Simple Unix tools
Simple Unix commandline tools written in Haskell.
This is intended as a beginner's tutorial for learning Haskell from a "Let's just solve things already!" point of view. The examples should help give a flavor of the beauty and expressiveness of Haskell programming.
These functions can be executed as one liners from a shell. For example, to use the Haskell version of 'wc':
$ cat file.txt | ghc -e 'wc_l' UnixTools.hs
Or, one could define 'main' to be a chosen tool/function (add a line to the effect that "main = wc_l") and then compile the tool with $ ghc --make UnixTools.hs
The given Haskell codes presents yet a third way of doing things: much like the BusyBox suite of Unix tools, it is possible to compile a single monolithic binary and have it detect what name it is run by and then act appropriately. This is the approach the following code takes: you can compile it and then make symbolic links (like "ln -s UnixTools echo") and then run those commands ("echo foo | ./echo" would produce output of "foo").
-- Some Unix-like tools written in simple, clean Haskell module Main where import Data.List ( sort, nub, intersperse, delete, isPrefixOf, foldl' ) import Data.Char ( toUpper, isSpace, ord ) import Text.Printf ( printf ) import System.Environment ( getProgName ) main = do calledBy <- getProgName case calledBy of "blank" -> io blank "cksum" -> interact (showln . cksum) "clean" -> io clean'' "echo" -> interact id -- not perfect. what if echoed thing is an argument and not on stdin? "drop" -> interact drop' "grep" -> io grep "grep -v" -> io grep_v "head" -> io (return . head') "join" -> io join "num" -> io num "remove" -> io (remove "str") "revw" -> io revw "reverse" -> io rev "reverseword" -> io revw "rpt" -> io rpt "sort" -> interact sort "space" -> io space "tac" -> interact tac "take" -> io take' "tail" -> io (return . tail') --"tr" -> interact tr --"tr -d" -> interact (trd . unwords) "unspace" -> io unspace "upper" -> interact upper "uniq" -> interact uniq "wc_c" -> interact wc_c "wc_l" -> interact wc_l "wc_w" -> interact wc_w -- First, two helpers io :: ([String] -> [String]) -> IO () io f = interact (unlines . f . lines) showln :: Int -> String showln = (++ "\n") . show -- Sort a file sort' :: String -> String sort' = sort -- remove duplicate lines from a file (like uniq) uniq :: String -> String uniq = nub -- repeat the input file infinitely rpt :: [a] -> [a] rpt = cycle -- Return the head -10 line of a file take' :: [String] -> [String] take' = take 10 -- Remove the first 10 lines of a file drop' :: String -> String drop' = drop 10 -- Return the head -1 line of a file head' :: [String] -> String head' = head -- Return the tail -1 line of a file tail' :: [String] -> String tail' = last -- Reverse lines in a file (tac) tac :: String -> String tac = reverse -- Reverse characters on each line (rev) rev :: [String] -> [String] rev = map reverse -- Reverse words on each line revw :: [String] -> [String] revw = map (unwords. reverse . words) -- Count number of characters in a file (like wc -c) wc_c :: String -> String wc_c = showln . length -- Count number of lines in a file, like wc -l wc_l :: String -> String wc_l = showln . length . lines -- Count number of words in a file (like wc -w) wc_w :: String -> String wc_w = showln . length . words -- double space a file space :: [String] -> [String] space = intersperse "" -- undo double space unspace :: [String] -> [String] unspace = filter (not.null) -- remove the first occurrence of the line "str" remove :: String -> [String] -> [String] remove a = delete a -- make a string all upper case upper :: String -> String upper = map toUpper -- remove leading space from each line clean :: [String] -> [String] clean = map (dropWhile isSpace) -- remove trailing whitespace clean' :: [String] -> [String] clean' = (map f) where f = reverse . dropWhile isSpace . reverse -- delete leading and trailing whitespace clean'' :: [String] -> [String] clean'' = map (f . f) where f = reverse . dropWhile isSpace -- insert blank space at beginning of each line blank :: [String] -> [String] blank = map (s ++) where s = replicate 8 ' ' -- join lines of a file join :: [String] -> [String] join = return . concat -- Translate the letter 'e' to '*', like tr 'e' '*' (or y// in sed) tr :: Char -> Char -> IO () tr a b = interact (map f) where f c = if c == a then b else c -- Delete characters from a string. trd :: Char -> IO () trd a = tr a ' ' -- grep lines matching "^foo" from a file grep :: [String] -> [String] grep = filter (isPrefixOf "foo") -- grep lines that don't match "^foo" (grep -v) grep_v :: [String] -> [String] grep_v = filter (not . isPrefixOf "foo") -- number each line of a file num :: [String] -> [String] num = zipWith (printf "%3d %s") [(1::Int)..] -- Compute a simple cksum of a file cksum :: [Char] -> Int cksum = foldl' k 5381 where k h c = h * 33 + ord c