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218 lines
9.1 KiB
Markdown
page_title: FAQ
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page_description: Most frequently asked questions.
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page_keywords: faq, questions, documentation, docker
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# FAQ
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## Most frequently asked questions.
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### How much does Docker cost?
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> Docker is 100% free, it is open source, so you can use it without
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> paying.
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### What open source license are you using?
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> We are using the Apache License Version 2.0, see it here:
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> [https://github.com/dotcloud/docker/blob/master/LICENSE](https://github.com/dotcloud/docker/blob/master/LICENSE)
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### Does Docker run on Mac OS X or Windows?
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> Not at this time, Docker currently only runs on Linux, but you can use
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> VirtualBox to run Docker in a virtual machine on your box, and get the
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> best of both worlds. Check out the [*Mac OS
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> X*](../installation/mac/#macosx) and [*Microsoft
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> Windows*](../installation/windows/#windows) installation guides. The
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> small Linux distribution boot2docker can be run inside virtual
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> machines on these two operating systems.
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### How do containers compare to virtual machines?
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> They are complementary. VMs are best used to allocate chunks of
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> hardware resources. Containers operate at the process level, which
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> makes them very lightweight and perfect as a unit of software
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> delivery.
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### What does Docker add to just plain LXC?
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> Docker is not a replacement for LXC. "LXC" refers to capabilities of
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> the Linux kernel (specifically namespaces and control groups) which
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> allow sandboxing processes from one another, and controlling their
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> resource allocations. On top of this low-level foundation of kernel
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> features, Docker offers a high-level tool with several powerful
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> functionalities:
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>
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> - *Portable deployment across machines.*
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> : Docker defines a format for bundling an application and all
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> its dependencies into a single object which can be transferred
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> to any Docker-enabled machine, and executed there with the
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> guarantee that the execution environment exposed to the
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> application will be the same. LXC implements process
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> sandboxing, which is an important pre-requisite for portable
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> deployment, but that alone is not enough for portable
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> deployment. If you sent me a copy of your application
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> installed in a custom LXC configuration, it would almost
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> certainly not run on my machine the way it does on yours,
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> because it is tied to your machine’s specific configuration:
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> networking, storage, logging, distro, etc. Docker defines an
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> abstraction for these machine-specific settings, so that the
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> exact same Docker container can run - unchanged - on many
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> different machines, with many different configurations.
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>
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> - *Application-centric.*
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> : Docker is optimized for the deployment of applications, as
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> opposed to machines. This is reflected in its API, user
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> interface, design philosophy and documentation. By contrast,
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> the `lxc` helper scripts focus on
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> containers as lightweight machines - basically servers that
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> boot faster and need less RAM. We think there’s more to
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> containers than just that.
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>
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> - *Automatic build.*
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> : Docker includes [*a tool for developers to automatically
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> assemble a container from their source
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> code*](../reference/builder/#dockerbuilder), with full control
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> over application dependencies, build tools, packaging etc.
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> They are free to use
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> `make, maven, chef, puppet, salt,` Debian
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> packages, RPMs, source tarballs, or any combination of the
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> above, regardless of the configuration of the machines.
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>
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> - *Versioning.*
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> : Docker includes git-like capabilities for tracking successive
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> versions of a container, inspecting the diff between versions,
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> committing new versions, rolling back etc. The history also
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> includes how a container was assembled and by whom, so you get
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> full traceability from the production server all the way back
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> to the upstream developer. Docker also implements incremental
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> uploads and downloads, similar to `git pull`
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> , so new versions of a container can be transferred
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> by only sending diffs.
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>
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> - *Component re-use.*
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> : Any container can be used as a [*"base
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> image"*](../terms/image/#base-image-def) to create more
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> specialized components. This can be done manually or as part
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> of an automated build. For example you can prepare the ideal
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> Python environment, and use it as a base for 10 different
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> applications. Your ideal Postgresql setup can be re-used for
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> all your future projects. And so on.
