#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <dirent.h>
#define normal 0        //一般的命令
#define out_redirect 1  //输出重定向
#define in_redirect 2   //输入重定向
#define have_pipe 3     //命令中有管道void print_prompt();         //打印提示符
void get_input(char*);       //得到输入的命令
void explain_input(char*,int*,char a[][256]);   //对输入命令进行解析
void do_cmd(int,char a[][256]); //执行命令
int find_command(char*);     //查找命令中的可执行程序int main(int argc,char *argv[])
{int i;int argcount= 0;char arglist[100][256];char *buf= NULL;char **arg= NULL;buf= (char*)malloc(256);if(buf == NULL){perror("malloc failed");exit(-1);}while(1){memset(buf,0,256);     //将buf所指向的空间清零
        print_prompt();get_input(buf);//若输入的命令为exit或logout则退出本程序if(strcmp(buf,"exit\n")== 0||strcmp(buf,"logout\n")== 0)break;for(i= 0;i< 100;i++){arglist[i][0]= '\0';}argcount= 0;explain_input(buf,&argcount,arglist);do_cmd(argcount,arglist);}if(buf!= NULL){free(buf);buf= NULL;}return 0;
}void print_prompt()
{printf("myshell$ ");
}//获取用户输入
void get_input(char *buf)
{int len= 0;int ch;ch= getchar();while(len< 256&&ch!= '\n'){buf[len++]= ch;ch= getchar();}if(len== 256){printf("command is too long\n");exit(-1);       //输入的命令过长则退出程序
    }buf[len]= '\n';len++;buf[len]= '\0';
}/*解析buf中的命令，将结果存入arglist中，命令以回车符号\n结束，如输入命令为*"ls -l /tmp"，则arglist[0]、arglist[1]、arglist[2]分别为ls、-l、/tmp */
void explain_input(char *buf,int *argcount,char arglist[100][256])
{char *p= buf;char *q= buf;int number= 0;while(1){if(p[0]== '\n')break;if(p[0]== ' ')p++;else{q = p;number= 0;while((q[0]!= ' ')&&(q[0]!= '\n')){number++;q++;}strncpy(arglist[*argcount],p,number+ 1);arglist[*argcount][number]= '\0';*argcount= *argcount+ 1;p = q;}}
}void do_cmd(int argcount,char arglist[100][256])
{int flag= 0;int how= 0;            //用于指示命令中是否含有>、<、|int background= 0;     //标识命令中是否有后台运行标识符&int status;int i;int fd;char* arg[argcount+ 1];char* argnext[argcount+ 1];char* file;pid_t pid;//将命令取出for(i= 0;i< argcount;i++){arg[i]= (char*)arglist[i];}arg[argcount]= NULL;//查看命令行是否有后台运行符for(i= 0;i< argcount;i++){if(strncmp(arg[i],"&",1)== 0){if(i== argcount-1){background= 1;arg[argcount-1]= NULL;break;}else{printf("wrong command\n");return;}}}for(i= 0;arg[i]!= NULL;i++){if(strcmp(arg[i],">")== 0){flag++;how= out_redirect;if(arg[i+ 1]== NULL)flag++;}if(strcmp(arg[i],"<")== 0){flag++;how= in_redirect;if(i== 0)flag++;}if(strcmp(arg[i],"|")== 0){flag++;how= have_pipe;if(arg[i+ 1]== NULL)flag++;if(i== 0)flag++;}}/* flag大于1，说明命令中含有多个>，<，|符号，本程序是不支持这样的命令的，或者命令格式不对，如"ls -l /tmp >" */if(flag> 1){printf("wrong command\n");return;}if(how== out_redirect){//命令只含有一个输出重定向符号for(i= 0;arg[i]!= NULL;i++){if(strcmp(arg[i],">")== 0){file= arg[i+ 1];arg[i]= NULL;}}}if(how== in_redirect){//命令中只含有一个输入重定向符号for(i= 0;arg[i]!= NULL;i++){if(strcmp(arg[i],"<")== 0){file= arg[i+ 1];arg[i]= NULL;}}}if(how== have_pipe){/*命令只含有一个管道符号|,把管道符号后面的部分存入argnext中，管道后面的部分是一个可执行的shell命令 */for(i= 0;arg[i]!= NULL;i++){if(strcmp(arg[i],"|")== 0){arg[i]= NULL;int j;for(j= i+ 1;arg[j]!= NULL;j++){argnext[j-i-1]= arg[j];}argnext[j-i-1]= arg[j];break;}}}if((pid= fork())< 0){printf("fork error\n");return;}switch(how){case 0:/*pid为0说明是子进程，在子进程中执行输入的命令，输入的命令不含>、<、和| */if(pid== 0){if(!(find_command(arg[0]))){printf("%s : command not found\n",arg[0]);exit(0);}execvp(arg[0],arg);exit(0);}break;case 1://输入的命令中含有输出重定向符>if(pid== 0){if(!(find_command(arg[0]))){printf("%s : command not found\n",arg[0]);exit(0);}fd= open(file,O_RDWR|O_CREAT|O_TRUNC,0644);dup2(fd,1);execvp(arg[0],arg);exit(0);}break;case 2://输入的命令中含有输入重定向符<if(pid== 0){if(!(find_command(arg[0]))){printf("%s : command not found\n",arg[0]);exit(0);}fd= open(file,O_RDONLY);dup2(fd,0);execvp(arg[0],arg);exit(0);}break;case 3://输入的命令中含有管道符|if(pid== 0){int pid2;int status2;int fd2;if((pid2= fork())< 0){printf("fork2 error\n");return;}else if(pid2== 0){if(!(find_command(arg[0]))){printf("%s : command not found\n",arg[0]);exit(0);}fd2= open("/tmp/youdonotknowfile",O_WRONLY|O_CREAT|O_TRUNC,0644);dup2(fd2,1);execvp(arg[0],arg);exit(0);}if(waitpid(pid2,&status2,0)== -1)printf("wait for child process error\n");if(!(find_command(argnext[0]))){printf("%s : command not found\n",argnext[0]);exit(0);}fd2= open("/tmp/youdonotknowfile",O_RDONLY);dup2(fd2,0);execvp(argnext[0],argnext);if(remove("/tmp/youdonotknowfile"))printf("remove error\n");exit(0);}break;default:break;}//若命令中有&，表示后台执行，父进程直接返回，不等待子进程结束if(background== 1){printf("[process id %d]\n",pid);return;}//父进程等待子进程结束if(waitpid(pid,&status,0)== -1)printf("wait for child process error\n");
}//查找命令中的可执行程序
int find_command(char *command)
{DIR *dp;struct dirent *dirp;char *path[]= {"./","/bin","/usr/bin",NULL};//使当前目录下的程序可以运行，如命令"./fork"可以被正确解释和执行if(strncmp(command,"./",2)== 0)command= command+ 2;//分别在当前目录、/bin和/usr/bin目录查找要执行的程序int i= 0;while(path[i]!= NULL){if((dp= opendir(path[i]))== NULL)printf("can not open /bin \n");while((dirp= readdir(dp))!= NULL){if(strcmp(dirp->d_name,command)== 0){closedir(dp);return 1;}}closedir(dp);i++;}return 0;
}