My C shell can successfully handle redirection (e.g. ls -al > output.txt, ./pre < input1.txt, etc.) and multiple pipes (i.e. cmd1 | cmd 2 | cmd 3). However, my code is not working when I try to do input and output redirection together with a pipe, such as ./pre < input.txt | ./sort > output.txt. No output file is ever made, although the ./pre does successfully execute.
pre is an executable that prints names with GPAs over 3.0
sort is an executable that alphabetizes a list of names input.txt is a file with names and GPAs (Name 3.1...).
CODE:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#ifndef READ
#define READ 0
#endif
#ifndef WRITE
#define WRITE 1
#endif
void clearArgumentContainer (int argumentContainer[]);
int main() {
/* professor-supplied variables for commands and command parsing */
char *iPath, *oPath, *argv[20], buf[80], n, *p;
int m, status, inword, continu;
int start[20];
/* flags for redirection (note: C does not have type bool; using integer value 0 or 1) */
int inputRedirectFlag, outputRedirectFlag;
/* variables for piping */
int count, pipes;
pid_t pid;
/* pipes */
int l_pipe[2], r_pipe[2];
/* required container for handling arguments */
int argumentContainer[20] = { 0 };
while (1) {
inword = m = continu = count = pipes = pid = 0;
p = buf;
/* redirection flags reset */
inputRedirectFlag = outputRedirectFlag = 0;
/* shell prompt */
printf("\nshhh> ");
/* command parsing */
while ((n = getchar()) != '\n' || continu)
{
if (n == ' ') {
if (inword)
{
inword = 0;
*p++ = 0;
}
}
else if (n == '\n')
continu = 0;
else if (n == '\\' && !inword)
continu = 1;
else {
if (!inword)
{
inword = 1;
argv[m++] = p;
*p++ = n;
}
else
*p++ = n;
}
} /* end of command parsing */
*p++ = 0;
argv[m] = 0;
/* user wishes to terminate program */
if (strcmp(argv[0], "exit") == 0)
exit(0);
/* manage redirection */
while (argv[count] != 0) {
if (strcmp(argv[count], "|") == 0) {
argv[count] = 0;
argumentContainer[pipes + 1] = count + 1;
++pipes;
}
else if (strcmp(argv[count], "<") == 0) {
iPath = strdup(argv[count + 1]); /* copy string argument (file string) */
argv[count] = 0;
argv[count + 1] = 0;
inputRedirectFlag = 1;
}
else if (strcmp(argv[count], ">") == 0) {
oPath = strdup(argv[count + 1]); /* copy string argument (file string) */
argv[count] = 0;
argv[count + 1] = 0;
outputRedirectFlag = 1;
}
else {
argumentContainer[count] = count;
}
++count;
} /* end of redirection management */
/* execute commands [<= in for-loop; n pipes require n+1 processes] */
for (int index = 0; index <= pipes; ++index) {
if (pipes > 0 && index != pipes) { /* if user has entered multiple commands with '|' */
pipe(r_pipe); /* no pipe(l_pipe); r_pipe becomes next child's l_pipe */
}
/* switch-statement for command execution */
switch (pid = fork()) {
/* fork() error */
case -1: perror("fork failed");
break;
case 0: /* child process manages redirection and executes */
if ((index == 0) && (inputRedirectFlag == 1)) {
int inputFileDescriptor = open(iPath, O_RDONLY , 0400);
if (inputFileDescriptor == -1) {
perror("input file failed to open\n");
return(EXIT_FAILURE);
}
close(READ);
dup(inputFileDescriptor);
close(inputFileDescriptor);
} /* end of input redirection management */
if ((index == pipes) && (outputRedirectFlag == 1)) {
int outputFileDescriptor = open(oPath, O_WRONLY | O_CREAT, 0755);
if (outputFileDescriptor < 0) {
perror("output file failed to open\n");
return(EXIT_FAILURE);
}
close(WRITE);
dup(outputFileDescriptor);
close(outputFileDescriptor);
} /* end of output redirection management */
/* manage pipes if (a) first child process, (b) middle child process, or (c) third/final child process */
if (pipes > 0) {
if (index == 0){ /* first child process */
close(WRITE);
dup(r_pipe[WRITE]);
close(r_pipe[WRITE]);
close(r_pipe[READ]);
}
else if (index < pipes) { /* middle child process */
close(READ);
dup(l_pipe[READ]);
close(l_pipe[READ]);
close(l_pipe[WRITE]);
close(WRITE);
dup(r_pipe[WRITE]);
close(r_pipe[READ]);
close(r_pipe[WRITE]);
}
else { /* third/final child process */
close(READ);
dup(l_pipe[READ]);
close(l_pipe[READ]);
close(l_pipe[WRITE]);
}
}
/* execute command */
execvp(argv[argumentContainer[index]], &argv[argumentContainer[index]]);
/* if execvp() fails */
perror("execution of command failed\n");
break;
default: /* parent process manages the pipes for child process(es) */
if (index > 0) {
close(l_pipe[READ]);
close(l_pipe[WRITE]);
}
l_pipe[READ] = r_pipe[READ];
l_pipe[WRITE] = r_pipe[WRITE];
/* parent waits for child process to complete */
wait(&status);
break;
} /* end of switch-statement for command execution */
} /* end of loop for all pipes */
// clear all executed commands
for (int i = 0; i < 20; ++i) {
argv[i] = 0;
}
}
}
void clearArgumentContainer (int argumentContainer[]){
// clear argument container
for (int i = 0; i < 20; ++i) {
argumentContainer[i] = 0;
}
}
Here is the input file I'm using:
Tim 3.5
Todd 2.1
Beth 3.9
Jason 3.5
Zander 3.3
Alex 3.5
Tyler 3.5
Lauren 3.6
Jack 2.3
Amir 3.4
Beth 3.2
pre executable will list only those names with GPAs higher than 3.0 sort executable will sort list of names in alphabetical order
