diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -205,6 +205,9 @@
 template that can be reused by niv when fetching a new URL (see the
 documentation for [add](#add) and [update](#update)).
 
+The type of the dependency is guessed from the provided URL template, if `-T`
+is not specified.
+
 For updating the version of GHC used run this command:
 
 ``` shell
@@ -216,12 +219,13 @@
 ```
 niv - dependency manager for Nix projects
 
-version: 0.2.18
+version: 0.2.19
 
-Usage: niv [-s|--sources-file FILE] COMMAND
+Usage: niv [-s|--sources-file FILE] [--no-colors] COMMAND
 
 Available options:
   -s,--sources-file FILE   Use FILE instead of nix/sources.json
+  --no-colors              Don't use colors in output
   -h,--help                Show this help text
   --version                Print version
 
diff --git a/niv.cabal b/niv.cabal
--- a/niv.cabal
+++ b/niv.cabal
@@ -4,10 +4,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: f33a4bacf65fdd3b3ad1ee550d862cd5394d00eb4c7f2bf5c51890eb76124fbb
+-- hash: 6551d56f4a527a8b959ff8a47f1bf12e96efb5c6f62bb82d4c61cfb92ff1544f
 
 name:           niv
-version:        0.2.18
+version:        0.2.19
 synopsis:       Easy dependency management for Nix projects
 description:    Easy dependency management for Nix projects.
 category:       Development
diff --git a/nix/sources.nix b/nix/sources.nix
--- a/nix/sources.nix
+++ b/nix/sources.nix
@@ -98,7 +98,10 @@
       saneName = stringAsChars (c: if isNull (builtins.match "[a-zA-Z0-9]" c) then "_" else c) name;
       ersatz = builtins.getEnv "NIV_OVERRIDE_${saneName}";
     in
-      if ersatz == "" then drv else ersatz;
+      if ersatz == "" then drv else
+        # this turns the string into an actual Nix path (for both absolute and
+        # relative paths)
+        if builtins.substring 0 1 ersatz == "/" then /. + ersatz else /. + builtins.getEnv "PWD" + "/${ersatz}";
 
   # Ports of functions for older nix versions
 
diff --git a/src/Niv/Cli.hs b/src/Niv/Cli.hs
--- a/src/Niv/Cli.hs
+++ b/src/Niv/Cli.hs
@@ -54,16 +54,17 @@
 cli :: IO ()
 cli = do
   warnGitHubEnvVars
-  (fsj, nio) <-
+  ((fsj, colors), nio) <-
     execParserPure' Opts.defaultPrefs opts <$> getArgs
       >>= Opts.handleParseResult
+  setColors colors
   runReaderT (runNIO nio) fsj
   where
     execParserPure' pprefs pinfo [] =
       Opts.Failure $
         Opts.parserFailure pprefs pinfo Opts.ShowHelpText mempty
     execParserPure' pprefs pinfo args = Opts.execParserPure pprefs pinfo args
-    opts = Opts.info ((,) <$> parseFindSourcesJson <*> (parseCommand <**> Opts.helper <**> versionflag)) $ mconcat desc
+    opts = Opts.info ((,) <$> ((,) <$> parseFindSourcesJson <*> parseColors) <*> (parseCommand <**> Opts.helper <**> versionflag)) $ mconcat desc
     desc =
       [ Opts.fullDesc,
         Opts.headerDoc $ Just $
@@ -80,6 +81,12 @@
               <> Opts.help "Use FILE instead of nix/sources.json"
           )
         <|> pure Auto
+    parseColors =
+      (\case True -> Never; False -> Always)
+        <$> Opts.switch
+          ( Opts.long "no-colors"
+              <> Opts.help "Don't use colors in output"
+          )
     versionflag :: Opts.Parser (a -> a)
     versionflag =
       Opts.abortOption (Opts.InfoMsg (showVersion version)) $
diff --git a/src/Niv/Git/Cmd.hs b/src/Niv/Git/Cmd.hs
--- a/src/Niv/Git/Cmd.hs
+++ b/src/Niv/Git/Cmd.hs
@@ -181,9 +181,9 @@
     abortNoOutput args =
       abortGitFailure
         args
-        "Git didn't produce any output."
+        $ "Git didn't produce any output. Does the branch '" <> branch <> "' exist?"
     abortTooMuchOutput args ls =
-      abortGitFailure args $ T.unlines $
+      abortGitBug args $ T.unlines $
         ["Git produced too much output:"] <> map ("  " <>) ls
 
