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- :title: Container
- :description: Definitions of a container
- :keywords: containers, lxc, concepts, explanation, image, container
- .. _container_def:
- Container
- =========
- .. image:: images/docker-filesystems-busyboxrw.png
- Once you start a process in Docker from an :ref:`image_def`, Docker
- fetches the image and its :ref:`parent_image_def`, and repeats the
- process until it reaches the :ref:`base_image_def`. Then the
- :ref:`ufs_def` adds a read-write layer on top. That read-write layer,
- plus the information about its :ref:`parent_image_def` and some
- additional information like its unique id, networking configuration,
- and resource limits is called a **container**.
- .. _container_state_def:
- Container State
- ...............
- Containers can change, and so they have state. A container may be
- **running** or **exited**.
- When a container is running, the idea of a "container" also includes a
- tree of processes running on the CPU, isolated from the other
- processes running on the host.
- When the container is exited, the state of the file system and
- its exit value is preserved. You can start, stop, and restart a
- container. The processes restart from scratch (their memory state is
- **not** preserved in a container), but the file system is just as it
- was when the container was stopped.
- You can promote a container to an :ref:`image_def` with ``docker
- commit``. Once a container is an image, you can use it as a parent for
- new containers.
- Container IDs
- .............
- All containers are identified by a 64 hexadecimal digit string (internally a 256bit
- value). To simplify their use, a short ID of the first 12 characters can be used
- on the commandline. There is a small possibility of short id collisions, so the
- docker server will always return the long ID.
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