执行一个命令，将其输出截获的代码－使用无名管道

使用popen库函数的版本：

    #include <unistd.h>

    #include <stdio.h>

    #include <stdlib.h>

    #include <fcntl.h>

    #include <limits.h>



    #define BUFSZ PIPE_BUF



    void err_quit(char *msg);



    int main(int argc, char *argv[])

    {

        FILE *fp;  // FILE stream for popen

        char *cmdstring = "ps -aux|grep root";

        char buf[BUFSZ];



        // create the pipe

        if ((fp = popen(cmdstring, "r")) == NULL)

            err_quit("popen");



        // read the cmdstring's output

        while ((fgets(buf, BUFSZ, fp)) != NULL)

            printf("%s", buf);



        // close and frap the exit status

        pclose(fp);

        exit(EXIT_SUCCESS);

    }



    void err_quit(char *msg)

    {

        perror(msg);

        exit(EXIT_FAILURE);

    }

不使用库函数，自己实现的版本（先fork，然后修改子进程的STDOUT文件描述符为管道，然后再exec），这个思路来自Linux Shells by Examples一书，因为BASH执行命令就是酱紫的：

    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/wait.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <fcntl.h>
    #include <limits.h>

    #define BUFSZ PIPE_BUF

    void err_quit(char *msg);

    int main(int argc, char *argv[])
    {
        int fd[2]; // File descriptor array for the pipe
        char buf[BUFSZ];
        char *cmdstring[] = {"/bin/ps", "-aux", NULL};
        int pid, len;

        // create the pipe
        if ((pipe(fd)) < 0)
            err_quit("pipe");

        // fork and close the appropriate descriptors
        if ((pid = fork()) < 0)
            err_quit("fork");
        if (pid == 0) {
            // child will invoke the cmdstring, so close stdout, dup pipewriter
            close(STDOUT_FILENO);
            dup(fd[1]);
            close(fd[0]);
            // invoke the cmdstring
            if (execve(cmdstring[0], cmdstring, NULL) == -1)
                err_quit("execve");
        } else {
            // parent process
            close(fd[1]);

            while ((len = read(fd[0], buf, BUFSZ)) > 0)
                printf("%s", buf);
            close(fd[0]);

            waitpid(pid, NULL, 0);

            exit(EXIT_SUCCESS);
        }
    }

    void err_quit(char *msg)
    {
        perror(msg);
        exit(EXIT_FAILURE);
    }