 defaultBranchAndRev ::
@@ -195,7 +195,7 @@
   case sout of
     (l1 : l2 : _) -> (,) <$> parseBranch l1 <*> parseRev l2
     _ ->
-      abortGitFailure args $ T.unlines $
+      abortGitBug args $ T.unlines $
         [ "Could not read reference and revision from stdout:"
         ]
           <> sout
@@ -211,10 +211,10 @@
     checkRev t = if isRev t then Just t else Nothing
 
 abortNoRev :: [T.Text] -> T.Text -> IO a
-abortNoRev args l = abortGitFailure args $ "Could not read revision from: " <> l
+abortNoRev args l = abortGitBug args $ "Could not read revision from: " <> l
 
 abortNoRef :: [T.Text] -> T.Text -> IO a
-abortNoRef args l = abortGitFailure args $ "Could not read reference from: " <> l
+abortNoRef args l = abortGitBug args $ "Could not read reference from: " <> l
 
 -- | Run the "git" executable
 runGit :: [T.Text] -> IO [T.Text]
@@ -223,7 +223,7 @@
   case (exitCode, lines sout) of
     (ExitSuccess, ls) -> pure $ T.pack <$> ls
     _ ->
-      abortGitFailure args $
+      abortGitBug args $
         T.unlines
           [ T.unwords ["stdout:", T.pack sout],
             T.unwords ["stderr:", T.pack serr]
@@ -239,6 +239,15 @@
 
 abortGitFailure :: [T.Text] -> T.Text -> IO a
 abortGitFailure args msg =
+  abort $
+    T.unlines
+      [ "Could not read the output of 'git'.",
+        T.unwords ("command:" : "git" : args),
+        msg
+      ]
+
+abortGitBug :: [T.Text] -> T.Text -> IO a
+abortGitBug args msg =
   abort $ bug $
     T.unlines
       [ "Could not read the output of 'git'.",
diff --git a/src/Niv/GitHub.hs b/src/Niv/GitHub.hs
--- a/src/Niv/GitHub.hs
+++ b/src/Niv/GitHub.hs
@@ -34,9 +34,9 @@
       <<< (useOrSet "url_template" <<< completeSpec) <+> (load "url_template") -<
       ()
   url <- update "url" -< urlTemplate
-  let isTar = (\u -> "tar.gz" `T.isSuffixOf` u || ".tgz" `T.isSuffixOf` u) <$> url
-  useOrSet "type" -< bool "file" "tarball" <$> isTar :: Box T.Text
-  let doUnpack = isTar
+  let isTarGuess = (\u -> "tar.gz" `T.isSuffixOf` u || ".tgz" `T.isSuffixOf` u) <$> url
+  type' <- useOrSet "type" -< bool "file" "tarball" <$> isTarGuess :: Box T.Text
+  let doUnpack = (== "tarball") <$> type'
   _sha256 <- update "sha256" <<< run (\(up, u) -> prefetch up u) -< (,) <$> doUnpack <*> url
   returnA -< ()
   where
diff --git a/src/Niv/Logger.hs b/src/Niv/Logger.hs
--- a/src/Niv/Logger.hs
+++ b/src/Niv/Logger.hs
@@ -4,7 +4,9 @@
 {-# LANGUAGE OverloadedStrings #-}
 
 module Niv.Logger
-  ( job,
+  ( Colors (Always, Never),
+    job,
+    setColors,
     bug,
     tsay,
     say,
@@ -35,6 +37,27 @@
 import System.IO.Unsafe (unsafePerformIO)
 import UnliftIO
 
+-- A somewhat hacky way of deciding whether or not to use SGR codes, by writing
+-- and reading a global variable unsafely.
+-- This should be fine as long as the IORef is written right after argument
+-- parsing, and as long as the value is never changed.
+-- NOTE: this won't work in GHCi.
+
+data Colors
+  = Always
+  | Never
+  deriving (Eq)
+
+colors :: IORef Colors
+colors = unsafePerformIO $ newIORef Always
+{-# NOINLINE colors #-}
+
+setColors :: Colors -> IO ()
+setColors = writeIORef colors
+
+useColors :: Bool
+useColors = unsafePerformIO $ (\c -> c == Always) <$> readIORef colors
+
 type S = String -> String
 
 type T = T.Text -> T.Text
@@ -85,61 +108,58 @@
 mkNote :: T.Text -> T.Text
 mkNote w = tbold (tblue "NOTE") <> ": " <> w
 
