git-annex/doc/git-annex-shell.mdwn

# NAME

git-annex-shell - Restricted login shell for git-annex only SSH access

# SYNOPSIS

git-annex-shell [-c] command [params ...]

# DESCRIPTION

git-annex-shell is a restricted shell, similar to git-shell, which
can be used as a login shell for SSH accounts.

Since its syntax is identical to git-shell's, it can be used as a drop-in
replacement anywhere git-shell is used. For example it can be used as a 
user's restricted login shell.

# COMMANDS

Any command not listed below is passed through to git-shell.

Note that the directory parameter should be an absolute path, otherwise
it is assumed to be relative to the user's home directory. Also the
first "/~/" or "/~user/" is expanded to the specified home directory.

* configlist directory

  This outputs a subset of the git configuration, in the same form as
  `git config --list`. This is used to get the annex.uuid of the remote
  repository.

  When run in a repository that does not yet have an annex.uuid, one
  will be created, as long as a git-annex branch has already been pushed to
  the repository, or if the autoinit= flag is used to indicate
  initialization is desired.

* inannex directory [key ...]

  This checks if all specified keys are present in the annex, 
  and exits zero if so.

  Exits 1 if the key is certainly not present in the annex.
  Exits 100 if it's unable to tell (perhaps the key is in the process of
  being removed from the annex).

* lockcontent directory key

  This locks a key's content in place in the annex, preventing it from
  being dropped.

  Once the content is successfully locked, outputs "OK". Then the content
  remains locked until a newline is received from the caller or the
  connection is broken.

  Exits nonzero if the content is not present, or could not be locked.

* dropkey directory [key ...]

  This drops the annexed data for the specified keys.

* recvkey directory key

  This runs rsync in server mode to receive the content of a key,
  and stores the content in the annex.

* sendkey directory key

  This runs rsync in server mode to transfer out the content of a key.

* transferinfo directory key

  This is typically run at the same time as sendkey is sending a key
  to the remote. Using it is optional, but is used to update
  progress information for the transfer of the key.

  It reads lines from standard input, each giving the number of bytes
  that have been received so far. 

* commit directory

  This commits any staged changes to the git-annex branch.
  It also runs the annex-content hook.

* notifychanges directory

  This is used by `git-annex remotedaemon` to be notified when
  refs in the remote repository are changed.

* gcryptsetup directory gcryptid

  Sets up a repository as a gcrypt repository.

* p2pstdio directory uuid

  This causes git-annex-shell to communicate using the git-annex p2p
  protocol over stdio. When supported by git-annex-shell, this allows
  multiple actions to be run over a single connection, improving speed.

  The uuid is the one belonging to the repository that will be
  communicating with git-annex-shell.

# OPTIONS

Most options are the same as in git-annex. The ones specific
to git-annex-shell are:

* --uuid=UUID

  git-annex uses this to specify the UUID of the repository it was expecting
  git-annex-shell to access, as a sanity check.

* Also the [[git-annex-common-options]](1) can be used.

* -- fields=val fields=val.. --

  Additional fields may be specified this way, to retain compatibility with
  past versions of git-annex-shell (that ignore these, but would choke
  on new dashed options).

  Currently used fields include remoteuuid=, associatedfile=,
  unlocked=, direct=, and autoinit=

# HOOK

After content is received or dropped from the repository by git-annex-shell,
it runs a hook, `.git/hooks/annex-content` (or `hooks/annex-content` on a bare
repository). The hook is not currently passed any information about what
changed.

# ENVIRONMENT

* GIT_ANNEX_SHELL_READONLY

  If set, disallows any action that could modify the git-annex 
  repository.

  Note that this does not prevent passing commands on to git-shell.
  For that, you also need ...

* GIT_ANNEX_SHELL_LIMITED

  If set, disallows running git-shell to handle unknown commands.

* GIT_ANNEX_SHELL_APPENDONLY

  If set, allows data to be written to the git-annex repository,
  but does not allow data to be removed from it.

  Note that this does not prevent passing commands on to git-shell,
  so you will have to separately configure git to reject pushes that
  overwrite branches or are otherwise not appends. The git pre-receive
  hook may be useful for accomplishing this.

  It's a good idea to enable annex.securehashesonly in a repository
  that's set up this way.

* GIT_ANNEX_SHELL_DIRECTORY

  If set, git-annex-shell will refuse to run commands that do not operate
  on the specified directory.

# EXAMPLES

To make a `~/.ssh/authorized_keys` file that only allows git-annex-shell
to be run, and not other commands, pass the original command to the -c
option:
    
	command="git-annex-shell -c \"$SSH_ORIGINAL_COMMAND\"",no-agent-forwarding,no-port-forwarding,no-X11-forwarding ssh-rsa AAAAB3NzaC1y[...] user@example.com

To further restrict git-annex-shell to a particular repository, 
and fully lock it down to read-only mode:

	command="GIT_ANNEX_SHELL_DIRECTORY=/srv/annex GIT_ANNEX_SHELL_LIMITED=true GIT_ANNEX_SHELL_READONLY=true git-annex-shell -c \"$SSH_ORIGINAL_COMMAND\"",restrict ssh-rsa AAAAB3NzaC1y[...] user@example.com

Obviously, `ssh-rsa AAAAB3NzaC1y[...] user@example.com` needs to
replaced with your SSH key. The above also assumes `git-annex-shell`
is available in your `$PATH`, use an absolute path if it is not the
case. Also note how the above uses the `restrict` option instead of an
explicit list of functionality to disallow. This only works in certain
OpenSSH releases, starting from 7.1p2.

To only allow adding new objects to the repository, the
`GIT_ANNEX_SHELL_APPENDONLY` variable can be used as well:

    command="GIT_ANNEX_SHELL_DIRECTORY=/srv/annex GIT_ANNEX_SHELL_APPENDONLY=true git-annex-shell -c \"$SSH_ORIGINAL_COMMAND\"",restrict ssh-rsa AAAAB3NzaC1y[...] user@example.com

This will not keep an attacker from destroying the git history, as
explained above. For this you might want to disallow certain
operations, like branch deletion and force-push, with options from
git-config(1). For example:

    git config receive.denyDeletes true
    git config receive.denyNonFastForwards true

With this configuration, git commits can still remove files, 
but they will still be available in the git history and git-annex will
retain their contents. Changes to `git-annex` branch, however, can
negatively impact git-annex's location tracking information and might
cause data loss. To work around this problem, more complex hooks
are required, see for example the `update-paranoid` hook in the git
source distribution.


# SEE ALSO

[[git-annex]](1)

git-shell(1)

# AUTHOR

Joey Hess <id@joeyh.name>

<http://git-annex.branchable.com/>

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