git-annex tries to ensure that the configured number of [[copies]] of your
data always exist, and leaves it up to you to use commands like `git annex
get` and `git annex drop` to move the content to the repositories you want
to contain it. But often, it can be good to have more fine-grained
control over which content is wanted by which repositories. Configuring
this allows the git-annex assistant as well as 
`git annex get --auto`, `git annex drop --auto`, `git annex sync --content`,
etc to do smarter things.

Preferred content settings can be edited using `git
annex vicfg`, or viewed and set at the command line with `git annex wanted`.
Each repository can have its own settings, and other repositories will
try to honor those settings when interacting with it.
(So there's no local `.git/config` for preferred content settings.)

The idea is that you write an expression that files are matched against.
If a file matches, the repository wants to store its content.
If it doesn't, the repository wants to drop its content
(if there are enough copies elsewhere to allow removing it).

## finding preferred content

To check at the command line which files are matched by preferred content
settings, you can use the --want-get and --want-drop options.

For example, `git annex find --want-get --not --in .` will find all the
files that `git annex get --auto` will want to get, and `git annex find
--want-drop --in .` will find all the files that `git annex drop --auto`
will want to drop.

## writing expressions

[[!template id=note text="""
### [[quickstart|standard_groups]]

Rather than writing your own preferred content expression, you can use
several standard ones included in git-annex that are tuned to cover different
common use cases.

You do this by putting a repository in a group,
and simply setting its preferred content to "standard" to match whatever
is standard for that group. See [[standard_groups]] for a list.
"""]]


The expressions are very similar to the matching options documented
on the [[git-annex-matching-options]] man page.
At the command line, you can use those options in commands like this:

	git annex get --include='*.mp3' --and -'(' --not --largerthan=100mb -')'

The equivalent preferred content expression looks like this:

	include=*.mp3 and (not largerthan=100mb)

So, just remove the dashes, basically. But, there are some differences
between the command line options and expressions, so see the documentation
below to get the full story.

* `include=glob` and `exclude=glob`

  Match files to include, or exclude.

  While --include=glob and --exclude=glob match files relative to the current
  directory, preferred content expressions always match files relative to the
  top of the git repository. 

  For example, suppose you put files into `archive` directories
  when you're done with them. Then you could configure your laptop to prefer
  to not retain those files, like this: `exclude=*/archive/*`

* `copies=number`

  Matches only files that git-annex believes to have the specified number
  of copies, or more. Note that it does not check remotes to verify that
  the copies still exist.

  To decide if content should be dropped, git-annex evaluates the preferred
  content expression under the assumption that the content has *already* been
  dropped. If the content would not be wanted then, the drop can be done.
  So, for example, `copies=2` in a preferred content expression lets
  content be dropped only when there are currently 3 copies of it, including
  the repo it's being dropped from. This is different than running `git annex
  drop --copies=2`, which will drop files that currently have 2 copies.

* `copies=trustlevel:number`

  Matches only files that git-annex believes have the specified number
  copies, on remotes with the specified trust level. For example,
  `copies=trusted:2`

  To match any trust level at or higher than a given level,
  use `trustlevel+`. For example, `--copies=semitrusted+:2`

* `copies=groupname:number`

  Matches only files that git-annex believes have the specified number of
  copies, on remotes in the specified group. For example,
  `copies=archive:2`

  Preferred content expressions have no equivilant to the `--in`
  option, but groups can accomplish similar things. You can add
  repositories to groups, and match against the groups in a
  preferred content expression. So rather than `--in=usbdrive`,
  put all the USB drives into a "transfer" group, and use
  `copies=transfer:1`

* `lackingcopies=number`

  Matches only files that git-annex believes need the specified number or
  more additional copies to be made in order to satisfy their numcopies
  settings.

* `approxlackingcopies=number`

  Like lackingcopies, but does not look at .gitattributes annex.numcopies
  settings. This makes it significantly faster.

* `inbackend=name`

  Matches only files whose content is stored using the specified key-value
  backend.

* `inallgroup=groupname`

  Matches only files that git-annex believes are present in all repositories
  in the specified group.

* `smallerthan=size` and `largerthan=size`

  Matches only files whose content is smaller than, or larger than the
  specified size.

  The size can be specified with any commonly used units, for example,
  "0.5 gb" or "100 KiloBytes"

* `metadata=field=glob`

  Matches only files that have a metadata field attached with a value that
  matches the glob. The values of metadata fields are matched case
  insensitively.

