shell本质就是一个进程，它提供了一个用户界面，用于接收用户输入的命令，并将这些命令解释成操作系统能够理解和执行的操作。它充当了用户和操作系统内核之间的中介。例如，在 Linux 系统中，当用户在终端输入ls命令时，shell 会解释这个命令，告诉操作系统去列出当前目录下的文件和目录信息。

下面是模拟实现shell的基本代码：

#include <iostream>
#include <string>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
using namespace std;const int basesize = 1024;
const int argvnum = 64;
const int envnum = 64;//全局的命令行参数表
char *gargv[argvnum];
int gargc = 0;//我自己的环境变量
char *genv[envnum];//全局变量,用来表示退出结果
int lastcode = 0;// 全局的工作路径
char pwd[basesize];
char pwdenv[basesize * 2];string GetName()
{string name = getenv("USER");return name.empty() ? "None" : name;
}string GetHostName()
{char hostname[basesize];gethostname(hostname, sizeof(hostname));;return gethostname(hostname, sizeof(hostname)) != 0 ? "None" : hostname;
}string GetPwd()
{// getcwd获取当前工作路径if (nullptr == getcwd(pwd, sizeof(pwd)))return "None";// 讲获取的当前路径输入到pwdenvsnprintf(pwdenv, sizeof(pwdenv), "PWD=%s", pwd);// 导入环境变量putenv(pwdenv);return pwd;// string pwd = getenv("PWD");// return pwd.empty() ? "None" : pwd;
}string LastDir()
{string curr = GetPwd();if(curr == "/" || curr == "None") return curr;// /home/xzl/xxxsize_t pos = curr.rfind("/");if(pos == std::string::npos) return curr;return curr.substr(pos+1);
}string MakeCommandLine()
{char Command_Line[basesize];snprintf(Command_Line, basesize, "[%s@%s %s]#",\GetName().c_str(), GetHostName().c_str(), LastDir().c_str());return Command_Line;
}// 打印命令行提示符
void PrintCommandLine()
{printf("%s", MakeCommandLine().c_str());fflush(stdout);
}
// 获取用户命令
bool GetCommandLine(char command_buffer[])
{// 获取字符串char *result = fgets(command_buffer, basesize, stdin);if (!result){return false;}command_buffer[strlen(command_buffer) - 1] = 0;if (strlen(command_buffer) == 0)return false;return true;
}//  分析命令
void ParseCommandLine(char command_buffer[])
{memset(gargv, 0, sizeof(gargc));gargc = 0;const char *seq = " ";gargv[gargc++] = strtok(command_buffer, seq);while (gargv[gargc++] = strtok(nullptr, seq));gargc--;
}void debug()
{printf("argc: %d\n", gargc);for (int i = 0; gargv[i]; i++){printf("argv[%d]: %s\n", i, gargv[i]);}
}//执行命令
bool ExecuteCommand()
{pid_t id = fork();if (id < 0)return false;else if (id == 0){// 子进程//执行命令execvpe(gargv[0], gargv,genv);//退出exit(1);}int status = 0;pid_t rid = waitpid(id, &status, 0);if (rid > 0){if(WIFEXITED(status)){lastcode = WEXITSTATUS(status);}else//表示代码异常退出{lastcode = 100;}return true;}return false;
}void AddEnv(const char *item)
{int index = 0;while(genv[index]){index++;}genv[index] = (char*)malloc(strlen(item)+1);strncpy(genv[index],item,strlen(item)+1);index++;genv[index] = nullptr;
}//在shell中
//有些命令，必须由子进程执行
//有些命令，不能由子进程执行，要由shell自己执行 -----内建命令 built command
bool CheckAndExecBuiltCommand()
{//不能让子进程进行，因为子进程退出就结束了，并不能影响下一个进程的工作路径if (strcmp(gargv[0], "cd") == 0){if (gargc == 2){chdir(gargv[1]);lastcode = 0;}else{lastcode = 1;}return true;}else if(strcmp(gargv[0],"export")==0){if(gargc == 2){AddEnv(gargv[1]);lastcode = 0;}else{lastcode = 2;}}else if(strcmp(gargv[0],"env") == 0){for(int i = 0 ;genv[i];i++){printf("%s\n",genv[i]);}lastcode = 0;return true;}else if(strcmp(gargv[0],"echo") == 0){if(gargc == 2){//echo $?//echo $PATH//echo helloif(gargv[1][0] == '$'){if(gargv[1][1] == '?'){printf("%d\n",lastcode);lastcode = 0;}}else{printf("%s\n",gargv[1]);lastcode = 0;}}else{lastcode = 3;}return true;}return false;
}//作为一个shell，获取环境变量应该从系统环境变量获取
//今天外面做不到就直接从父进程shell中获取环境变量
void InitEnv()
{extern char **environ;int index = 0;while (environ[index]){genv[index] = (char*)malloc(strlen(environ[index])+1);strncpy(genv[index],environ[index],strlen(environ[index]));index++;}genv[index] = nullptr;}int main()
{//初始化环境变量表InitEnv();char command_buffer[basesize];while (true){// 打印命令行提示符PrintCommandLine();// 获取用户命令if (!GetCommandLine(command_buffer)){continue;}// printf("%s\n", command_buffer);//  分析命令ParseCommandLine(command_buffer);// 判断是不是内建命令if (CheckAndExecBuiltCommand()){continue;}// debug();// 执行命令ExecuteCommand();}return 0;
}