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moby--moby/docs/sources/terms/image.rst

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:title: Images
:description: Definition of an image
:keywords: containers, lxc, concepts, explanation, image, container
.. _image_def:
Image
=====
.. image:: images/docker-filesystems-debian.png
In Docker terminology, a read-only :ref:`layer_def` is called an
**image**. An image never changes.
Since Docker uses a :ref:`ufs_def`, the processes think the whole file
system is mounted read-write. But all the changes go to the top-most
writeable layer, and underneath, the original file in the read-only
image is unchanged. Since images don't change, images do not have state.
.. image:: images/docker-filesystems-debianrw.png
.. _parent_image_def:
Parent Image
............
.. image:: images/docker-filesystems-multilayer.png
Each image may depend on one more image which forms the layer beneath
it. We sometimes say that the lower image is the **parent** of the
upper image.
.. _base_image_def:
Base Image
..........
An image that has no parent is a **base image**.
Image IDs
.........
All images 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 command line. There is a small possibility of short id collisions, so the
docker server will always return the long ID.