git-annex/doc/preferred_content.mdwn

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.)

[[!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 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).

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.

The expressions are very similar to the matching options documented
on the [[git-annex]] 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 equivilant preferred content expression looks like this:

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

So, just remove the dashes, basically. However, there are some differences
from the command line options to keep in mind:

### difference: file matching

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

### difference: no "in="

Preferred content expressions have no direct equivilant to `--in`.

Often, it's best to 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"

### difference: dropping

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.

### difference: "present"

There's a special "present" keyword you can use in a preferred content
expression. This means that content is wanted if it's present,
and 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..

### difference: "inpreferreddir"

There's a special "inpreferreddir" keyword you can use in a
preferred content expression of a special remote. This means that the
content is preferred if it's in a directory (located anywhere in the tree)
with a special 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.)

### difference: "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/*`"

### difference: "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 amoung 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

### difference: metadata matching

This:

	git annex get --metadata tag=done

becomes

	metadata=tag=done

### difference: unused

The --unused option makes git-annex operate on every key that `git annex
unused` has determined to be unused. The corresponding `unused` keyword
in a preferred content expression also matches those keys. 

However, the latter doesn't make git-annex consider those 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?

The git-annex assistant periodically scans for unused files, and
moves them to some repository whose preferred content expression
matches "unused". (Or, if annex.expireunused is set, it may just delete
them.)

## 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.

* "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