---
layout: docs
page_title: Vault Agent's Process Supervisor Mode
description: >-
  Vault Agent's Process Supervisor Mode allows Vault secrets to be injected
  into a process via environment variables using Consul Template markup.
---

# Vault agent's process supervisor mode

Vault Agent's Process Supervisor Mode allows Vault secrets to be injected into
a process via environment variables using
[Consul Template markup][consul-templating-language].

-> If you are running your applications in a Kubernetes cluster, we recommend
  evaluating the [Vault Secrets Operator](/vault/docs/platform/k8s/vso) and
  the [Vault Agent Sidecar Injector](/vault/docs/platform/k8s/injector)
  instead.

## Functionality

Vault Agent will inject secrets referenced in the `env_template` configuration
blocks as environment variables into the child process specified in the `exec` block.

When you start Vault Agent in process supervisor mode, it will wait until each
environment variable template has rendered at least once before starting the
process. If `restart_on_secret_changes` is set to `always` (default), Agent
will restart the process whenever an update to an injected secret is detected.
This could be either a static secret update (done on
[`static_secret_render_interval`](/vault/docs/agent-and-proxy/agent/template#static_secret_render_interval))
or dynamic secret being close to its expiration.

In many ways, Vault Agent will mirror the child process. Standard intput and
output streams (`stdin` / `stdout` / `stderr`) are all forwarded to the child
process. Additionally, Vault Agent will exit when the child process exits on
its own with the same exit code.

## Configuration

-> Agent's [generate-config](/vault/docs/agent-and-proxy/agent/generate-config)
   tool will help you get started by generating a valid agent configuration
   file from the given inputs.

The process supervisor mode requires at least one `env_template` block and
exactly one top level `exec` block. It is incompatible with regular file
`template` entries.

### `env_template`

`env_template` stanza maps the template specified in the `contents` field or
referenced in the `source` field to the environment variable name in the title
of the stanza. It uses the same
[templating language](/vault/docs/agent-and-proxy/agent/template#templating-language)
as file templates but permits only a subset of
[its configuration parameters](/vault/docs/agent-and-proxy/agent/template#template_configurations):

- environment variable name `(string: <required>)` - the name of the
  environment variable to which the contents of the template should map.

- `contents` `(string: "")` - This option allows embedding the contents of
  a template in the configuration file rather then supplying the `source` path to
  the template file. This is useful for short templates. This option is mutually
  exclusive with the `source` option.

- `source` `(string: "")` - Path on disk to use as the input template. This
  option is required if not using the `contents` option.

- `error_on_missing_key` `(bool: false)` - Exit with an error when accessing
  a struct or map field/key that does notexist. The default behavior will print `<no value>`
  when accessing a field that does not exist. It is highly recommended you set this
  to "true". Also see
  [`exit_on_retry_failure` in global Vault Agent Template Config](/vault/docs/agent-and-proxy/agent/template#interaction-between-exit_on_retry_failure-and-error_on_missing_key).

- `left_delimiter` `(string: "{{")` - Delimiter to use in the template. The
  default is "{{" but for some templates, it may be easier to use a different
  delimiter that does not conflict with the output file itself.

- `right_delimiter` `(string: "}}")` - Delimiter to use in the template. The
  default is "}}" but for some templates, it may be easier to use a different
  delimiter that does not conflict with the output file itself.

### `exec`

The top level `exec` block has the following configuration entries.

- `command` `(string array: required)` - Specify the command for the child
  process with optional arguments. The executable's path must be either
  absolute or relative to the current working directory.

- `restart_on_secret_changes` `(string: "always")` - Controls whether agent
  will restart the child process on secret changes. There are two types of
  secret changes relevant to this configuration: a static secret update (on
  [static_secret_render_interval`](/vault/docs/agent-and-proxy/agent/template#static_secret_render_interval))
  and dynamic secret being close to its expiration. The configuration supports
  two options: `always` and `never`.

- `restart_stop_signal` `(string: "SIGTERM")` - Signal to send to the child
  process when a secret has been updated and the process needs to be restarted.
  The process has 30 seconds after this signal is sent until `SIGKILL` is sent
  to force the child process to stop.


## Configuration example

The following example was generated using
[`vault agent generate-config`](/vault/docs/agent-and-proxy/agent/generate-config),
a configuration helper tool. Given this configuration, Vault Agent will run
the child process (`./my-app arg1 arg2`) with two additional environment
variables (`FOO_USER` and `FOO_PASSWORD`) populated with secrets from Vault.

```hcl
auto_auth {

  method {
    type = "token_file"

    config {
      token_file_path = "/Users/avean/.vault-token"
    }
  }
}

template_config {
  static_secret_render_interval = "5m"
  exit_on_retry_failure         = true
}

vault {
  address = "http://localhost:8200"
}

env_template "FOO_PASSWORD" {
  contents             = "{{ with secret \"secret/data/foo\" }}{{ .Data.data.password }}{{ end }}"
  error_on_missing_key = true
}
env_template "FOO_USER" {
  contents             = "{{ with secret \"secret/data/foo\" }}{{ .Data.data.user }}{{ end }}"
  error_on_missing_key = true
}

exec {
  command                   = ["./my-app", "arg1", "arg2"]
  restart_on_secret_changes = "always"
  restart_stop_signal       = "SIGTERM"
}
```

[consul-templating-language]: https://github.com/hashicorp/consul-template/blob/v0.28.1/docs/templating-language.md
[template-config]: /vault/docs/agent-and-proxy/agent/template#template-configurations


## Tutorial

Refer to the [Vault Agent - secrets as environment
variables](/vault/tutorials/vault-agent/agent-env-vars) tutorial for an
end-to-end example.
