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"

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.

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.
