41 lines
1.3 KiB
Markdown
41 lines
1.3 KiB
Markdown
page_title: Images
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page_description: Definition of an image
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page_keywords: containers, lxc, concepts, explanation, image, container
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# Image
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## Introduction
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In Docker terminology, a read-only [*Layer*](/terms/layer/#layer-def) is
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called an **image**. An image never changes.
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Since Docker uses a [*Union File System*](/terms/layer/#ufs-def), the
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processes think the whole file system is mounted read-write. But all the
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changes go to the top-most writeable layer, and underneath, the original
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file in the read-only image is unchanged. Since images don't change,
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images do not have state.
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## Parent Image
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Each image may depend on one more image which forms the layer beneath
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it. We sometimes say that the lower image is the **parent** of the upper
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image.
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## Base Image
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An image that has no parent is a **base image**.
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## Image IDs
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All images are identified by a 64 hexadecimal digit string (internally a
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256bit value). To simplify their use, a short ID of the first 12
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characters can be used on the command line. There is a small possibility
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of short id collisions, so the docker server will always return the long
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ID.
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