学个damn！

给我狠狠玩好嘛别卷我啊

知道的是图书馆，不知道的还以为开演唱会呢！

八点啊，那可是八点，才八点！

不是哥们下午两点半开馆即满人啊我靠？

只能去儿童区学我说不是？

别拷打我了我真碎了哥

说说自主shell编写吧（要不然进程控制不白学了）

自主shell首先得有打印命令行提示符吧，命令行提示符构成是这样的：用户名+主机名+当前所处路径，我们都封装成函数：

const char* GetUserName() { 	const char* name = getenv("USER"); 	if (name == NULL) 	{ 		return "None"; 	} 	return name; }  const char* GetHostName() { 	const char* hostname = getenv("HOSTNAME"); 	if (hostname == NULL) 	{ 		return "None"; 	} 	return hostname; }  const char* GetCwd() { 	const char* cwd = getenv("PWD"); 	if (cwd == NULL) 	{ 		return "None"; 	} 	return cwd; }

分别获取到了对应的字符串，我们还需要一个函数把这些字符串拼接起来，这个功德圆满的函数就是：sprintf！

sprintf是个怎样的函数呢？

sprintf是一个可以向内存空间写入的printf

而snprintf是给定大小的更安全的向内存空间中写入的printf（写到缓冲区）

void MakeCommandLine(char line[], size_t size) { 	const char* username = GetUserName(); 	const char* hostname = GetHostName(); 	const char* cwd = GetCwd(); 	snprintf(line, size, "[%s@%s %s]#", username, hostname, cwd); 	fflush(stdout); }

搞完哩， ，，

命令行已经输出了，我们需要获取用户的命令 

而像ls -a -l -i这种命令本质上是字符串嘞，不能直接用scanf获取，因为scanf空格作为分割，但是我们输入命令是不知道有多少空格的输入，按照行获取有木有？

有哇，fgets啊

为何偏偏是它 

 乐意，，，

#include #include #include #include #include  #define SIZE 512  const char* GetUserName() { 	const char* name = getenv("USER"); 	if (name == NULL) 	{ 		return "None"; 	} 	return name; }  const char* GetHostName() { 	const char* hostname = getenv("HOSTNAME"); 	if (hostname == NULL) 	{ 		return "None"; 	} 	return hostname; }  const char* GetCwd() { 	const char* cwd = getenv("PWD"); 	if (cwd == NULL) 	{ 		return "None"; 	} 	return cwd; }  void MakeCommandLine() { 	char line[SIZE]; 	const char* username = GetUserName(); 	const char* hostname = GetHostName(); 	const char* cwd = GetCwd(); 	snprintf(line, sizeof(line), "[%s@%s %s]#", username, hostname, cwd); 	printf("%s", line); 	fflush(stdout); }  int main() { 	MakeCommandLine();  	char usercommand[SIZE]; 	char* s = fgets(usercommand, sizeof(usercommand), stdin); 	if (s == NULL) 	{ 		return 1; 	} 	printf("echo:%s\n", usercommand); 	return 0; }

为什么这多输出了一个空行（因为多有了一个\n啊）

为了剔除这个\n，可以这样干：

#include #include #include #include #include #include  #define SIZE 512 #define ZERO '\0'  const char* GetUserName() { 	const char* name = getenv("USER"); 	if (name == NULL) 	{ 		return "None"; 	} 	return name; }  const char* GetHostName() { 	const char* hostname = getenv("HOSTNAME"); 	if (hostname == NULL) 	{ 		return "None"; 	} 	return hostname; }  const char* GetCwd() { 	const char* cwd = getenv("PWD"); 	if (cwd == NULL) 	{ 		return "None"; 	} 	return cwd; }  void MakeCommandLine() { 	char line[SIZE]; 	const char* username = GetUserName(); 	const char* hostname = GetHostName(); 	const char* cwd = GetCwd(); 	snprintf(line, sizeof(line), "[%s@%s %s]#", username, hostname, cwd); 	printf("%s", line); 	fflush(stdout); }  int main() { 	MakeCommandLine();  	char usercommand[SIZE]; 	char* s = fgets(usercommand, sizeof(usercommand), stdin); 	if (s == NULL) 	{ 		return 1; 	} 	usercommand[strlen(usercommand) - 1] = ZERO; 	printf("echo:%s\n", usercommand); 	return 0; }

