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git-annex allows you to store arbitrary metadata about the content of files
stored in the git-annex repository. The metadata is stored in the
`git-annex` branch, and so is automatically kept in sync with the rest of
git-annex's state, such as [[location_tracking]] information.
Some of the things you can do with metadata include:
* Using `git annex metadata file` to show all
the metadata associated with a file.
* [[tips/metadata_driven_views]]
* Limiting the files git-annex commands act on to those with
or without particular metadata.
For example `git annex find --metadata tag=foo --or --metadata tag=bar`
* Using it in [[preferred_content]] expressions.
For example "metadata=tag=important or not metadata=author=me"
* Editing and viewing it with git-annex-metadata-gui,
[[tips/a_gui_for_metadata_operations]].
Each file (actually the underlying key) can have any number of metadata
fields, which each can have any number of values. For example, to tag
files, the `tag` field is typically used, with values set to each tag that
applies to the file.
The field names are limited to alphanumerics (and `[_-.]`), and are case
insensitive. The metadata values can contain absolutely anything you
like -- but you're recommended to keep it simple and [[reasonably short|forum/metadata_limits]]
Here are some metadata fields that git-annex has special support for:
* `tag` - With each tag being a different value.
* `year`, `month` - When this particular version of the file came into
being.
* `$field-lastchanged` - This is automatically maintained for each
field that's set, and gives the date and time of the most recent
change to the field. It cannot be modified directly.
* `lastchanged` - This is automatically maintained, giving the data and time
of the last change to any of the metadata of a file.
To make git-annex automatically set the year and month when adding files,
run `git config annex.genmetadata true`. Also, see
[[tips/automatically_adding_metadata]].
git-annex's metadata can be updated in a distributed fashion. For example,
two users, each with their own clone of a repository, can set and unset
metadata at the same time, even for the same field of the same file.
When they push their changes, `git annex merge` will combine their
metadata changes in a consistent and (probably) intuitive way.
See [[the metadata design page|design/metadata]] for more details.
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