将 n 个命令与 shell 中的管道连接起来?

问题描述：

我正在尝试用 C 实现一个 shell.我可以用一个简单的 execvp() 执行简单的命令，但要求之一是管理这样的命令:ls -l | head | tail -4"一个for"循环和只有一个pipe()"语句重定向标准输入和标准输出.现在过了几天，我有点迷失了.

I am trying to implement a shell in C. I can execute simple commands just fine with a simple execvp() but one of the requirements is to manage commands like this: "ls -l | head | tail -4" with a 'for' loop and only one 'pipe()' statement redirecting stdin and stdout. Now after days I'm a bit lost.

N = 简单命令的数量(示例中为 3:ls、head、tail)命令 = 带有命令的结构列表，如下所示:

N = Number of simple commands (3 in the example: ls, head, tail) commands = a list of structs with the commands, like this:

commands[0].argv[0]: ls
commands[0].argv[1]: -l
commands[1].argv[0]: head
commands[2].argv[0]: tail
commands[2].argv[1]: -4

所以，我创建了 for 循环，并开始重定向 stdin 和 stdout，以便将所有命令与管道连接起来，但是......我只是不知道为什么它不起作用.

So, I made the for loop, and started to redirect stdin and stdout in order to connect all the commands with pipes, but...I'm just clueless why it doesn't work.

for (i=0; i < n; i++){

pipe(pipe);
if(fork()==0){  // CHILD

    close(pipe[0]);
    close(1);
    dup(pipe[1]);
    close(pipe[1]);

    execvp(commands[i].argv[0], &commands[i].argv[0]);
    perror("ERROR: ");
    exit(-1);

}else{      // FATHER

    close(pipe[1]);
    close(0);
    dup(pipe[0]);
    close(pipe[0]);

}
}

我想要创建的是子进程的行":

What I want to create is a 'line' of childed processes:

[ls -l] ----pipe----> [head] ----pipe----> [tail -4]

[ls -l] ----pipe----> [head] ----pipe----> [tail -4]

所有这些进程都有一个root(运行我的shell的进程)所以，第一个父亲也是shell进程的孩子，我已经有点累了，有人可以帮我吗?

All this processes have a root (the process runing my shell) so, the first father is also a child of the shell process, I'm a bit exhausted already, can anyone help me here please?

我什至不确定孩子是否应该执行命令.

I'm not even sure if the childs should be the ones executing the commands.

谢谢你们！！

答

这里没什么复杂的，只要记住最后一个命令应该输出到原始进程的文件描述符 1，第一个应该从原始进程的文件描述符 0 读取. 您只需按顺序生成进程，并沿用前一个 pipe 调用的输入端.

Nothing complex here, just have in mind that the last command should output to the original process' file descriptor 1 and the first should read from original process file descriptor 0. You just spawn the processes in order, carrying along the input side of the previous pipe call.

所以，这里是类型:

#include <unistd.h>

struct command
{
  const char **argv;
};

使用简单的定义良好的语义制作辅助函数:

Make a helper function with a simple well defined semantics:

int
spawn_proc (int in, int out, struct command *cmd)
{
  pid_t pid;

  if ((pid = fork ()) == 0)
    {
      if (in != 0)
        {
          dup2 (in, 0);
          close (in);
        }

      if (out != 1)
        {
          dup2 (out, 1);
          close (out);
        }

      return execvp (cmd->argv [0], (char * const *)cmd->argv);
    }

  return pid;
}

这里是主要的 fork 例程:

And here's the main fork routine:

int
fork_pipes (int n, struct command *cmd)
{
  int i;
  pid_t pid;
  int in, fd [2];

  /* The first process should get its input from the original file descriptor 0.  */
  in = 0;

  /* Note the loop bound, we spawn here all, but the last stage of the pipeline.  */
  for (i = 0; i < n - 1; ++i)
    {
      pipe (fd);

      /* f [1] is the write end of the pipe, we carry `in` from the prev iteration.  */
      spawn_proc (in, fd [1], cmd + i);

      /* No need for the write end of the pipe, the child will write here.  */
      close (fd [1]);

      /* Keep the read end of the pipe, the next child will read from there.  */
      in = fd [0];
    }

  /* Last stage of the pipeline - set stdin be the read end of the previous pipe
     and output to the original file descriptor 1. */  
  if (in != 0)
    dup2 (in, 0);

  /* Execute the last stage with the current process. */
  return execvp (cmd [i].argv [0], (char * const *)cmd [i].argv);
}

还有一个小测试:

int
main ()
{
  const char *ls[] = { "ls", "-l", 0 };
  const char *awk[] = { "awk", "{print $1}", 0 };
  const char *sort[] = { "sort", 0 };
  const char *uniq[] = { "uniq", 0 };

  struct command cmd [] = { {ls}, {awk}, {sort}, {uniq} };

  return fork_pipes (4, cmd);
}

似乎可以工作.:)

将 n 个命令与 shell 中的管道连接起来?

相关推荐