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>
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> - *Sharing.*
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> : Docker has access to a [public
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> registry](http://index.docker.io) where thousands of people
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> have uploaded useful containers: anything from Redis, CouchDB,
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> Postgres to IRC bouncers to Rails app servers to Hadoop to
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> base images for various Linux distros. The
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> [*registry*](../reference/api/registry_index_spec/#registryindexspec)
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> also includes an official "standard library" of useful
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> containers maintained by the Docker team. The registry itself
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> is open-source, so anyone can deploy their own registry to
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> store and transfer private containers, for internal server
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> deployments for example.
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>
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> - *Tool ecosystem.*
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> : Docker defines an API for automating and customizing the
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> creation and deployment of containers. There are a huge number
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> of tools integrating with Docker to extend its capabilities.
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> PaaS-like deployment (Dokku, Deis, Flynn), multi-node
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> orchestration (Maestro, Salt, Mesos, Openstack Nova),
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> management dashboards (docker-ui, Openstack Horizon,
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> Shipyard), configuration management (Chef, Puppet), continuous
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> integration (Jenkins, Strider, Travis), etc. Docker is rapidly
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> establishing itself as the standard for container-based
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> tooling.
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>
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### What is different between a Docker container and a VM?
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There’s a great StackOverflow answer [showing the
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differences](http://stackoverflow.com/questions/16047306/how-is-docker-io-different-from-a-normal-virtual-machine).
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### Do I lose my data when the container exits?
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Not at all! Any data that your application writes to disk gets preserved
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in its container until you explicitly delete the container. The file
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system for the container persists even after the container halts.
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### How far do Docker containers scale?
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Some of the largest server farms in the world today are based on
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containers. Large web deployments like Google and Twitter, and platform
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providers such as Heroku and dotCloud all run on container technology,
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at a scale of hundreds of thousands or even millions of containers
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running in parallel.
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### How do I connect Docker containers?
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Currently the recommended way to link containers is via the link
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primitive. You can see details of how to [work with links
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here](http://docs.docker.io/en/latest/use/working_with_links_names/).
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Also of useful when enabling more flexible service portability is the
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[Ambassador linking
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pattern](http://docs.docker.io/en/latest/use/ambassador_pattern_linking/).
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### How do I run more than one process in a Docker container?
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Any capable process supervisor such as
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[http://supervisord.org/](http://supervisord.org/), runit, s6, or
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daemontools can do the trick. Docker will start up the process
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management daemon which will then fork to run additional processes. As
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long as the processor manager daemon continues to run, the container
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will continue to as well. You can see a more substantial example [that
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uses supervisord
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here](http://docs.docker.io/en/latest/examples/using_supervisord/).
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### What platforms does Docker run on?
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Linux:
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- Ubuntu 12.04, 13.04 et al
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- Fedora 19/20+
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- RHEL 6.5+
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- Centos 6+
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- Gentoo
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- ArchLinux
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- openSUSE 12.3+
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- CRUX 3.0+
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Cloud:
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- Amazon EC2
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- Google Compute Engine
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- Rackspace
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### How do I report a security issue with Docker?
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You can learn about the project’s security policy
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[here](http://www.docker.io/security/) and report security issues to
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this [mailbox](mailto:security%40docker.com).
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### Why do I need to sign my commits to Docker with the DCO?
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Please read [our blog
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post](http://blog.docker.io/2014/01/docker-code-contributions-require-developer-certificate-of-origin/)
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on the introduction of the DCO.
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### Can I help by adding some questions and answers?
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Definitely! You can fork [the
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repo](http://www.github.com/dotcloud/docker) and edit the documentation
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sources.
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### Where can I find more answers?
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You can find more answers on:
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- [Docker user
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mailinglist](https://groups.google.com/d/forum/docker-user)
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- [Docker developer
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mailinglist](https://groups.google.com/d/forum/docker-dev)
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- [IRC, docker on freenode](irc://chat.freenode.net#docker)
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- [GitHub](http://www.github.com/dotcloud/docker)
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- [Ask questions on
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Stackoverflow](http://stackoverflow.com/search?q=docker)
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- [Join the conversation on Twitter](http://twitter.com/docker)
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Looking for something else to read? Checkout the [*Hello
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World*](../examples/hello_world/#hello-world) example.
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