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[1] 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 System.IO --not needed yet
import Text.Printf ( printf )
import System.Environment ( getProgName )

main = do
  calledBy <- getProgName
  case calledBy of
    "blank" -> io blank
    "cksum" -> 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 :: IO ()
cksum   = interact $ showln . foldl' k 5381
 where k h c = h * 33 + ord c

Where to now?

• Haskell.org
• The Haskell standard list library, with docs
• Alternative implementations of the wc program
• Learn how to test Haskell code
• More Haskell code
• Haskell for shell scripting
• Export list functions to the shell with h4sh