Simple Unix tools

Simple unix tools written in Haskell.

This is intended as a beginners tutorial for learning Haskell from a "Lets just solve things already!" point of view. The examples should help give a flavour of the beauty and expressiveness of Haskell programming.

--
-- Some unix-like tools written in simple, clean Haskell
--

import Data.List
import Data.Char
import System.IO
import Text.Printf

-- 
-- First, a useful helper
--
input f = interact (unlines . f . lines)

--
-- The 'cat' program
--
cat     = interact id

--
-- Sort a file
--
sort'   = input sort

-- 
-- Reverse a file (tac)
--
tac     = input reverse

--
-- Return the head -10 line of a file
--
head'   = input $ take 10

--
-- Remove the first 10 lines of a file
--
drop'   = input $ drop 10

--
-- Return the tail -1 line of a file
--
tail'   = input $ (:[]) . last

--
-- remove duplicate lines from a file (like uniq)
--
uniq    = input nub

--
-- double space a file
--
space   = input $ intersperse ""

-- 
-- repeat the input file infinitely 
--
rpt     = interact cycle

--
-- remove the first occurence of the line "str"
--
remove  = input $ delete "str"

--
-- make a file all upper case
--
upper   = interact $ map toUpper

--
-- remove leading space from each line
--
clean   = input $ map (dropWhile isSpace)

--
-- join lines of a file
--
join    = input $ (:[]) . concat

--
-- Translate the letter 'e' to '*', like tr 'e' '*' (or y// in sed)
--
y       = interact $ map f
    where f 'e' = '*'
          f  c  =  c
--
-- Filter the letter 'e' from a file, like tr -d 'e'
--
tr      = interact $ filter (/= 'e')

--
-- Count number of characters in a file (like wc -c)
--
wc_c    = interact $ show . length

--
-- Count number of lines in a file, like wc -l
--
wc_l    = interact $ show . length . lines

--
-- Count number of words in a file (like wc -w)
--
wc_w    = interact $ show . length . words

--
-- grep lines matching "^foo" from a file
--
grep    = input $ filter (isPrefixOf "foo")

--
-- grep lines that don't match "^foo" (grep -v)
--
grep_v  = input $ filter (not . isPrefixOf "foo")

--
-- number each line of a file
--
num     = input $ zipWith (printf "%3d %s") [1::Int..]

--
-- Compute a simple hash of a file
--
cksum   = interact $ printf "%u\n" . foldl' k 5381
    where k h c = h * 33 + fromIntegral (ord c) :: Int

Where to now?

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