Normally, git-annex stores annexed files in the repository, locked down, which prevents the content of the file from being modified. That's a good thing, because it might be the only copy, you wouldn't want to lose it in a fumblefingered mistake.

# git annex add some_file
add some_file
# echo oops > some_file
bash: some_file: Permission denied

Sometimes though you want to modify a file. Maybe once, or maybe repeatedly. To support this, git-annex also supports unlocked files. They are stored in the git repository differently, and they appear as regular files in the working tree, instead of the symbolic links used for locked files.

using unlocked files

You can unlock any annexed file:

# git annex unlock my_cool_big_file

That changes what's stored in git between a git-annex symlink (locked) and a git-annex pointer file (unlocked). You can commit the change, if you want that file to be unlocked in other clones of the repository. To lock the file again, use git annex lock.

The nice thing about an unlocked file is that you can modify it in place -- it's a regular file. And you can commit your changes.

# echo more stuff >> my_cool_big_file
# git commit -a -m "some changes"
[master 196c0e2] some changes
 1 files changed, 1 insertion(+), 1 deletion(-)

Notice that git commit -a added the new content of the file to the annex, and only committed a change to the pointer. That happened because git-annex knows this was an annexed file before. Git leaves the file unlocked, so you can continue to make modifications to it.

By default, using git to add a file that has not been annexed before will still add its contents to git, not to the annex. If you tell git-annex what files are large, it will arrange for the large files to be added to the annex, and the small ones to be added to git. This is done by configuring annex.largefiles. See largefiles for full documentation of that.

All the regular git-annex commands (find, get, drop, etc) can be used on unlocked files as well as locked files. When you drop the content of an unlocked file, it will be replaced by a pointer file, which looks like "/annex/objects/...". So if you open a file and see that, you'll need to use git annex get.

Under the hood, unlocked files use git's smudge/clean filter interface, and git-annex converts between the content of the big file and a pointer file, which is what gets committed to git.

adjusted branches

If you want to mostly keep files locked, but be able to locally switch to having them all unlocked, you can do so using git annex adjust --unlock. See git-annex-adjust for details. This is particularly useful when using filesystems like FAT, and OS's like Windows that don't support symlinks. Indeed, git-annex init detects such filesystems and automatically sets up a repository to use all unlocked files.

finding unlocked files

While it's easy to see when a file is a git-annex symlink, unlocked files look the same as files stored in git. To see what files are unlocked or locked, many git-annex commands support --unlocked and --locked options.

git annex find --unlocked

imperfections

Unlocked files mostly work very well, but there are a few imperfections which you should be aware of when using them.

1. git stash, git cherry-pick and git reset --hard don't update the working tree with the content of unlocked files. The files will contain pointers, the same as if the content was not in the repository. So after running these commands, you will need to manually run git annex smudge --update.

2. When git-annex is running a command that gets or drops the content of an unlocked file, git's index will briefly be locked, which might prevent you from running a git commit at the same time.

3. Conversely, if you have a git commit in progress, running git-annex may complain that the index is locked, though this will not prevent it from working.

4. When an operation such as a checkout or merge needs to update a large number of unlocked files, it can become slow. So can be git add of a large number of files (git annex add is faster).

(The technical reasons behind these imperfections are explained in detail in git smudge clean interface suboptiomal.)

using less disk space

Unlocked files are handy, but they have one significant disadvantage compared with locked files: On most filesystems, they use more disk space.

While only one copy of a locked file has to be stored, often two copies of an unlocked file are stored on disk. One copy is in the git work tree, where you can use and modify it, and the other is stashed away in .git/annex/objects (see internals).

The reason for that second copy is to preserve the old version of the file, when you modify the unlocked file in the work tree. Being able to access old versions of files is an important part of git after all!

(Some filesystems including btrfs and xfs support reflinks, and on those, the extra copy is a reflink, and takes up no additional space.)

So two copies is a good safe default. But there are ways to use git-annex that make the second copy not be worth keeping:

• When you're using git-annex to sync the current version of files across devices, and don't care much about previous versions.
• When you have set up a backup repository, and use git-annex to copy your files to the backup.

In situations like these, you may want to avoid the overhead of the second local copy of unlocked files. There's a config setting for that.

git config annex.thin true

After changing annex.thin, you'll want to fix up the work tree to match the new setting:

git annex fix

Setting annex.thin can save a lot of disk space, but it's a tradeoff between disk usage and safety.

Keeping files locked is safer and also avoids using unnecessary disk space, but trades off easy modification of files.

Pick the tradeoff that's right for you.