Thread: programming unix shell piping problem

Well, the first problem I see is that you fail to close the file descriptors. Next, you redirect ls to STDIN when it should be STDOUT to pipe properly. Something more like this:

Code:

#include <stdio.h> 
#include <unistd.h> 
#include <errno.h> 
#include <sys/types.h> 
#include <sys/wait.h>

enum
{
    STDIN = STDIN_FILENO,
    STDOUT = STDOUT_FILENO,
    STDERR = STDERR_FILENO,
    FD_READ = 0,
    FD_WRITE = 1
};

int main(void)
{
    pid_t pid;
    int fd[2];

    char *lessArg[] =  
    { 
        "/bin/less", 
        NULL 
    }; 
 
    char *lsArg[] = 
    { 
        "/bin/ls", 
        NULL 
    }

    if (pipe(fd) == -1) {
        perror("pipe -- main");
        return 1;
    }

    if ((pid = fork()) == 0) { /* ls is the child */
        if (dup2(fd[FD_WRITE], STDOUT) == -1)
            perror("child dup2 -- main");
        else if (close(fd[FD_READ]) == -1 || close(fd[FD_WRITE) == -1)
            perror("child close -- main");
        else
            execve("/bin/ls", lsArg, envp);
    }

    if (dup2(fd[FD_READ], STDIN) == -1) /* less is the parent */
        perror("parent dup2 -- main");
    else if (close(fd[FD_READ]) == -1 || close(fd[FD_WRITE]) == -1)
        perror("parent close -- main" );
    else
        execve("/bin/less", lessArg, envp);

    return 0;
}

Thx for that prelude it works, but after looking at it there's one problem, the program doesn't fork before executing less.

And now, i think i know the initial problem which is now what i need to solve.

I think i am shut out of the input because less is still running in the background and the parent was what was running. ANd since the parent is waiting of the child to finish i wasn't able to do anything. The input was connected to the parent not the child (less).

So now i need to have control of the child (less) then after less has finished, gain control to the parent.