diff --git a/src/Niv/Git/Cmd.hs b/src/Niv/Git/Cmd.hs
index a58be11..0a3ec75 100644
--- a/src/Niv/Git/Cmd.hs
+++ b/src/Niv/Git/Cmd.hs
@@ -1,8 +1,20 @@
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE Arrows #-}
+{-# LANGUAGE QuasiQuotes #-}
 
-module Niv.Git.Cmd (gitCmd) where
+module Niv.Git.Cmd {- (gitCmd) -} where
 
+import Control.Arrow
+import Data.String.QQ (s)
+import Data.Text.Extended as T
 import Niv.Cmd
+import Niv.Sources
+import Niv.Update
+import System.Exit (ExitCode(ExitSuccess))
+import System.Process (readProcessWithExitCode)
+import qualified Data.HashMap.Strict as HMS
+import qualified Data.Text as T
 import qualified Options.Applicative as Opts
 import qualified Options.Applicative.Help.Pretty as Opts
 
@@ -10,11 +22,14 @@ gitCmd :: Cmd
 gitCmd = Cmd
   { description = describeGit
   , parseCmdShortcut = pure Nothing
-  , parsePackageSpec = pure mempty
-  , updateCmd = error "git update is not implemented yet"
+  , parsePackageSpec = parseGitPackageSpec
+  , updateCmd = gitUpdate
   , name = "git"
   }
 
+parseGitPackageSpec :: Opts.Parser PackageSpec
+parseGitPackageSpec = pure $ PackageSpec $ HMS.singleton "repo" "git@github.com:nmattia/niv"
+
 describeGit :: Opts.InfoMod a
 describeGit = mconcat
   [ Opts.fullDesc
@@ -27,13 +42,91 @@ describeGit = mconcat
       "  niv add https://github.com/stedolan/jq.git"
   ]
 
--- for git:
---  default branch:
---  ~/niv$ git ls-remote --symref git@github.com:NixOS/nixpkgs HEAD
---  ref: refs/heads/master  HEAD
---  3dd8e8e7faa87fc45c2492f88643bb363572180e        HEAD
---  0a46a71a6ec41764b118a24e4cbf1b4bc4be906e        refs/remotes/origin/HEAD
---
---  lastest rev:
---  ~/niv$ git ls-remote git@github.com:NixOS/nixpkgs refs/heads/master
---  3dd8e8e7faa87fc45c2492f88643bb363572180e        refs/heads/master
+gitUpdate :: Update () ()
+gitUpdate = proc () -> do
+    useOrSet "type" -< ("git" :: Box T.Text)
+    repository <- load "repo" -< ()
+    refAndRev <- (discoverRev <+> discoverRefAndRev) -< repository
+    update "ref" -< fst <$> refAndRev
+    update "rev" -< snd <$> refAndRev
+    returnA -< ()
+  where
+    discoverRefAndRev = proc repository -> do
+      run defaultRefAndHEAD -< repository
+    discoverRev = proc repository -> do
+      ref <- load "ref" -< ()
+      rev <- run (\(r1,r2) -> latestRev r1 r2)-< (,) <$> repository <*> ref
+      returnA -< (,) <$> ref <*> rev
+
+latestRev
+    :: T.Text -- ^ the repository
+    -> T.Text -- ^ the ref/branch
+    -> IO T.Text
+latestRev repo ref = do
+    let gitArgs = [ "ls-remote", repo, "refs/heads/" <> ref ]
+    sout <- runGit gitArgs
+    case sout of
+      ls@(_:_:_) -> abortTooMuchOutput ls
+      (l1:[]) -> parseRev l1
+      [] -> abortNoOutput
+  where
+    parseRev l = maybe (abortNoRev l) pure $ do
+      checkRev $ T.takeWhile (/= '\t') l
+    checkRev t = if isRev t then Just t else Nothing
+    abortNoOutput = abort "foo" -- TODO: args + abortBugIn
+
+defaultRefAndHEAD
+    :: T.Text -- ^ the repository
+    -> IO (T.Text, T.Text)
+defaultRefAndHEAD repo = do
+    sout <- runGit [ "ls-remote", "--symref", repo, "HEAD" ]
+    case sout of
+      (l1:l2:_) -> (,) <$> parseRef l1 <*> parseRev l2
+      _ -> abortNoRefAndRev
+  where
+    parseRef l = maybe (abortNoRef l) pure $ do
+      -- ref: refs/head/master\tHEAD -> master\tHEAD
+      refAndSym <- T.stripPrefix "ref: refs/heads/" l
+      let ref = T.takeWhile (/= '\t') refAndSym
+      if T.null ref then Nothing else Just ref
+    parseRev l = maybe (abortNoRev l) pure $ do
+      checkRev $ T.takeWhile (/= '\t') l
+    checkRev t = if isRev t then Just t else Nothing
+
+-- | Run the "git" executable
+runGit :: [T.Text] -> IO [T.Text]
+runGit args = do
+    (exitCode, sout, serr) <- readProcessWithExitCode "git" (T.unpack <$> args) ""
+    case (exitCode, lines sout) of
+      (ExitSuccess, ls)  -> pure $ T.pack <$> ls
+      _ -> abortGitFailure args (T.pack sout) (T.pack serr)
+
+isRev :: T.Text -> Bool
+isRev t =
+  -- commit hashes a comprised of abcdef0123456789
+  T.all (\c -> (c >= 'a' && c <= 'f') || (c >= '0' && c <= '9')) t &&
+  -- commit _should_ be 40 chars long, but to be sure we pick 7
+  T.length t >= 7
+
+abortTooMuchOutput :: [T.Text] -> IO a
+abortTooMuchOutput = abort .  T.unwords
+
+abortNoRef :: T.Text -> IO a
+abortNoRef = abort -- TODO
+
+abortNoRev :: T.Text -> IO a
+abortNoRev = abort -- TODO
+
+abortNoRefAndRev :: IO a
+abortNoRefAndRev = error "foo"
+
+-- TODO: mention error code
+abortGitFailure :: [T.Text] -> T.Text -> T.Text -> IO a
+abortGitFailure args sout serr = abort $ [s|
+Could not read the output of 'git'. This is a bug. Please create a
+ticket:
+
+  https://github.com/nmattia/niv/issues/new
+
+Thanks! I'll buy you a beer.
+|] <> T.unlines ["command: ", T.unwords args, "stdout: ", sout, "stderr: ", serr]
diff --git a/src/Niv/GitHub/Cmd.hs b/src/Niv/GitHub/Cmd.hs
index dba412b..c5b9fbf 100644
--- a/src/Niv/GitHub/Cmd.hs
+++ b/src/Niv/GitHub/Cmd.hs
@@ -132,7 +132,6 @@ parseAddShortcutGitHub str =
 githubUpdate' :: Update () ()
 githubUpdate' = githubUpdate nixPrefetchURL githubLatestRev githubRepo
 
--- TODO: dedup
 nixPrefetchURL :: Bool -> T.Text -> IO T.Text
 nixPrefetchURL unpack (T.unpack -> url) = do
     (exitCode, sout, serr) <- runNixPrefetch