#!/usr/bin/python

from os import *
import sys

def GetCommand(prompt):
    cmd = raw_input(prompt)
    path = cmd.split(" ", 1)[0]

    return(cmd, path)

def Run():
    (cmd, path) = GetCommand("What command do you want to run? ")

    pid = fork()

    if pid == 0 :
       execvp(path, cmd.split())
    else:
       wait()


def Pipe():
    (cmd1, path1) = GetCommand("What is the first command? ")
    (cmd2, path2) = GetCommand("What is the second command? ")

    pid = fork()
    if pid == 0:
       (readEnd, writeEnd) = pipe()

       pid = fork()

       if pid == 0:
           dup2(readEnd, 0)
           close (writeEnd)
           execvp(path2, cmd2.split())
       else:
           dup2(writeEnd, 1)
           close(readEnd)
           execvp(path1, cmd1.split())
    else :
       waitpid(pid,0)



def Redirect():
    inFile = raw_input("What is the input file? ");
    outFile = raw_input("What is the output file? ");

    (cmd, path) = GetCommand("What command do you want to run? ")

    pid = fork()
    if pid == 0:

        if inFile != "":
           inFD = open(inFile, O_RDONLY)
           dup2(inFD, 0)

        if outFile != "":
           outFD = open(outFile, O_WRONLY|O_CREAT|O_TRUNC, 0o644)
           dup2(outFD, 1)

        execvp(path, cmd.split())

    else:
        wait()

def Error(command):
    print "Unknown command ", command
    print "Known commands are RUN, PIPE, REDIRECT, QUIT"

command = raw_input("Enter a command: ")
while command != "QUIT":
    print "Executing ", command

    if command == "RUN":
        Run()
    elif command =="PIPE":
        Pipe()
    elif command == "REDIRECT":
        Redirect()
    else :
        Error(command)
 
    print
    command = raw_input("Enter a command: ")