[[environment-variables]]
=== Using environment variables

==== Overview

* You can set environment variable references in the configuration for Logstash plugins by using `${var}`.
* At Logstash startup, each reference is replaced by the value of the environment variable.
* The replacement is case-sensitive.
* References to undefined variables raise a Logstash configuration error.
* You can give a default value by using the form `${var:default value}`. Logstash uses the default value if the
environment variable is undefined.
* You can add environment variable references in any plugin option type: string, number, boolean, array, or hash.
* Environment variables for list-type URI parameters can support lists of space-delimited values. Currently, other non-URI based options do not support lists of values. See <<plugin-concepts>>
* Environment variables are immutable. If you update the environment variable, you'll have to restart Logstash to pick up the updated value.

==== Examples

These examples show you how to use environment variables to set the values of some commonly used
configuration options.

===== Setting the TCP port

Here's an example that uses an environment variable to set the TCP port:

[source,ruby]
----------------------------------
input {
  tcp {
    port => "${TCP_PORT}"
  }
}
----------------------------------

Now let's set the value of `TCP_PORT`:

[source,shell]
----
export TCP_PORT=12345
----

At startup, Logstash uses this configuration:

[source,ruby]
----------------------------------
input {
  tcp {
    port => 12345
  }
}
----------------------------------

If the `TCP_PORT` environment variable is not set, Logstash returns a configuration error.

You can fix this problem by specifying a default value:

[source,ruby]
----
input {
  tcp {
    port => "${TCP_PORT:54321}"
  }
}
----

Now, instead of returning a configuration error if the variable is undefined, Logstash uses the default:

[source,ruby]
----
input {
  tcp {
    port => 54321
  }
}
----

If the environment variable is defined, Logstash uses the value specified for the variable instead of the default.

===== Setting the value of a tag

Here's an example that uses an environment variable to set the value of a tag:

[source,ruby]
----
filter {
  mutate {
    add_tag => [ "tag1", "${ENV_TAG}" ]
  }
}
----

Let's set the value of `ENV_TAG`:

[source,shell]
----
export ENV_TAG="tag2"
----

At startup, Logstash uses this configuration:

[source,ruby]
----
filter {
  mutate {
    add_tag => [ "tag1", "tag2" ]
  }
}
----

===== Setting a file path

Here's an example that uses an environment variable to set the path to a log file:

[source,ruby]
----
filter {
  mutate {
    add_field => {
      "my_path" => "${HOME}/file.log"
    }
  }
}
----

Let's set the value of `HOME`:

[source,shell]
----
export HOME="/path"
----

At startup, Logstash uses the following configuration:

[source,ruby]
----
filter {
  mutate {
    add_field => {
      "my_path" => "/path/file.log"
    }
  }
}
----