> For the complete documentation index, see [llms.txt](https://a7246c5516ab4c80cdfe21ca2be3e40c.gitbook.io/docker-basics/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://a7246c5516ab4c80cdfe21ca2be3e40c.gitbook.io/docker-basics/docker-file.md).

# docker file

A Dockerfile is a script used to build a Docker image. It contains a set of instructions for assembling a containerized application. Below is a comprehensive reference for various instructions that can be used in a Dockerfile:

#### **Dockerfile Instructions:**

**1. FROM:**

* Defines the base image for subsequent instructions.

```Dockerfile
FROM image_name:tag
```

**2. WORKDIR:**

* Sets the working directory for subsequent instructions.

```Dockerfile
WORKDIR /path/to/directory
```

**3. COPY:**

* Copies files or directories from the build context to the container.

```Dockerfile
COPY source destination
```

**4. ADD:**

* Similar to COPY but allows for additional features like URL support and automatic extraction of compressed files.

```Dockerfile
ADD source destination
```

**5. RUN:**

* Executes commands in a new layer on top of the current image.

```Dockerfile
RUN command
```

**6. CMD:**

* Provides default command for the container when it starts. It can be overridden by command-line arguments.

```Dockerfile
CMD ["executable", "arg1", "arg2"]
```

**7. ENTRYPOINT:**

* Configures a container that will run as an executable.

```Dockerfile
ENTRYPOINT ["executable", "arg1", "arg2"]
```

**8. EXPOSE:**

* Informs Docker that the container will listen on the specified network ports at runtime.

```Dockerfile
EXPOSE port
```

**9. ENV:**

* Sets environment variables in the form key=value.

```Dockerfile
ENV key=value
```

**10. ARG:**

* Defines variables that users can pass at build-time to the builder with the docker build command.

```Dockerfile
ARG variable_name=default_value
```

**11. VOLUME:**

* Creates a mount point with specified name for externally mounted volumes.

```Dockerfile
VOLUME /path/to/mountpoint
```

**12. USER:**

* Sets the username or UID and optionally the user group or GID to use when running the image.

```Dockerfile
USER user[:group]
```

**13. HEALTHCHECK:**

* Tells Docker how to test a container to check that it is still working.

```Dockerfile
HEALTHCHECK [options] CMD command
```

**14. LABEL:**

* Adds metadata to an image in the form of key-value pairs.

```Dockerfile
LABEL key="value" key2="value2"
```

**15. ONBUILD:**

* Adds a trigger instruction to the image that will be executed at a later time.

```Dockerfile
ONBUILD instruction
```

**16. SHELL:**

* Overrides the default shell used for the RUN instruction.

```Dockerfile
SHELL ["executable", "parameters"]
```

#### **Example Dockerfile:**

```Dockerfile
# Use an official base image
FROM ubuntu:latest

# Set the working directory
WORKDIR /app

# Copy the current directory contents into the container at /app
COPY . /app

# Install dependencies
RUN apt-get update && apt-get install -y python3

# Set environment variable
ENV NAME World

# Expose port
EXPOSE 80

# Run the command when the container starts
CMD ["python3", "app.py"]
```

Remember that Dockerfiles are case-sensitive, and instructions must be written in uppercase. Additionally, Dockerfiles are usually kept in the root of the application code. The `docker build` command is used to build an image from a Dockerfile, and `docker run` is used to run a container from that image.


---

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