path: root/docs/sources/reference/builder.md

page_title: Dockerfile Reference
page_description: Dockerfiles use a simple DSL which allows you to automate the steps you would normally manually take to create an image.
page_keywords: builder, docker, Dockerfile, automation, image creation

# Dockerfile Reference

**Docker can act as a builder** and read instructions from a text *Dockerfile*
to automate the steps you would otherwise take manually to create an image.
Executing `docker build` will run your steps and commit them along the way,
giving you a final image.

## Usage

To [*build*](../commandline/cli/#cli-build) an image from a source repository,
create a description file called Dockerfile at the root of your repository.
This file will describe the steps to assemble the image.

Then call `docker build` with the path of you source repository as argument
(for example, `.`):

    $ sudo docker build .

The path to the source repository defines where to find the *context* of
the build. The build is run by the Docker daemon, not by the CLI, so the
whole context must be transferred to the daemon. The Docker CLI reports
"Sending build context to Docker daemon" when the context is sent to the daemon.

You can specify a repository and tag at which to save the new image if
the build succeeds:

    $ sudo docker build -t shykes/myapp .

The Docker daemon will run your steps one-by-one, committing the result
to a new image if necessary, before finally outputting the ID of your
new image. The Docker daemon will automatically clean up the context you
sent.

Note that each instruction is run independently, and causes a new image
to be created - so `RUN cd /tmp` will not have any effect on the next
instructions.

Whenever possible, Docker will re-use the intermediate images,
accelerating `docker build` significantly (indicated by `Using cache`):

    $ docker build -t SvenDowideit/ambassador .
    Uploading context 10.24 kB
    Uploading context
    Step 1 : FROM docker-ut
     ---> cbba202fe96b
    Step 2 : MAINTAINER SvenDowideit@home.org.au
     ---> Using cache
     ---> 51182097be13
    Step 3 : CMD env | grep _TCP= | sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/'  | sh && top
     ---> Using cache
     ---> 1a5ffc17324d
    Successfully built 1a5ffc17324d

When you're done with your build, you're ready to look into
[*Pushing a repository to its registry*](
/userguide/dockerrepos/#image-push).

## Format

Here is the format of the Dockerfile:

    # Comment
    INSTRUCTION arguments

The Instruction is not case-sensitive, however convention is for them to
be UPPERCASE in order to distinguish them from arguments more easily.

Docker evaluates the instructions in a Dockerfile in order. **The first
instruction must be \`FROM\`** in order to specify the [*Base
Image*](/terms/image/#base-image-def) from which you are building.

Docker will treat lines that *begin* with `#` as a
comment. A `#` marker anywhere else in the line will
be treated as an argument. This allows statements like:

    # Comment
    RUN echo 'we are running some # of cool things'

Here is the set of instructions you can use in a Dockerfile
for building images.

## FROM

    FROM <image>

Or

    FROM <image>:<tag>

The `FROM` instruction sets the [*Base Image*](/terms/image/#base-image-def)
for subsequent instructions. As such, a valid Dockerfile must have `FROM` as
its first instruction. The image can be any valid image – it is especially easy
to start by **pulling an image** from the [*Public Repositories*](
/userguide/dockerrepos/#using-public-repositories).

`FROM` must be the first non-comment instruction in the Dockerfile.

`FROM` can appear multiple times within a single Dockerfile in order to create
multiple images. Simply make a note of the last image id output by the commit
before each new `FROM` command.

If no `tag` is given to the `FROM` instruction, `latest` is assumed. If the
used tag does not exist, an error will be returned.

## MAINTAINER

    MAINTAINER <name>

The `MAINTAINER` instruction allows you to set the *Author* field of the
generated images.

## RUN

RUN has 2 forms:

- `RUN <command>` (the command is run in a shell - `/bin/sh -c`)
- `RUN ["executable", "param1", "param2"]` (*exec* form)

The `RUN` instruction will execute any commands in a new layer on top of the
current image and commit the results. The resulting committed image will be
used for the next step in the Dockerfile.

Layering `RUN` instructions and generating commits conforms to the core
concepts of Docker where commits are cheap and containers can be created from
any point in an image's history, much like source control.

The *exec* form makes it possible to avoid shell string munging, and to `RUN`
commands using a base image that does not contain `/bin/sh`.

