diff --git a/cless.cabal b/cless.cabal
--- a/cless.cabal
+++ b/cless.cabal
@@ -1,5 +1,5 @@
 name:                cless
-version:             0.2.0.0
+version:             0.3.0.0
 synopsis:            Colorized LESS
 description:         Print file contents with syntax highlighting
 homepage:            https://github.com/tanakh/cless
diff --git a/src/Main.hs b/src/Main.hs
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -4,9 +4,11 @@
 
 import           Control.Exception
 import           Control.Monad
+import           Data.Char
 import           Data.Maybe
 import           Data.Monoid
 import           Data.String
+import           Data.Version
 import           Options.Applicative
 import           System.Console.Terminfo
 import           System.Console.Terminfo.Color       as Terminfo
@@ -18,6 +20,8 @@
 import           Text.PrettyPrint.Free               hiding ((<>))
 import           Text.Printf
 
+import           Paths_cless                         (version)
+
 main :: IO ()
 main = join $ execParser opts where
   opts = info (helper <*> cmd)
@@ -26,21 +30,18 @@
         <> header "cless: Colorized LESS" )
 
   cmd = process
-        <$> switch ( long "list-langs"
-                  <> short 'L'
+        <$> switch ( long "version" <> short 'v'
+                  <> help "Show version information" )
+        <*> switch ( long "list-langs" <> short 'L'
                   <> help "Show the list of supported languages" )
-        <*> switch ( long "list-styles"
-                  <> short 'S'
+        <*> switch ( long "list-styles" <> short 'S'
                   <> help "Show the list of supported styles" )
-        <*> switch ( long "LINE-NUMBERS"
-                  <> short 'N'
+        <*> switch ( long "LINE-NUMBERS" <> short 'N'
                   <> help "Show line numbers" )
-        <*> optional (strOption ( long "lang"
-                  <> short 'l'
+        <*> optional (strOption ( long "lang" <> short 'l'
                   <> metavar "LANG"
                   <> help "Specify language name" ) )
-        <*> optional (strOption ( long "style"
-                  <> short 's'
+        <*> optional (strOption ( long "style" <> short 's'
                   <> metavar "STYLE"
                   <> help "Specify style name (default 'pygments')" ) )
         <*> optional (argument str (metavar "FILE"))
@@ -65,34 +66,38 @@
 defaultStyle :: Style
 defaultStyle = pygments
 
-process :: Bool -> Bool -> Bool -> Maybe String -> Maybe String -> Maybe FilePath -> IO ()
-process True _ _ _ _ _ =
-  mapM_ putStrLn languages
-
-process _ True _ _ _ _ =
-  mapM_ (putStrLn . fst) styles
+process :: Bool -> Bool -> Bool -> Bool -> Maybe String -> Maybe String -> Maybe FilePath -> IO ()
+process showVer showLangs showStyles linum mb_lang mb_stylename mb_file
+  | showVer =
+    putStrLn $ "cless version " ++ showVersion version
+  | showLangs =
+    mapM_ putStrLn languages
+  | showStyles =
+    mapM_ (putStrLn . fst) styles
+  | otherwise = do
+    process' linum mb_lang mb_stylename mb_file
 
-process _ _ linum mb_lang mb_stylename mb_file = do
+process' :: Bool -> Maybe String -> Maybe String -> Maybe FilePath -> IO ()
+process' linum mb_lang mb_stylename mb_file = do
   con <- case mb_file of
-      Just file -> readFile file
-      Nothing   -> getContents
-
-  let lang = fromMaybe "plain"
-             $ mb_lang <|> do file <- mb_file; listToMaybe (languagesByFilename file)
-
-      ss = highlightAs lang con
+    Just file -> readFile file
+    Nothing   -> do
+      isTerm <- hIsTerminalDevice stdin
+      when isTerm $
+        error "Missing filename (\"cless --help\" for help)"
+      getContents
 
+  let lang  = determineLanguage mb_lang mb_file con
       style = maybe defaultStyle findStyle mb_stylename
 
-      findStyle name =
-        fromMaybe (error $ "invalid style name: " ++ name)
-        $ lookup name styles
+  -- to raise error eagerly
+  evaluate lang
+  evaluate style
 
-      doc = ppr linum style ss <> linebreak
+  let ss   = highlightAs lang con
+      doc  = ppr linum style ss <> linebreak
       sdoc = renderPretty 0.6 80 (prettyTerm doc)
 
-  evaluate style -- to raise error eagerly
-
   termType <- fromMaybe defaultTerm <$> lookupEnv "TERM"
   pager <- fromMaybe defaultPager <$> lookupEnv "PAGER"
   term  <- setupTerm $ if termType == "screen" then defaultTerm else termType
@@ -105,6 +110,43 @@
         Just output -> hRunTermOutput h term output
         Nothing -> displayIO h sdoc
       hClose h
+
+-- determin using language:
+--   1. user specified (must be correct)
+--   2. filename
+--   3. content
+determineLanguage :: Maybe String -> Maybe String -> String -> String
+determineLanguage mb_lang mb_file content = fromMaybe "plain" $
+  (isValid <$> mb_lang) <|>
+  (listToMaybe . languagesByFilename =<< mb_file) <|>
+  (findSupportedLanguage =<< detectLanguage content)
+  where
+    isValid lang
+      | Just lang' <- findSupportedLanguage lang = lang'
+      | otherwise = error $ "Unsupported language: " ++ lang
+
+-- detect language from shebang
+detectLanguage :: String -> Maybe String
+detectLanguage ss
+  | take 2 ss == "#!" =
+    let sb = head $ lines ss
+    in listToMaybe $ catMaybes
+       [ findSupportedLanguage w
+       | w <- words $ map (\c -> if c == '/' then ' ' else c) sb
+       ]
+  | otherwise =
+     Nothing
+
+findSupportedLanguage :: String -> Maybe String
+findSupportedLanguage lang
+  | map toLower lang `elem` map (map toLower) languages = Just lang
+  | (lang': _) <- languagesByExtension lang             = Just lang'
+  | otherwise = Nothing
+
+findStyle :: String -> Style
+findStyle name =
+  fromMaybe (error $ "invalid style name: " ++ name)
+  $ lookup name styles
 
 ppr :: Bool -> Style -> [SourceLine] -> TermDoc
 ppr linum Style{..} = vcat . zipWith addLinum [1..] . map (hcat . map token) where