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目标

• 能处理普通命令
• 能处理内建命令

实现原理

用下面的时间轴来表示时间发生次序。时间从左向右。shell由标识为sh的方块，它随着时间从左向右移动。
shell从用户读入字符串“ls”。shell建立一个新的进程，然后等待进程中运行ls程序并等待进程结束。

然后shell读取新的一行输入，建立一个新的进程在这个进程中运行程序，等待进程结束。
所以写一个shell，需要循环一下过程

1. 获取命令行
2. 解析命令行
3. 建立子进程(fork)
4. 替换子进程(execvp)
5. 父进程等待子进程退出(wait)

源码

#include<iostream>
#include<cstdio>
#include<cstring>
#include<unistd.h>
#include<sys/types.h>
#include<sys/wait.h>
#include<string>#define COMMAND_SIZE 1024
#define FORMAT "[%s@%s %s]# "//下面是shell定义的全局数据
// 1.命令行参数表
#define MAXARGC 128
char *g_argv[MAXARGC];
int g_argc = 0;// 2.环境变量表
#define MAX_ENV 100
char* g_env[MAX_ENV];
int g_envs = 0;char cwd[1024];
char cwdenv[1024];// last code exit
int lastcode = 0;const char* GetUserName()
{const char* name=getenv("USER");return name == NULL ? "None" : name;
}const char* GetHostName()
{const char* host=getenv("HOSTNAME");return host == NULL ? "None" : host;
}const char* GetPwd()
{//const char* pwd=getenv("PWD");const char* pwd = getcwd(cwd,sizeof(cwd));if(pwd != NULL){snprintf(cwdenv,sizeof(cwdenv),"PWD=%s",cwd);putenv(cwdenv);}return pwd == NULL ? "None" : pwd;
}const char* GetHome()
{const char* home=getenv("HOME");return home == NULL ? "None" : home;
}const char* GetOldpwd()
{const char* oldpwd=getenv("OLDPWD");return oldpwd == NULL ? "None" : oldpwd;
}// 处理目录
std::string DirName(const char* pwd)
{
#define SLASH "/"std::string dir = pwd;if(dir == SLASH) return SLASH;auto pos = dir.rfind(SLASH);if(pos == std::string::npos) return pwd;return dir.substr(pos+1);
}void InitEnv()
{extern char** environ;memset(g_env,0,sizeof(g_env));g_envs = 0;//简化操作，从父进程获得环境变量for(int i = 0; environ[i]; i++){g_env[i]=(char*)malloc(strlen(environ[i])+1);strcpy(g_env[i],environ[i]);g_envs++;}g_env[g_envs] = NULL;//2.导成环境变量for(int i = 0; g_env[i]; i++){putenv(g_env[i]);}environ = g_env;
}void MakeCommandLine(char cmd_prompt[], int size)
{snprintf(cmd_prompt, size, FORMAT, GetUserName(), GetHostName(), DirName(GetPwd()).c_str());
}void PrintfCommandPrompt()
{char command[COMMAND_SIZE];MakeCommandLine(command,COMMAND_SIZE);printf("%s",command);fflush(stdout);
}// ls -a -l => "ls -a -l"字符串
bool GetCommandLine(char* out, int size)
{char* c = fgets(out, size, stdin);if(c==NULL) return false;out[strlen(out)-1]=0; // 清理\nif(strlen(out) <= 0) return false;return true;
}// 命令行分析 "ls -a -l" => "ls" "-a" "-l"
bool CommandParse(char* commandline)
{
#define SEP " "g_argc = 0;g_argv[g_argc++] = strtok(commandline, SEP);while((bool)(g_argv[g_argc++] = strtok(nullptr,SEP)));g_argc--;return g_argc > 0 ? true :false;
}// command
bool cd()
{//cdif(g_argc == 1){std::string home = GetHome();if(home.empty()) return true;chdir(home.c_str());}else {std::string where = g_argv[1];//cd ~if(where == "~"){std::string home = GetHome();chdir(home.c_str());}//cd -if(where == "-"){std::string oldpwd = GetOldpwd();chdir(oldpwd.c_str());}chdir(where.c_str());}return true;
}
void PrintEnv()
{extern char** environ;for(int i = 0; environ[i];++i){printf("%-2d->%s\n",i,environ[i]);}
}bool CheckAndExcuteBuildin()
{std::string cmd = g_argv[0];if(cmd == "cd"){cd();return true;}if(cmd == "test"){PrintEnv();return true;}return false;
}void Excute()
{pid_t id = fork();if(id == 0) // child{execvp(g_argv[0], g_argv);exit(1);}// fatherint status = 0;pid_t rid = waitpid(id,&status,0);if(rid > 0){lastcode=WEXITSTATUS(status);        }
}void test_GetCommandLine(char* str)
{printf("%s\n", str);
}
void test_CommandParse()
{for(int i = 0; i < g_argc; ++i)std::cout << g_argv[i] << " " ;
}
void test_InitEnv()
{for(int i = 0; g_env[i]; i++){printf("env[%2d]->%s\n",i,g_env[i]);}printf("envs: %d\n",g_envs);
}int main()
{InitEnv();while(true){// 1. 输出命令行提示符  [jfs@superg-alicloud myshell]$PrintfCommandPrompt();// 2. 获取用户输入的命令 ls -a -lchar commandline[COMMAND_SIZE];if(!GetCommandLine(commandline, sizeof(commandline))) continue;//test_GetCommandLine(commandline);// 3. 命令行分析，"ls -a -l" => "ls" "-a" "-l"if(!CommandParse(commandline))continue;//test_CommandParse();// 4.检测并处理内建命令if(CheckAndExcuteBuildin())continue;// 5. 执行命令Excute();}return 0;
}