Cat
June 8, 2012
One of the on-going themes here at Programming Praxis involves the re-implementation of the text utilities from Unix V7. Today we look at the cat command:
NAME
cat – catenate and print
SYNOPSIS
cat [ –u ] file …
DESCRIPTION
Cat reads each file in sequence and writes it on the standard output. Thus
cat file
prints the file and
cat file1 file2 >file3
concatenates the first two files and places the result on the third.
If no file is given, or if the argument ‘-‘ is encountered, cat reads from the standard input. Output is buffered in 512-byte blocks unless the standard output is a terminal or the –u option is present.
SEE ALSO
pr(1), cp(1)
BUGS
Beware of ‘cat a b >a’ and ‘cat a b >b’, which destroy input files before reading them.
Your task is to write the Unix V7 cat program. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.
Programming Praxis 
My Haskell solution. Ignores the “-u” switch.
import System.Environment
import System.IO
import Control.Exception
parseCmd :: [String] -> IO [Handle]
parseCmd [] = return []
parseCmd ["-"] = return [stdin]
parseCmd ("-u":xs) = parseCmd xs
parseCmd xs = mapM (`openFile` ReadMode) xs
main :: IO ()
main = do
args hGetContents h >>= putStr))
Ok, something went wrong while posting. Let’s try again.
import System.Environment import System.IO import Control.Exception parseCmd :: [String] -> IO [Handle] parseCmd [] = return [] parseCmd ["-"] = return [stdin] parseCmd ("-u":xs) = parseCmd xs parseCmd xs = mapM (`openFile` ReadMode) xs main :: IO () main = do args <- getArgs bracket (parseCmd args) (mapM_ hClose) (mapM_ (\h -> hGetContents h >>= putStr))