The cache for `RUN` instructions isn't invalidated automatically during the
next build. The cache for an instruction like `RUN apt-get dist-upgrade -y`
will be reused during the next build.
The cache for `RUN` instructions can be invalidated by using the `--no-cache`
flag, for example `docker build --no-cache`.

The first encountered `ADD` instruction will invalidate the cache for all
following instructions from the 'Dockerfile' if the contents of the context
have changed. This will also invalidate the cache for `RUN` instructions.

### Known Issues (RUN)

- [Issue 783](https://github.com/dotcloud/docker/issues/783) is about file
  permissions problems that can occur when using the AUFS file system. You
  might notice it during an attempt to `rm` a file, for example. The issue
  describes a workaround.
- [Issue 2424](https://github.com/dotcloud/docker/issues/2424) Locale will
  not be set automatically.

## CMD

CMD has three forms:

- `CMD ["executable","param1","param2"]` (like an *exec*, this is the preferred form)
- `CMD ["param1","param2"]` (as *default parameters to ENTRYPOINT*)
- `CMD command param1 param2` (as a *shell*)

There can only be one CMD in a Dockerfile. If you list more than one CMD
then only the last CMD will take effect.

**The main purpose of a CMD is to provide defaults for an executing
container.** These defaults can include an executable, or they can omit
the executable, in which case you must specify an ENTRYPOINT as well.

When used in the shell or exec formats, the `CMD` instruction sets the command
to be executed when running the image.

If you use the *shell* form of the CMD, then the `<command>` will execute in
`/bin/sh -c`:

    FROM ubuntu
    CMD echo "This is a test." | wc -

If you want to **run your** `<command>` **without a shell** then you must
express the command as a JSON array and give the full path to the executable.
**This array form is the preferred format of CMD.** Any additional parameters
must be individually expressed as strings in the array:

    FROM ubuntu
    CMD ["/usr/bin/wc","--help"]

If you would like your container to run the same executable every time, then
you should consider using `ENTRYPOINT` in combination with `CMD`. See
[*ENTRYPOINT*](#entrypoint).

If the user specifies arguments to `docker run` then they will override the
default specified in CMD.

> **Note**:
> don't confuse `RUN` with `CMD`. `RUN` actually runs a command and commits
> the result; `CMD` does not execute anything at build time, but specifies
> the intended command for the image.

## EXPOSE

    EXPOSE <port> [<port>...]

The `EXPOSE` instructions informs Docker that the container will listen on the
specified network ports at runtime. Docker uses this information to interconnect
containers using links (see the [Docker User
Guide](/userguide/dockerlinks)).

## ENV

    ENV <key> <value>

The `ENV` instruction sets the environment variable `<key>` to the value
`<value>`. This value will be passed to all future `RUN` instructions. This is
functionally equivalent to prefixing the command with `<key>=<value>`

The environment variables set using `ENV` will persist when a container is run
from the resulting image. You can view the values using `docker inspect`, and
change them using `docker run --env <key>=<value>`.

> **Note**:
> One example where this can cause unexpected consequences, is setting
> `ENV DEBIAN_FRONTEND noninteractive`. Which will persist when the container
> is run interactively; for example: `docker run -t -i image bash`

## ADD

    ADD <src> <dest>

The `ADD` instruction will copy new files from `<src>` and add them to the
container's filesystem at path `<dest>`.

`<src>` must be the path to a file or directory relative to the source directory
being built (also called the *context* of the build) or a remote file URL.

`<dest>` is the absolute path to which the source will be copied inside the
destination container.

All new files and directories are created with a uid and gid of 0.

In the case where `<src>` is a remote file URL, the destination will have permissions 600.

> **Note**:
> If you build using STDIN (`docker build - < somefile`), there is no
> build context, so the Dockerfile can only contain an URL based ADD
> statement.

> **Note**:
> If your URL files are protected using authentication, you will need to
> use an `RUN wget` , `RUN curl`
> or other tool from within the container as ADD does not support
> authentication.

The copy obeys the following rules:

- The `<src>` path must be inside the *context* of the build;
  you cannot `ADD ../something /something`, because the first step of a
  `docker build` is to send the context directory (and subdirectories) to the
  docker daemon.

- If `<src>` is a URL and `<dest>` does not end with a trailing slash, then a
  file is downloaded from the URL and copied to `<dest>`.

