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.

These functions can be executed as one liners from the bash shell with, for example (like wc, but in haskell):

$ cat file.txt | ghc -e 'wc_l' UnixTools.hs

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

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

-- 
-- First, two helpers
--
io f    = interact (unlines . f . lines)
showln  = (++ "\n") . show

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

--
-- Sort a file
--
sort'   = io sort

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

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

--
-- Return the head -10 line of a file
--
take'   = io (take 10)

--
-- Remove the first 10 lines of a file
--
drop'   = io (drop 10)

--
-- Return the head -1 line of a file
--
head'   = io (return . head)

--
-- Return the tail -1 line of a file
--
tail'   = io (return . last)

-- 
-- Reverse lines in a file (tac)
--
tac     = io reverse

--
-- Reverse characters on each line (rev)
--
rev     = io (map reverse)

--
-- Reverse words on each line
--
revw    = io $ map (unwords. reverse . words)

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

--
-- Count number of lines in a file, like wc -l
--
wc_l    = interact (showln . length . lines)

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

--
-- double space a file
--
space   = io (intersperse "")

-- 
-- undo double space
--
unspace = io $ filter (not.null)

--
-- remove the first occurence of the line "str"
--
remove  = io (delete "str")

--
-- make a file all upper case
--
upper   = interact (map toUpper)

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

--
-- remove trailing whitespace
--
clean'  = io (map f)
    where f = reverse . dropWhile isSpace . reverse

--
-- delete leading and trailing whitespace
--
clean'' = io $ map (f . f)
    where f = reverse . dropWhile isSpace

--
-- insert blank space at beginning of each line
--
blank   = io $ map (s ++)
    where s = replicate 8 ' '

--
-- join lines of a file
--
join    = io (return . 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')

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

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

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

--
-- Compute a simple cksum of a file
--
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