How do I use dup2 to perform the following command?
ls -al | grep alpha | more
You would use pipe(2,3p)
as well. Create the pipe, fork, duplicate the appropriate end of the pipe onto FD 0 or FD 1 of the child, then exec.
A Little example with the first two commands. You need to create a pipe with the pipe() function that will go between ls and grep and other pipe between grep and more. What dup2 does is copy a file descriptor into another. Pipe works by connecting the input in fd[0] to the output of fd[1]. You should read the man pages of pipe and dup2. I may try and simplify the example later if you have some other doubts.
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#define READ_END 0
#define WRITE_END 1
int
main(int argc, char* argv[])
{
pid_t pid;
int fd[2];
pipe(fd);
pid = fork();
if(pid==0)
{
printf("i'm the child used for ls \n");
dup2(fd[WRITE_END], STDOUT_FILENO);
close(fd[WRITE_END]);
execlp("ls", "ls", "-al", NULL);
}
else
{
pid=fork();
if(pid==0)
{
printf("i'm in the second child, which will be used to run grep\n");
dup2(fd[READ_END], STDIN_FILENO);
close(fd[READ_END]);
execlp("grep", "grep", "alpha",NULL);
}
}
return 0;
}