- If `<src>` is a URL and `<dest>` does end with a trailing slash, then the
  filename is inferred from the URL and the file is downloaded to
  `<dest>/<filename>`. For instance, `ADD http://example.com/foobar /` would
  create the file `/foobar`. The URL must have a nontrivial path so that an
  appropriate filename can be discovered in this case (`http://example.com`
  will not work).

- If `<src>` is a directory, the entire directory is copied, including
  filesystem metadata.

- If `<src>` is a *local* tar archive in a recognized compression format
  (identity, gzip, bzip2 or xz) then it is unpacked as a directory. Resources
  from *remote* URLs are **not** decompressed. When a directory is copied or
  unpacked, it has the same behavior as `tar -x`: the result is the union of:

    1. whatever existed at the destination path and
    2. the contents of the source tree, with conflicts resolved in favor of 
       "2." on a file-by-file basis.

- If `<src>` is any other kind of file, it is copied individually along with
  its metadata. In this case, if `<dest>` ends with a trailing slash `/`, it
  will be considered a directory and the contents of `<src>` will be written
  at `<dest>/base(<src>)`.

- If `<dest>` does not end with a trailing slash, it will be considered a
  regular file and the contents of `<src>` will be written at `<dest>`.

- If `<dest>` doesn't exist, it is created along with all missing directories
  in its path.

## COPY

    COPY <src> <dest>

The `COPY` instruction will copy new files from `<src>` and add them to the
container's filesystem at path `<dest>`.

`<src>` must be the path to a file or directory relative to the source directory
being built (also called the *context* of the build).

`<dest>` is the absolute path to which the source will be copied inside the
destination container.

All new files and directories are created with a uid and gid of 0.

> **Note**:
> If you build using STDIN (`docker build - < somefile`), there is no
> build context, so `COPY` can't be used.

The copy obeys the following rules:

- The `<src>` path must be inside the *context* of the build;
  you cannot `COPY ../something /something`, because the first step of a
  `docker build` is to send the context directory (and subdirectories) to the
  docker daemon.

- If `<src>` is a directory, the entire directory is copied, including
  filesystem metadata.

- If `<src>` is any other kind of file, it is copied individually along with
  its metadata. In this case, if `<dest>` ends with a trailing slash `/`, it
  will be considered a directory and the contents of `<src>` will be written
  at `<dest>/base(<src>)`.

- If `<dest>` does not end with a trailing slash, it will be considered a
  regular file and the contents of `<src>` will be written at `<dest>`.

- If `<dest>` doesn't exist, it is created along with all missing directories
  in its path.

## ENTRYPOINT

ENTRYPOINT has two forms:

- `ENTRYPOINT ["executable", "param1", "param2"]`
  (like an *exec*, preferred form)
- `ENTRYPOINT command param1 param2`
  (as a *shell*)

There can only be one `ENTRYPOINT` in a Dockerfile. If you have more than one
`ENTRYPOINT`, then only the last one in the Dockerfile will have an effect.

An `ENTRYPOINT` helps you to configure a container that you can run as an
executable. That is, when you specify an `ENTRYPOINT`, then the whole container
runs as if it was just that executable.

The `ENTRYPOINT` instruction adds an entry command that will **not** be
overwritten when arguments are passed to `docker run`, unlike the behavior
of `CMD`. This allows arguments to be passed to the entrypoint. i.e. 
`docker run <image> -d` will pass the "-d" argument to the ENTRYPOINT.

You can specify parameters either in the ENTRYPOINT JSON array (as in
"like an exec" above), or by using a CMD statement. Parameters in the
ENTRYPOINT will not be overridden by the `docker run`
arguments, but parameters specified via CMD will be overridden
by `docker run` arguments.

Like a `CMD`, you can specify a plain string for the `ENTRYPOINT` and it will
execute in `/bin/sh -c`:

    FROM ubuntu
    ENTRYPOINT wc -l -

For example, that Dockerfile's image will *always* take stdin as input
("-") and print the number of lines ("-l"). If you wanted to make this
optional but default, you could use a CMD:

    FROM ubuntu
    CMD ["-l", "-"]
    ENTRYPOINT ["/usr/bin/wc"]

## VOLUME

    VOLUME ["/data"]

The `VOLUME` instruction will create a mount point with the specified name
and mark it as holding externally mounted volumes from native host or other
containers. The value can be a JSON array, `VOLUME ["/var/log/"]`, or a plain
string, `VOLUME /var/log`. For more information/examples and mounting
instructions via the Docker client, refer to [*Share Directories via Volumes*](
/userguide/dockervolumes/#volume-def) documentation.

