compose/docs/yml.md

---
layout: default
title: docker-compose.yml reference
page_title: docker-compose.yml reference
page_description: docker-compose.yml reference
page_keywords: fig, composition, compose, docker
---

# docker-compose.yml reference

Each service defined in `docker-compose.yml` must specify exactly one of
`image` or `build`. Other keys are optional, and are analogous to their
`docker run` command-line counterparts.

As with `docker run`, options specified in the Dockerfile (e.g., `CMD`,
`EXPOSE`, `VOLUME`, `ENV`) are respected by default - you don't need to
specify them again in `docker-compose.yml`.

### image

Tag or partial image ID. Can be local or remote - Compose will attempt to
pull if it doesn't exist locally.

```
image: ubuntu
image: orchardup/postgresql
image: a4bc65fd
```

### build

Path to a directory containing a Dockerfile. This directory is also the
build context that is sent to the Docker daemon.

Compose will build and tag it with a generated name, and use that image thereafter.

```
build: /path/to/build/dir
```

### command

Override the default command.

```
command: bundle exec thin -p 3000
```

<a name="links"></a>
### links

Link to containers in another service. Either specify both the service name and
the link alias (`SERVICE:ALIAS`), or just the service name (which will also be
used for the alias).

```
links:
 - db
 - db:database
 - redis
```

An entry with the alias' name will be created in `/etc/hosts` inside containers
for this service, e.g:

```
172.17.2.186  db
172.17.2.186  database
172.17.2.187  redis
```

Environment variables will also be created - see the [environment variable
reference](env.md) for details.

### external_links

Link to containers started outside this `docker-compose.yml` or even outside
of Compose, especially for containers that provide shared or common services.
`external_links` follow semantics similar to `links` when specifying both the
container name and the link alias (`CONTAINER:ALIAS`).

```
external_links:
 - redis_1
 - project_db_1:mysql
 - project_db_1:postgresql
```

### ports

Expose ports. Either specify both ports (`HOST:CONTAINER`), or just the container
port (a random host port will be chosen).

> **Note:** When mapping ports in the `HOST:CONTAINER` format, you may experience
> erroneous results when using a container port lower than 60, because YAML will
> parse numbers in the format `xx:yy` as sexagesimal (base 60). For this reason,
> we recommend always explicitly specifying your port mappings as strings.

```
ports:
 - "3000"
 - "8000:8000"
 - "49100:22"
 - "127.0.0.1:8001:8001"
```

### expose

Expose ports without publishing them to the host machine - they'll only be
accessible to linked services. Only the internal port can be specified.

```
expose:
 - "3000"
 - "8000"
```

### volumes

Mount paths as volumes, optionally specifying a path on the host machine
(`HOST:CONTAINER`), or an access mode (`HOST:CONTAINER:ro`).

```
volumes:
 - /var/lib/mysql
 - cache/:/tmp/cache
 - ~/configs:/etc/configs/:ro
```

### volumes_from

Mount all of the volumes from another service or container.

```
volumes_from:
 - service_name
 - container_name
```

### environment

Add environment variables. You can use either an array or a dictionary.

Environment variables with only a key are resolved to their values on the
machine Compose is running on, which can be helpful for secret or host-specific values.

```
environment:
  RACK_ENV: development
  SESSION_SECRET:

environment:
  - RACK_ENV=development
  - SESSION_SECRET
```

### env_file

Add environment variables from a file. Can be a single value or a list.

If you have specified a Compose file with `docker-compose -f FILE`, paths in
`env_file` are relative to the directory that file is in.

Environment variables specified in `environment` override these values.

```
env_file: .env

env_file:
  - ./common.env
  - ./apps/web.env
  - /opt/secrets.env
```

```
RACK_ENV: development
```

### extends

Extend another service, in the current file or another, optionally overriding
configuration.

Here's a simple example. Suppose we have 2 files - **common.yml** and
**development.yml**. We can use `extends` to define a service in
**development.yml** which uses configuration defined in **common.yml**:

**common.yml**

```
webapp:
  build: ./webapp
  environment:
    - DEBUG=false
    - SEND_EMAILS=false
```

**development.yml**

```
web:
  extends:
    file: common.yml
    service: webapp
  ports:
    - "8000:8000"
  links:
    - db
  environment:
    - DEBUG=true
db:
  image: postgres
```

Here, the `web` service in **development.yml** inherits the configuration of
the `webapp` service in **common.yml** - the `build` and `environment` keys -
and adds `ports` and `links` configuration. It overrides one of the defined
environment variables (DEBUG) with a new value, and the other one
(SEND_EMAILS) is left untouched. It's exactly as if you defined `web` like
this:

```yaml
web:
  build: ./webapp
  ports:
    - "8000:8000"
  links:
    - db
  environment:
    - DEBUG=true
    - SEND_EMAILS=false
```

The `extends` option is great for sharing configuration between different
apps, or for configuring the same app differently for different environments.
You could write a new file for a staging environment, **staging.yml**, which
binds to a different port and doesn't turn on debugging:

```
web:
  extends:
    file: common.yml
    service: webapp
  ports:
    - "80:8000"
  links:
    - db
db:
  image: postgres
```

> **Note:** When you extend a service, `links` and `volumes_from`
> configuration options are **not** inherited - you will have to define
> those manually each time you extend it.

### net

Networking mode. Use the same values as the docker client `--net` parameter.

```
net: "bridge"
net: "none"
net: "container:[name or id]"
net: "host"
```

### dns

Custom DNS servers. Can be a single value or a list.

```
dns: 8.8.8.8
dns:
  - 8.8.8.8
  - 9.9.9.9
```

### cap_add, cap_drop

Add or drop container capabilities.
See `man 7 capabilities` for a full list.

```
cap_add:
  - ALL

cap_drop:
  - NET_ADMIN
  - SYS_ADMIN
```

### dns_search

Custom DNS search domains. Can be a single value or a list.

```
dns_search: example.com
dns_search:
  - dc1.example.com
  - dc2.example.com
```

### working\_dir, entrypoint, user, hostname, domainname, mem\_limit, privileged, restart, stdin\_open, tty, cpu\_shares

Each of these is a single value, analogous to its
[docker run](https://docs.docker.com/reference/run/) counterpart.

```
cpu_shares: 73

working_dir: /code
entrypoint: /code/entrypoint.sh
user: postgresql

hostname: foo
domainname: foo.com

mem_limit: 1000000000
privileged: true

restart: always

stdin_open: true
tty: true
```

## Compose documentation

- [Installing Compose](install.md)
- [User guide](index.md)
- [Command line reference](cli.md)
- [Compose environment variables](env.md)
- [Compose command line completion](completion.md)