Use the env-injector extension¶

The env-injector extension lets you expose environment variables within the snap to the user. These variables are accessible by the snap’s apps and can modify their behavior as they would in a bare host environment.

The user has multiple available methods for setting these environment variables. Your snap’s apps and design should be oriented toward the most optimal method, and its documentation should cover which variables are available and the correct method of assigning them.

Set up the env-injector extension¶

To add the env-injector extension to an app in your snap:

Since env-injector is an experimental extension, it’s blocked by default. To enable experimental extensions during build, your host must set the following environment variable when packing with Snapcraft:
```
SNAPCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=1
```
Your snap must enable configurations with a configure hook. If your snap doesn’t use this hook yet, it needs at minimum an executable file named configure, without an extension, in snap/hooks/. It should contain at minimum:
```
#!/bin/sh

# Optional validation logic
```
In your snap’s project file, the target app’s extensions key must list env-injector. For example, if your snap had an app named server, the key would declare:
snapcraft.yaml¶
```
apps:
  server:
    command: run.sh
    daemon: simple
    extensions: [ env-injector ]
```

Once set up, the user can set any available environment variables for the snap’s apps.

Set an environment variable¶

When an app in a snap has behavior bound to an environment variable, the user can set it either through the snap’s configuration or by reading an environment (.env) file.

Environment variables are applied to apps in one of two ways:

Globally, where the environment variable is passed to all apps that use env-injector.
Locally, where the environment variable is passed to a specific app that uses env-injector. The app’s name is taken from its definition in the snap project file. The name according to the extension can be overridden with an alias to avoid naming conflicts.

As a snap configuration option¶

The user can set environment variables one at a time as snap configuration options with the snap set command.

To set an environment variable for all apps in a snap, the user can call snap set and target the snap and its app. The passed environment variable name must be lowercase. For example, to set HTTP_PORT=8080 for all apps in a snap that use the env-injector, the user would run:

sudo snap set <snap-name> env.http-port=8080

To set a local environment variable and target a specific app, they can call snap set and prefix the option name with apps.<app-name>. To target only the server app in the previous example, the user would run:

sudo snap set <snap-name> apps.server.env.http-port=8080

The app’s name is taken from the snap’s project file.

When running snap set, the user must adjust the environment variable name. For the complete details on how snap options interpret environment variables, see Snap option naming and rules.

With an environment file¶

The user can pass environment variables in .env files to the snap with the snap set command.

If a snap is confined, its file system needs access to the file, either by storing the file in its writable area or through a file interface.

For a simple example, to globally export the contents of an environment file stored in the local host, the user would run:

sudo snap set <my-snap> envfile=/var/snap/my-snap/common/config.env

The environment variables inside config.env are then exported to all apps that use the extension.

To export the contents of the same file as local environment variables of the server app, the user would run:

bash sudo snap set <my-snap> apps.server.envfile=/var/snap/my-snap/common/server.env

Give an app an alias for the environment¶

The app’s name is taken from its definition in the snap’s project file. You can override how the app is referred to in the environment by setting its env_alias key.

For example, to override an app named server with web-server, the project file would declare:

snapcraft.yaml¶

apps:
  server:
    command: run.sh
    daemon: simple
    extensions: [ env-injector ]
    environments:
      env_alias: web-server

Then, the user could set a local environment variable on the app with:

sudo snap set <my-name> apps.web-server.env.http-port=8080

Similarly, the user could override the app’s local .env file with:

sudo snap set <my-name> apps.web-server.envfile=/var/snap/my-snap/common/server.env