## USER

    USER daemon

The `USER` instruction sets the username or UID to use when running the image
and for any following `RUN` directives.

## WORKDIR

    WORKDIR /path/to/workdir

The `WORKDIR` instruction sets the working directory for the `RUN`, `CMD` and
`ENTRYPOINT` Dockerfile commands that follow it.

It can be used multiple times in the one Dockerfile. If a relative path
is provided, it will be relative to the path of the previous `WORKDIR`
instruction. For example:

    WORKDIR /a WORKDIR b WORKDIR c RUN pwd

The output of the final `pwd` command in this
Dockerfile would be `/a/b/c`.

## ONBUILD

    ONBUILD [INSTRUCTION]

The `ONBUILD` instruction adds to the image a
"trigger" instruction to be executed at a later time, when the image is
used as the base for another build. The trigger will be executed in the
context of the downstream build, as if it had been inserted immediately
after the *FROM* instruction in the downstream Dockerfile.

Any build instruction can be registered as a trigger.

This is useful if you are building an image which will be used as a base
to build other images, for example an application build environment or a
daemon which may be customized with user-specific configuration.

For example, if your image is a reusable python application builder, it
will require application source code to be added in a particular
directory, and it might require a build script to be called *after*
that. You can't just call *ADD* and *RUN* now, because you don't yet
have access to the application source code, and it will be different for
each application build. You could simply provide application developers
with a boilerplate Dockerfile to copy-paste into their application, but
that is inefficient, error-prone and difficult to update because it
mixes with application-specific code.

The solution is to use *ONBUILD* to register in advance instructions to
run later, during the next build stage.

Here's how it works:

1. When it encounters an *ONBUILD* instruction, the builder adds a
   trigger to the metadata of the image being built. The instruction
   does not otherwise affect the current build.
2. At the end of the build, a list of all triggers is stored in the
   image manifest, under the key *OnBuild*. They can be inspected with
   *docker inspect*.
3. Later the image may be used as a base for a new build, using the
   *FROM* instruction. As part of processing the *FROM* instruction,
   the downstream builder looks for *ONBUILD* triggers, and executes
   them in the same order they were registered. If any of the triggers
   fail, the *FROM* instruction is aborted which in turn causes the
   build to fail. If all triggers succeed, the FROM instruction
   completes and the build continues as usual.
4. Triggers are cleared from the final image after being executed. In
   other words they are not inherited by "grand-children" builds.

For example you might add something like this:

    [...]
    ONBUILD ADD . /app/src
    ONBUILD RUN /usr/local/bin/python-build --dir /app/src
    [...]

> **Warning**: Chaining ONBUILD instructions using ONBUILD ONBUILD isn't allowed.

> **Warning**: ONBUILD may not trigger FROM or MAINTAINER instructions.

## Dockerfile Examples

    # Nginx
    #
    # VERSION               0.0.1

    FROM      ubuntu
    MAINTAINER Victor Vieux <victor@docker.com>

    # make sure the package repository is up to date
    RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
    RUN apt-get update

    RUN apt-get install -y inotify-tools nginx apache2 openssh-server

    # Firefox over VNC
    #
    # VERSION               0.3

    FROM ubuntu
    # make sure the package repository is up to date
    RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
    RUN apt-get update

    # Install vnc, xvfb in order to create a 'fake' display and firefox
    RUN apt-get install -y x11vnc xvfb firefox
    RUN mkdir /.vnc
    # Setup a password
    RUN x11vnc -storepasswd 1234 ~/.vnc/passwd
    # Autostart firefox (might not be the best way, but it does the trick)
    RUN bash -c 'echo "firefox" >> /.bashrc'

    EXPOSE 5900
    CMD    ["x11vnc", "-forever", "-usepw", "-create"]

    # Multiple images example
    #
    # VERSION               0.1

    FROM ubuntu
    RUN echo foo > bar
    # Will output something like ===> 907ad6c2736f

    FROM ubuntu
    RUN echo moo > oink
    # Will output something like ===> 695d7793cbe4

    # You᾿ll now have two images, 907ad6c2736f with /bar, and 695d7793cbe4 with
    # /oink.