 写成接口就是：

int GetUserCommand(char command[], size_t n) { 	char* s = fgets(command, n, stdin); 	if (s == NULL) 	{ 		return -1; 	} 	command[strlen(command) - 1] = ZERO; 	return strlen(command); }

 接下来需要做的是命令行字符串分割

我们无需再进行手搓函数，有个函数已经提供了分割的功能了：strtok

字符串函数们的使用和模拟实现-CSDN博客https://blog.csdn.net/chestnut_orenge/article/details/135351316 这样赋值，先判断，分割后strtok会返回NULL，刚好让gArgv的最后一个元素是NULL，并且while判断结束

void splitCommand(char command[], size_t n) { 	gArgv[0] = strtok(command, SEP); 	int index = 1; 	while ((gArgv[index++] = strtok(NULL, SEP))); }

命令行字符串分割后要执行相应的命令，但是我们不能让该程序执行（代码替换完之后就无了），所以要创建紫禁城帮助我们执行相应的命令

之前学过exec相关函数，我们所获得的命令串都是系统中的，所以默认去环境变量中寻找即可（带p的），又因为获取的是命令串，所以要带v的（容器）

现在大致是这样的：

#include #include #include #include #include #include #include  #define SIZE 512 #define ZERO '\0' #define SEP " " #define NUM 32  void Die() {  }  const char* GetUserName() { 	const char* name = getenv("USER"); 	if (name == NULL) 	{ 		return "None"; 	} 	return name; }  const char* GetHostName() { 	const char* hostname = getenv("HOSTNAME"); 	if (hostname == NULL) 	{ 		return "None"; 	} 	return hostname; }  const char* GetCwd() { 	const char* cwd = getenv("PWD"); 	if (cwd == NULL) 	{ 		return "None"; 	} 	return cwd; }  void MakeCommandLine() { 	char line[SIZE]; 	const char* username = GetUserName(); 	const char* hostname = GetHostName(); 	const char* cwd = GetCwd(); 	snprintf(line, sizeof(line), "[%s@%s %s]#", username, hostname, cwd); 	printf("%s", line); 	fflush(stdout); }  int GetUserCommand(char command[], size_t n) { 	char* s = fgets(command, n, stdin); 	if (s == NULL) 	{ 		return -1; 	} 	command[strlen(command) - 1] = ZERO; 	return strlen(command); }  char* gArgv[NUM];  void splitCommand(char command[], size_t n) { 	gArgv[0] = strtok(command, SEP); 	int index = 1; 	while ((gArgv[index++] = strtok(NULL, SEP))); }  int main() { 	MakeCommandLine();  	char usercommand[SIZE]; 	GetUserCommand(usercommand, sizeof(usercommand)); 	 	splitCommand(usercommand, sizeof(usercommand));  	for (int i = 0; gArgv[i]; i++) 	{ 		printf("gArgv[%d]:%s\n", i, gArgv[i]); 	}  	pid_t id = fork(); 	if (id < 0) 	{ 		Die(); 	} 	else if (id == 0) 	{ 		execvp(gArgv[0], gArgv); 		exit(errno); 	} 	else 	{ 		int status = 0; 		pid_t rid = waitpid(id,&status,0); 		if (rid > 0) 		{ 			 		} 	} 	return 0; }

