Sending Output of One Command (execv) to Another

Question

void execute_command_pipe(char * command_from, char * command_to, char ** args_from, char ** args_to) {
    pipe(pipefd);

    int pid = fork();
    close(pipefd[0]);
    if (pid == 0) {
        //close(STDOUT_FILENO);
        dup2(pipefd[1], STDOUT_FILENO);
        int rv1 = execv(get_contain_dir(command_from), args_from);
        close(pipefd[1]);
    } else {
        close(pipefd[1]);
        dup2(pipefd[0], STDIN_FILENO);
        int rv2 = execv(get_contain_dir(command_to), args_to);
        close(pipefd[0]);
    }
}

for example, if I wanted to do the equivalent of ls | grep test, the parent thread would run grep listening for input on STDIN, and the child thread would write the output of ls to STDTOUT.

Answer 1

Is using low-level pipe/fork is necessary? If no - more easy way - to use popen/pclose system calls.

For your example with ls | grep, this is:

FILE *f = popen("ls");
char buf[1000];
while(fgets(buf, sizeof(buf), f) 
  call_my_grep(buf);
pclose(f);

This is easy and efficient.

Answer 2

void execute_command_pipe(char * command_from, char * command_to, char ** args_from, char ** args_to) {
    pipe(pipefd);

    int pid = fork();
    if (pid != 0) {
        dup2(pipefd[0], STDIN_FILENO);
        close(pipefd[0]);
        int rv2 = execv(get_contain_dir(command_to), args_to);
    } else {
        dup2(pipefd[1], STDOUT_FILENO);
        close(pipefd[1]);
        int rv1 = execv(get_contain_dir(command_from), args_from);
        close(pipefd[0]);
    }
}