sh2.c

#include<stdio.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<unistd.h>
#include<sys/wait.h>
#include<string.h>
#include<stdlib.h>

#define buff 1024

void getcommand(int *argc, char *argv[buff])
{
	char tmp;
	int cur = 0, text_flag = 0, space_flag = 0;
	*argc = 0;
    while (1)
	{
		scanf("%c", &tmp);
		//换行命令结束
		if (tmp == '\n')
		{
			if (cur != 0)
			{
				argv[*argc][cur] = '\0';
				(*argc)++;
			}
			break;
		}
		//当前字符不为空格
		if (tmp != ' ')
		{
			if (cur == 0)
			{
				argv[*argc] = (char *)malloc(sizeof(char)*buff);
				memset(argv[*argc], 0, buff);
			}
			text_flag = 1;
			space_flag = 0;
			argv[*argc][cur++] = tmp;
		}
		else
			//读取到空格
			if (!space_flag && text_flag)
			{
				space_flag = 1;
				argv[*argc][cur] = '\0';
				cur = 0;
				(*argc)++;
			}

	}
}

void mysys(int argc, char *argv[])
{
	pid_t pid;
	char tmp[buff],t;
	int err,fd[2],file,flag=0;
	if (argc > 3 && strcmp(argv[argc-2],">")==0)
    {	//判断是否有重定向的情况
		flag=1;
		strcpy(tmp,argv[argc-1]);
	}
	pid=fork();
	if (pid==0)
	{
		if ( argc > 3 && strcmp(argv[argc-2],">")==0)
		{
			pid_t ppid;
			pipe(fd);		//创建管道标识符
			ppid=fork();	//避免修改主进程的标准输入输出端口，需要在子进程中创建子进程用于处理管道的输入输出
			if (ppid==0)
			{
				dup2(fd[1],1);		//将标准输出端口接至管道输入那口
				close(fd[0]);		//关闭标准输入输出端口
				close(fd[1]);
				argv[argc-2]=NULL;
				argv[argc-1]=NULL;
				err=execvp(argv[0], argv);
				if (err<0)
					perror("execvp");
			}
			else
			{
				wait(NULL);
				if (flag)
				{
					dup2(fd[0],0);		//将标准输入端口接至管道输出端口
					close(fd[0]);		//关闭标准输入输出端口
					close(fd[1]);
					file=open(tmp,O_RDWR|O_CREAT,0666);
					if (file<0)
					{
						printf("cannot open file!\n");
						exit(0);
					}
					while(1)
					{	//从管道中逐个读取字符，输出至目标文件
						if (!read(0,&t,sizeof(char)))
							break;
						//if (&t==NULL)
						//   break;
						write(file,&t,sizeof(char));
					}
				}
			}
            return;
		}
		else
		{
			err=execvp(argv[0], argv);
			if (err<0)
				perror("execvp");
		}
	}
	else
	{
		wait(NULL);
	}	
	return;	
}

int main()
{
	int i, argc;
	char pwd[buff], *argv[buff];
	FILE *shell;
	while (1)
	{
        argc=0;
		//输出每行前的表示符
		shell=popen("/bin/pwd","r");
		fscanf(shell,"%s",pwd);
		pclose(shell);
		printf("$%s>",pwd);		
		//用户输入命令
		getcommand(&argc, argv);
        //用户未输入
		if (argc==0)
			continue;		
		//输入exit指令 退出当前程序
		if (strcmp(argv[0],"exit")==0)
			return 0;
		if (strcmp(argv[0],"cd")==0)
		{	
			if (chdir(argv[1]))
				printf("no such direction : %s\n",argv[1]);
		}	
		else
			if (strcmp(argv[0],"pwd")==0)
			{
				char ppwd[buff];
        		printf("%s\n",getcwd(ppwd,sizeof(ppwd)));
			}
			else
			{
				mysys(argc,argv);
			}
		//for (i=0;i<argc;i++)
        //    argv[i]=NULL;
		memset(argv,0,sizeof(argv));
  	}
	return 0;
}