但是我们自己编写的这个shell只能运行一次，我们想要让它一直运行执行命令 

于是封装＋循环写个简单的shell就好：

#include #include #include #include #include #include #include  #define SIZE 512 #define ZERO '\0' #define SEP " " #define NUM 32  void Die() {  }  const char* GetUserName() { 	const char* name = getenv("USER"); 	if (name == NULL) 	{ 		return "None"; 	} 	return name; }  const char* GetHostName() { 	const char* hostname = getenv("HOSTNAME"); 	if (hostname == NULL) 	{ 		return "None"; 	} 	return hostname; }  const char* GetCwd() { 	const char* cwd = getenv("PWD"); 	if (cwd == NULL) 	{ 		return "None"; 	} 	return cwd; }  void MakeCommandLine() { 	char line[SIZE]; 	const char* username = GetUserName(); 	const char* hostname = GetHostName(); 	const char* cwd = GetCwd(); 	snprintf(line, sizeof(line), "[%s@%s %s]#", username, hostname, cwd); 	printf("%s", line); 	fflush(stdout); }  int GetUserCommand(char command[], size_t n) { 	char* s = fgets(command, n, stdin); 	if (s == NULL) 	{ 		return -1; 	} 	command[strlen(command) - 1] = ZERO; 	return strlen(command); }  char* gArgv[NUM];  void splitCommand(char command[], size_t n) { 	gArgv[0] = strtok(command, SEP); 	int index = 1; 	while ((gArgv[index++] = strtok(NULL, SEP))); }  void ExecuteCommand() { 	pid_t id = fork(); 	if (id < 0) 	{ 		Die(); 	} 	else if (id == 0) 	{ 		execvp(gArgv[0], gArgv); 		exit(errno); 	} 	else 	{ 		int status = 0; 		pid_t rid = waitpid(id, &status, 0); 		if (rid > 0) 		{  		} 	} }  int main() { 	int quit = 0; 	while (!quit) 	{ 		MakeCommandLine();  		char usercommand[SIZE]; 		GetUserCommand(usercommand, sizeof(usercommand));  		splitCommand(usercommand, sizeof(usercommand));  		ExecuteCommand(); 	} 	return 0; }

大致没问题，但是有个命令跑起来有问题：cd 

我们的shell没法进行路径的切换和回退 

每个进程都会记录自己当前所处的路径（父进程有，紫禁城也有）

紫禁城确实是执行了路径切换的命令，但是和父进程没什么关系，而打印命令行是父进程负责的

我们需要让cd交给父进程去执行，也就是所谓的内建命令

所以我们在执行命令前要检查是否是内建命令，由于命令已经被分割，所以直接检查

在获取完用户命令后，进行切换命令需要用到系统调用的接口chdir

 写完了一串代码，会发现cd倒是能用，只不过命令行打印不正常罢了，所以这意味着我们需要对环境变量进行更新

#include #include #include #include #include #include #include  #define SIZE 512 #define ZERO '\0' #define SEP " " #define NUM 32  void Die() {  }  const char* Home() { 	const char* home = getenv("HOME"); 	if (home == NULL) 	{ 		return "/"; 	} 	return home; }  void Cd() { 	const char* path = gArgv[1]; 	if (path == NULL) 	{ 		path = Home(); 	} 	chdir(path); }  const char* GetUserName() { 	const char* name = getenv("USER"); 	if (name == NULL) 	{ 		return "None"; 	} 	return name; }  const char* GetHostName() { 	const char* hostname = getenv("HOSTNAME"); 	if (hostname == NULL) 	{ 		return "None"; 	} 	return hostname; }  const char* GetCwd() { 	const char* cwd = getenv("PWD"); 	if (cwd == NULL) 	{ 		return "None"; 	} 	return cwd; }  void MakeCommandLine() { 	char line[SIZE]; 	const char* username = GetUserName(); 	const char* hostname = GetHostName(); 	const char* cwd = GetCwd(); 	snprintf(line, sizeof(line), "[%s@%s %s]#", username, hostname, cwd); 	printf("%s", line); 	fflush(stdout); }  int GetUserCommand(char command[], size_t n) { 	char* s = fgets(command, n, stdin); 	if (s == NULL) 	{ 		return -1; 	} 	command[strlen(command) - 1] = ZERO; 	return strlen(command); }  char* gArgv[NUM];  void splitCommand(char command[], size_t n) { 	gArgv[0] = strtok(command, SEP); 	int index = 1; 	while ((gArgv[index++] = strtok(NULL, SEP))); }  void ExecuteCommand() { 	pid_t id = fork(); 	if (id < 0) 	{ 		Die(); 	} 	else if (id == 0) 	{ 		execvp(gArgv[0], gArgv); 		exit(errno); 	} 	else 	{ 		int status = 0; 		pid_t rid = waitpid(id, &status, 0); 		if (rid > 0) 		{  		} 	} }  int CheckBuildin() { 	int yes = 0; 	const char* enter_cmd = gArgv[0]; 	if (strcmp(enter_cmd, "cd") == 0) 	{ 		yes = 1; 		Cd(); 	} 	return yes; }  int main() { 	int quit = 0; 	while (!quit) 	{ 		MakeCommandLine();  		char usercommand[SIZE]; 		int n = GetUserCommand(usercommand, sizeof(usercommand)); 		if (n < 0) 		{ 			return 1; 		}  		splitCommand(usercommand, sizeof(usercommand));  		n = CheckBuildin(); 		if (n) 		{ 			continue; 		}  		ExecuteCommand(); 	} 	return 0; }

