy to a production environment, you can make use of environment variables, like so:

DATABASE_URL=jdbc:postgres://localhost/prod java -jar standalone.jar

Or use Java system properties:

java -Ddatabase.url=jdbc:postgres://localhost/prod -jar standalone.jar

Note that Environ automatically lowercases keys, and replaces the characters "_" and "." with "-". The environment variable DATABASE_URL and the system property database.url are therefore both converted to the same keyword :database-url.


Copyright © 2016 James Reeves

Distributed under the Eclipse Public License, the same as Clojure.

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