pre-commit-annex hook script to automatically extract metadata from lots of types of files

Using the extract(1) program to do the heavy lifting.

Decided to make git-annex run pre-commit-annex when committing. Since
git-annex pre-commit also runs it, it'll be run when git commit is run too,
via the pre-commit hook. This basically gives back the pre-commit hook
that git-annex took away. The implementation avoids repeatedly looking
for the hook script when the assistant is running and committing
repeatedly; only checks if the hook is available once.

To make the script simpler, made git-annex metadata -s field?=value
only set a field when it's not already got a value.

This commit was sponsored by bak.

1 files changed, 24 insertions, 0 deletions

diff --git a/doc/tips/automatically_adding_metadata.mdwn b/doc/tips/automatically_adding_metadata.mdwn
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+git-annex's [[metadata]] works best when files have a lot of useful
+metadata attached to them.
+
+To make git-annex automatically set the year and month when adding files,
+run `git config annex.genmetadata true`.
+
+A git commit hook can be set up to extract lots of metadata from files
+like photos, mp3s, etc.
+
+* Install the `extract` utility, from <http://www.gnu.org/software/libextractor/>  
+  `apt-get install extract`
+* Download [[pre-commit-annex]] and install it in your git-annex repository
+  as `.git/hooks/pre-commit-annex`.  
+  Remember to make the script executable!
+* Run: `git config metadata.extract "artist album title camera_make video_dimensions"`
+
+Now any fields you list in metadata.extract to will be extracted and
+stored when files are committed.
+
+To get a list of all possible fields, run: `extract -L | sed ' ' _`
+
+By default, if a git-annex already has a metadata field for a file,
+its value will not be overwritten with metadata taken from files.
+To allow overwriting, run: `git config metadata.overwrite true`