转换cat file.txt | wc -l程序代码c

Question

I'm very newbie about pipes and I want develop a little program for understand and learn about this. My idea consist to communicate command shell cat to wc using c. I was doing a very simple program that use an exiting file (test.txt for example) but for the moment I can only display the content. I only want count the number of lines about 1 specific file.

Is this possible to implement? Or maybe I must do another option? Here my basic code:

int main(int argc, char *argv[]) {
    pid_t pid;
    int fd[2];

    pipe(fd);
    pid = fork();

    if (pid == -1) {
        perror("fork");
        exit(1);    
    }

    if (pid == 0) {
        /* Child process closes up input side of pipe */
        close(fd[0]);
        execlp("cat", "cat", "test.txt", NULL);
        //I don't know how communicate this process with the other process
    } else {
        /* Parent process closes up output side of pipe */
        close(fd[1]);
        execlp("wc", "wc", "-l", NULL);
    }
}

Answer 1

You must redirect the appropriate end of the pipe to standard input and/or standard output before the call to execlp() . If this call succeeds, the current process has been replaced with the new one, no further code is executed, but if it fails, you should complain with a perror() .

Here is a corrected version of the code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main (int argc, char *argv[]) {
    pid_t pid;
    int fd[2];

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

    pid = fork();
    if (pid == -1) {
        perror("fork");
        return 1;
    }

    if (pid == 0) {
        /* Child process redirects its output to the pipe */
        dup2(fd[1], 1);
        close(fd[0]);
        close(fd[1]);
        execlp("cat", "cat", "test.txt", NULL);
        perror("exec cat");
        return 1;
    } else {
        /* Parent process redirects its input from the pipe */
        dup2(fd[0], 0);
        close(fd[0]);
        close(fd[1]);
        execlp("wc", "wc", "-l", NULL);
        perror("exec wc");
        return 1;
    }
}

Answer 2

if you only care about the number of lines in the file you can just run the entire command using popen and then read the output for output or any error

e.g. fd = popen("cat test.txt | wc -l", "r");

then use read method to read the output. you can also use return value from pclose(fd) to check if process completed successfully.