calckey: New plumbing command, calculates the key that would be used to refer to a file

Command/CalcKey.hs

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+{- git-annex command
+ -
+ - Copyright 2016 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Command.CalcKey where
+
+import Command
+import Backend (genKey)
+import Types.KeySource
+
+cmd :: Command
+cmd = noCommit $ noMessages $ dontCheck repoExists $
+	command "calckey" SectionPlumbing 
+		"calculates the key that would be used to refer to a file"
+		(paramRepeating paramFile)
+		(batchable run (pure ()))
+
+run :: () -> String -> Annex Bool
+run _ file = do
+	mkb <- genKey (KeySource file file Nothing) Nothing
+	case mkb of
+		Just (k, _) -> do
+			liftIO $ putStrLn $ key2file k
+			return True
+		Nothing -> return False