18.c

/*
============================================================================
Name : 18
Author : Piyush Singh
Description : Write a program to find out total number of directories on the pwd.
execute ls -l | grep ^d | wc ? Use only dup2.
Date: 19th Sep, 2025.
============================================================================
*/

#include <stdio.h>      // For perror()
#include <stdlib.h>     // For exit()
#include <unistd.h>     // For fork(), pipe(), execlp(), dup2()

int main() 
{
    int pipe1[2], pipe2[2];

    // Step 1: Create first pipe for ls -l → grep ^d
    // pipe1[1] will carry output of ls -l to grep
    pipe(pipe1);

    // Step 2: Create second pipe for grep ^d → wc
    // pipe2[1] will carry output of grep to wc
    pipe(pipe2);

    // Step 3: First child process runs ls -l
    pid_t pid1 = fork();
    if (pid1 == 0) 
    {
        // Redirect stdout to pipe1's write end
        dup2(pipe1[1], STDOUT_FILENO);  // STDOUT_FILENO = 1

        // Close unused pipe ends
        close(pipe1[0]);  // read end not used by ls
        close(pipe1[1]);  // already duplicated
        close(pipe2[0]); close(pipe2[1]);  // not used in this process

        // Execute ls -l
        execlp("ls", "ls", "-l", NULL);

        // If execlp fails
        perror("❌ ls failed");
        exit(1);
    }

    // Step 4: Second child process runs grep ^d
    pid_t pid2 = fork();
    if (pid2 == 0) 
    {
        // Redirect stdin to pipe1's read end
        dup2(pipe1[0], STDIN_FILENO);   // STDIN_FILENO = 0

        // Redirect stdout to pipe2's write end
        dup2(pipe2[1], STDOUT_FILENO);  // STDOUT_FILENO = 1

        // Close unused pipe ends
        close(pipe1[0]); close(pipe1[1]);  // already duplicated
        close(pipe2[0]); close(pipe2[1]);  // already duplicated

        // Execute grep ^d
        execlp("grep", "grep", "^d", NULL);

        // If execlp fails
        perror("❌ grep failed");
        exit(1);
    }

    // Step 5: Parent process runs wc
    // Redirect stdin to pipe2's read end
    dup2(pipe2[0], STDIN_FILENO);       // STDIN_FILENO = 0

    // Close unused pipe ends
    close(pipe1[0]); close(pipe1[1]);   // not used in parent
    close(pipe2[0]); close(pipe2[1]);   // already duplicated

    // Execute wc
    execlp("wc", "wc", NULL);

    // If execlp fails
    perror("❌ wc failed");
    exit(1);
}

/*
Output:
╰─ ./18                                                                       ─╯
       1       9      66
*/