使用的strtok（），并在后台执行UNIX命令 [英] Use strtok() and execute UNIX command in background

问题描述

我试图写它创建一个UNIX外壳的C程序。 （;指定，当与放大器背景）在当UNIX命令键入这个壳，外壳应在前台或后台执行。我得到在前台运行的命令，但我不能在后台运行。
这里是我的code：

I'm trying to write a C program which creates a UNIX shell. In this shell when a UNIX command is typed, the shell should execute it in the foreground or background (background when & is specified). I'm getting the command to run in the foreground but I can't run it in the background. Here is my code:

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

#define MAX_LENGTH 1024
#define DELIMS " \t\r\n"

int main(int argc, char *argv[])
{
    char *cmd, *bg;
    char line[MAX_LENGTH];
    pid_t fpid,bpid;
    int status;
    while (1)
    {
           fpid=10;
           bpid=10;
            printf("myshell > ");
            if (!fgets(line, MAX_LENGTH, stdin))
                    break;
             int j=0;

            if(cmd = strtok(line, DELIMS))
            {

                    bg = strtok(line," ");
                    while(bg!=NULL)
                    {
                            printf("%s",bg);
                            bg = strtok(NULL, " ");
                            if(strcmp(bg, "&") == 0)
                                    break;
                    }

                    printf("%s", bg);
                    if(strcmp(cmd,"exit")==0)
                            break;

                    else if(strcmp(bg,"&")==0)
                    {
                            bpid=fork();
                            //waitpid(bpid,&status,0);
                            system(line);
                            exit(0);
                    }
                    else
                   {
                            //fpid=fork();
                            //if(fpid==0)
                            //{
                                    system(line);
                            //      exit(0);
                            //}
                            //else 
                            //{
                            //      waitpid(fpid,&status,0);
                            //}
                    }
            }
    }

  return(0);
}

有人可以帮我这个项目？我真的需要这为我的家庭作业。

Can someone please help me in this program? I really need this for my homework.

推荐答案

阅读手册页叉（）。返回0 code意味着你的孩子，非零（非负），意味着你的父。您应该基于具有不同的逻辑，并使用系统（）（或子分支更好的exec *（）

Read the manpage for fork(). The return code of 0 means that you are in the child, non-zero (non-negative) means you are the parent. You should have different logic based on that and use system() (or better exec*() in the child branch.

下面是你应该有典型的逻辑是：

Here's the typical logic you should have:

tokenize(line)

if (last token is '&') {
    rc = fork();

    if (rc < 0)
        handle error;

    else if (rc > 0) {  /* in parent, rc = child pid */
        do whatever you planned to do in the parent process
    }
    else {  /* in child */
        use exec*() to start the child command
    }
}
else {  /* foreground execution */
    use system() to run command
}

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