+color :: ANSI.Color -> String -> String
+color c str =
+  if useColors
+    then
+      ANSI.setSGRCode [ANSI.SetConsoleIntensity ANSI.BoldIntensity]
+        <> ANSI.setSGRCode [ANSI.SetColor ANSI.Foreground ANSI.Vivid c]
+        <> str
+        <> ANSI.setSGRCode [ANSI.Reset]
+    else str
+
+colorFaint :: ANSI.Color -> String -> String
+colorFaint c str =
+  if useColors
+    then
+      ANSI.setSGRCode [ANSI.SetConsoleIntensity ANSI.FaintIntensity]
+        <> ANSI.setSGRCode [ANSI.SetColor ANSI.Foreground ANSI.Vivid c]
+        <> str
+        <> ANSI.setSGRCode [ANSI.Reset]
+    else str
+
 green :: S
-green str =
-  ANSI.setSGRCode [ANSI.SetConsoleIntensity ANSI.BoldIntensity]
-    <> ANSI.setSGRCode [ANSI.SetColor ANSI.Foreground ANSI.Vivid ANSI.Green]
-    <> str
-    <> ANSI.setSGRCode [ANSI.Reset]
+green = color ANSI.Green
 
 tgreen :: T
 tgreen = t green
 
 yellow :: S
-yellow str =
-  ANSI.setSGRCode [ANSI.SetConsoleIntensity ANSI.BoldIntensity]
-    <> ANSI.setSGRCode [ANSI.SetColor ANSI.Foreground ANSI.Vivid ANSI.Yellow]
-    <> str
-    <> ANSI.setSGRCode [ANSI.Reset]
+yellow = color ANSI.Yellow
 
 tyellow :: T
 tyellow = t yellow
 
 blue :: S
-blue str =
-  ANSI.setSGRCode [ANSI.SetConsoleIntensity ANSI.BoldIntensity]
-    <> ANSI.setSGRCode [ANSI.SetColor ANSI.Foreground ANSI.Vivid ANSI.Blue]
-    <> str
-    <> ANSI.setSGRCode [ANSI.Reset]
+blue = color ANSI.Blue
 
 tblue :: T
 tblue = t blue
 
 red :: S
-red str =
-  ANSI.setSGRCode [ANSI.SetColor ANSI.Foreground ANSI.Vivid ANSI.Red]
-    <> str
-    <> ANSI.setSGRCode [ANSI.Reset]
+red = color ANSI.Red
 
 tred :: T
 tred = t red
 
 bold :: S
-bold str =
-  ANSI.setSGRCode [ANSI.SetConsoleIntensity ANSI.BoldIntensity]
-    <> ANSI.setSGRCode [ANSI.SetColor ANSI.Foreground ANSI.Vivid ANSI.White]
-    <> str
-    <> ANSI.setSGRCode [ANSI.Reset]
+bold = color ANSI.White
 
 tbold :: T
 tbold = t bold
 
 faint :: String -> String
-faint str =
-  ANSI.setSGRCode [ANSI.SetConsoleIntensity ANSI.FaintIntensity]
-    <> ANSI.setSGRCode [ANSI.SetColor ANSI.Foreground ANSI.Vivid ANSI.White]
-    <> str
-    <> ANSI.setSGRCode [ANSI.Reset]
+faint = colorFaint ANSI.White
 
 tfaint :: T
 tfaint = t faint
diff --git a/src/Niv/Sources.hs b/src/Niv/Sources.hs
--- a/src/Niv/Sources.hs
+++ b/src/Niv/Sources.hs
@@ -170,6 +170,8 @@
     V22
   | -- Allow to pass custom system to bootstrap niv in pure mode
     V23
+  | -- Fix NIV_OVERRIDE_{name} for sandbox
+    V24
   deriving stock (Bounded, Enum, Eq)
 
 -- | A user friendly version
@@ -198,6 +200,7 @@
   V21 -> "21"
   V22 -> "22"
   V23 -> "23"
+  V24 -> "24"
 
 latestVersionMD5 :: T.Text
 latestVersionMD5 = sourcesVersionToMD5 maxBound
@@ -233,6 +236,7 @@
   V21 -> "c501eee378828f7f49828a140dbdbca3"
   V22 -> "935d1d2f0bf95fda977a6e3a7e548ed4"
   V23 -> "4111204b613ec688e2669516dd313440"
+  V24 -> "116c2d936f1847112fef0013771dab28"
 
 -- | The MD5 sum of ./nix/sources.nix
 sourcesNixMD5 :: IO T.Text