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 sometimes, it can be good to have more fine-grained
control over which repositories prefer to have which content. 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="""
### [[canned preferred content|standard_groups]]

Rather than writing your own preferred content expression, you can use
several canned 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, it's preferred to have its content stored in the
repository. If it doesn't, it's preferred to drop its content from
the repository (if there are enough copies elsewhere).

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 preferred 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 preferred 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 prefer 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 standard preferred content expressions, that
can be used with repositories that are in some pre-defined groups,
as listed in [[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 preferrs to have. (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/*`"