Add plumbing-level lookupkey command.

Annex/CatFile.hs

@@ -119,7 +119,7 @@ catKeyChecked needhead ref@(Ref r) =
  - For command-line git-annex use, that doesn't matter. It's perfectly
  - reasonable for things staged in the index after the currently running
  - git-annex process to not be noticed by it. However, we do want to see
- - what's in the index, since it may have uncommitted changes not in HEAD>
+ - what's in the index, since it may have uncommitted changes not in HEAD
  -
  - For the assistant, this is much more of a problem, since it commits
  - files and then needs to be able to immediately look up their keys.

Command/LookupKey.hs

@@ -0,0 +1,26 @@
+{- git-annex command
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Command.LookupKey where
+
+import Common.Annex
+import Command
+import Annex.CatFile
+import Types.Key
+
+def :: [Command]
+def = [notBareRepo $
+	command "lookupkey" (paramRepeating paramFile) seek
+		SectionPlumbing "looks up key used for file"]
+
+seek :: [CommandSeek]
+seek = [withStrings start]
+
+start :: String -> CommandStart
+start file = do
+	liftIO . maybe exitFailure (putStrLn . key2file) =<< catKeyFile file
+	stop

GitAnnex.hs

@@ -20,6 +20,7 @@ import qualified Command.Drop
 import qualified Command.Move
 import qualified Command.Copy
 import qualified Command.Get
+import qualified Command.LookupKey
 import qualified Command.FromKey
 import qualified Command.DropKey
 import qualified Command.TransferKey
@@ -124,6 +125,7 @@ cmds = concat
 	, Command.Schedule.def
 	, Command.Ungroup.def
 	, Command.Vicfg.def
+	, Command.LookupKey.def
 	, Command.FromKey.def
 	, Command.DropKey.def
 	, Command.TransferKey.def

debian/changelog

@@ -1,6 +1,7 @@
 git-annex (5.20131214) UNRELEASED; urgency=low
 
   * Fix test suite to cover lock --force change.
+  * Add plumbing-level lookupkey command.
 
  -- Joey Hess <joeyh@debian.org>  Sun, 15 Dec 2013 13:32:49 -0400
 

doc/git-annex.mdwn

@@ -727,6 +727,13 @@ subdirectories).
   point to annexed content. Also handles injecting changes to unlocked
   files into the annex.
 
+* `lookupkey [file ...]`
+
+  This plumbing-level command looks up the key used for a file in the
+  index. The key is output to stdout. If there is no key (because
+  the file is not added to git, or is not a git-annex managed file),
+  nothing is output, and it exits nonzero.
+
 * `fromkey key file`
 
   This plumbing-level command can be used to manually set up a file