Creating multiple pipe in C - c

Question

I'm writing a simple shell and I'd like to change my program to add the possibility of multiple pipe commands like "echo foo | cat | cat | cat | cat | wc". I have written for two commands but for multiple i can't.
Here is the source code of my program:
if (pid == 0) // in the child process
{
for (i = 0; i < command; i++) // for each cmd
{
if (argv[i][0] == '|')
{
j = i;
}
}
if (j > 0)
{
if (pipe(p))
{
fprintf(stderr, "pipe");
exit(1);
}
argv[j] = NULL;
if (fork() == 0) // child
{
j = -1;
close(p[0]);
dup2(p[1],1);
close(p[1]);
}
// parent
close(p[1]);
dup2(p[0], 0);
close(p[0]);
}
for (i = 0; dirs[i] != 0; i++)
{
snprintf(pathname, sizeof(pathname), "%s/%s", dirs[i], argv[j+1]);
execv(pathname, &argv[j+1]);
}
}
else
{
while (wait(0) != pid) // parent: wait child
}
Thank you in advance for help.

Answer 1

I come with an example of what you are trying to do. I use constants as commands, I leave the command line parsing to you.
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <sys/wait.h>
static char *my_command0[] = {"cat", "stackoverflow.c", NULL};
static char *my_command1[] = {"grep", "return", NULL};
static char *my_command2[] = {"sed", "s/^ *//g", NULL};
static char **const my_commands[] = {
my_command0,
my_command1,
my_command2,
NULL
};
int create_sub_process(char *const command[], int input_stream)
{
int pipefd[2] = {-1, -1};
pid_t fk;
if (pipe(pipefd) < 0)
{
perror("pipe");
close(input_stream);
return -1;
}
if ((fk = fork()) < 0)
{
perror("fork");
close(pipefd[0]);
close(pipefd[1]);
close(input_stream);
return -1;
}
if (fk == 0)
{
close(pipefd[0]);
close(0);
dup(input_stream);
close(input_stream);
close(1);
dup(pipefd[1]);
close(pipefd[1]);
execvp(command[0], command);
perror("execvp");
exit(1);
}
close(input_stream);
close(pipefd[1]);
return pipefd[0];
}
int main()
{
int fd = dup(0);
for (int i = 0; my_commands[i] != NULL; i++)
{
fd = create_sub_process(my_commands[i], fd); // replace my_commands[i] by whatever you need to execute - check: man execvp
if (fd < 0)
{
exit(1);
}
}
// Also adapt the following lines to whatever you want to do with last child results
close(0);
dup(fd);
close(fd);
execlp("cat", "cat", (char *)NULL);
perror("execlp");
return 1;
}
create_sub_process() creates a pipe and creates a sub process to execute given command, taking inputs from given input stream and sending output to the stream it returns to parent.