  To match a tag "done", use `metadata=tag=done`

  To match author metadata, use `metadata=author=* Smith`

* `present`

  Makes content be wanted if it's present, but not otherwise.

  This leaves it up to you to use git-annex manually
  to move content around. You can use this to avoid preferred content
  settings from affecting a subdirectory. For example:
  `auto/* or (include=ad-hoc/* and present)`

  Note that `not present` is a very bad thing to put in a preferred content 
  expression. It'll make it want to get content that's not present, and
  drop content that is present! Don't go there..

* `inpreferreddir`

  Makes content be preferred if it's in a directory (located anywhere
  in the tree) with a particular name. 

  The name of the directory can be configured using 
  `git annex enableremote $remote preferreddir=$dirname`

  (If no directory name is configured, it uses "public" by default.)

* `standard`

  git-annex comes with some built-in preferred content expressions, that
  can be used with repositories that are in some [[standard_groups]].

  When a repository is in exactly one such group, you can use the "standard"
  keyword in its preferred content expression, to match whatever content
  the group's expression matches.
  (If a repository is put into multiple standard
  groups, "standard" will match anything.. so don't do that!)

  Most often, the whole preferred content expression is simply "standard".
  But, you can do more complicated things, for example:
  `standard or include=otherdir/*`

* `groupwanted`

  The "groupwanted" keyword can be used to refer to a preferred content
  expression that is associated with a group. This is like the "standard"
  keyword, but you can configure the preferred content expressions
  using `git annex groupwanted`.

  Note that when writing a groupwanted preferred content expression,
  you can use all of the keywords listed above, including "standard".
  (But not "groupwanted".)

  For example, to make a variant of the standard client preferred content
  expression that does not want files in the "out" directory, you
  could run: `git annex groupwanted client "standard and exclude=out/*"`

  Then repositories that are in the client group and have their preferred
  content expression set to "groupwanted" will use that, while
  other client repositories that have their preferred content expression
  set to "standard" will use the standard expression.

  Or, you could make a new group, with your own custom preferred content
  expression tuned for your needs, and every repository you put in this
  group and make its preferred content be "groupwanted" will use it.

  For example, the archive group only wants to archive 1 copy of each file,
  spread among every repository in the group.
  Here's how to configure a group named redundantarchive, that instead
  wants to contain 3 copies of each file:
  
	git annex groupwanted redundantarchive "not (copies=redundantarchive:3)"
	for repo in foo bar baz; do
		git annex group $repo redundantarchive
		git annex wanted $repo groupwanted
	done

* `unused`

  Matches only keys that `git annex unused` has determined to be unused.

  This is related the the --unused option.
  However, putting `unused` in a preferred content expression 
  doesn't make git-annex consider those unused keys. So when git-annex is
  only checking preferred content expressions against files in the
  repository (which are obviously used), `unused` in a preferred
  content expression won't match anything.

  So when is `unused` useful in a preferred content expression?

  1. Using `git annex sync --content --all` will operate on all files,
    including unused ones, and take `unused` in preferred content expressions
    into account.
  2. The git-annex assistant periodically scans for unused files, and
    moves them to some repository whose preferred content expression
    says it wants them. (Or, if annex.expireunused is set, it may just delete
    them.)

* `anything`

  Matches any version of any file.

* `not expression`

  Inverts what the expression matches. For example, `not include=archive/*`
  is the same as `exclude=archive/*`

* `and` / `or` / `( expression )`

  These can be used to build up more complicated expressions.

## upgrades

It's important that all clones of a repository can understand one-another's
preferred content expressions, especially when using the git-annex
assistant. So using newly added keywords can cause a problem if
an older version of git-annex is in use elsewhere.

Before git-annex version 5.20140320, when git-annex saw a keyword it
did not understand, it defaulted to assuming *all* files were
preferred content. From version 5.20140320, git-annex has a nicer fallback
behavior: When it is unable to parse a preferred content expression,
it assumes all files that are currently present are preferred content.

Here are recent changes to preferred content expressions, and the version
they were added in.

* "anything" 5.20150616
* "standard" 5.20140314  
  (only when used in a more complicated expression; "standard" by
  itself has been supported for a long time)
* "groupwanted=" 5.20140314
* "metadata=" 5.20140221
* "lackingcopies=", "approxlackingcopies=", "unused=" 5.20140127
* "inpreferreddir=" 4.20130501