 需要注意的是，我们需要刷新环境变量，以保证出来的都是绝对路径

 改完之后除了cd -不支持，其他都支持（cd -需要保存最近一次所处路径）

#include #include #include #include #include #include #include  #define SIZE 512 #define ZERO '\0' #define SEP " " #define NUM 32  char cwd[SIZE * 2]; char* gArgv[NUM];  void Die() { 	exit(1); }  const char* Home() { 	const char* home = getenv("HOME"); 	if (home == NULL) 	{ 		return "/"; 	} 	return home; }  void Cd() { 	const char* path = gArgv[1]; 	if (path == NULL) 	{ 		path = Home(); 	} 	chdir(path); 	char temp[SIZE * 2]; 	getcwd(temp, sizeof(temp)); 	snprintf(cwd, sizeof(cwd), "PWD=%s", temp); 	putenv(cwd); }  const char* GetUserName() { 	const char* name = getenv("USER"); 	if (name == NULL) 	{ 		return "None"; 	} 	return name; }  const char* GetHostName() { 	const char* hostname = getenv("HOSTNAME"); 	if (hostname == NULL) 	{ 		return "None"; 	} 	return hostname; }  const char* GetCwd() { 	const char* cwd = getenv("PWD"); 	if (cwd == NULL) 	{ 		return "None"; 	} 	return cwd; }  void MakeCommandLine() { 	char line[SIZE]; 	const char* username = GetUserName(); 	const char* hostname = GetHostName(); 	const char* cwd = GetCwd(); 	snprintf(line, sizeof(line), "[%s@%s %s]#", username, hostname, cwd); 	printf("%s", line); 	fflush(stdout); }  int GetUserCommand(char command[], size_t n) { 	char* s = fgets(command, n, stdin); 	if (s == NULL) 	{ 		return -1; 	} 	command[strlen(command) - 1] = ZERO; 	return strlen(command); }   void splitCommand(char command[], size_t n) { 	gArgv[0] = strtok(command, SEP); 	int index = 1; 	while ((gArgv[index++] = strtok(NULL, SEP))); }  void ExecuteCommand() { 	pid_t id = fork(); 	if (id < 0) 	{ 		Die(); 	} 	else if (id == 0) 	{ 		execvp(gArgv[0], gArgv); 		exit(errno); 	} 	else 	{ 		int status = 0; 		pid_t rid = waitpid(id, &status, 0); 		if (rid > 0) 		{  		} 	} }  int CheckBuildin() { 	int yes = 0; 	const char* enter_cmd = gArgv[0]; 	if (strcmp(enter_cmd, "cd") == 0) 	{ 		yes = 1; 		Cd(); 	} 	return yes; }  int main() { 	int quit = 0; 	while (!quit) 	{ 		MakeCommandLine();  		char usercommand[SIZE]; 		int n = GetUserCommand(usercommand, sizeof(usercommand)); 		if (n < 0) 		{ 			return 1; 		}  		splitCommand(usercommand, sizeof(usercommand));  		n = CheckBuildin(); 		if (n) 		{ 			continue; 		}  		ExecuteCommand(); 	} 	return 0; }

插个眼我很好奇为什么编译不过去

我们自己创建的shell在打印命令行的时候保存的是一串路径，但是正常的shell在保存路径的时候只会保存当前路径，那应该怎么办捏？

就是设个指针让它前移，但是需要做一些特殊处理，在一般情况下打印路径分割符后的内容，在特殊情况下只打印路径分隔符

使用宏是因为要单独对指针进行操作 

用do...while(0)是因为可以形成代码块，且do...while(0)后面可加;

在使用时可当做函数，且可应对分支情况的出现

然后还需要接着判断其他的内建命令

#include #include #include #include #include #include #include  #define SkipPath(p) do{ p += (strlen(p) - 1); while (*p != '/')p--;}while(0)  #define SIZE 512 #define ZERO '\0' #define SEP " " #define NUM 32   char cwd[SIZE * 2]; char* gArgv[NUM];  void Die() { 	exit(1); }  const char* Home() { 	const char* home = getenv("HOME"); 	if (home == NULL) 	{ 		return "/"; 	} 	return home; }  void Cd() { 	const char* path = gArgv[1]; 	if (path == NULL) 	{ 		path = Home(); 	} 	chdir(path); 	char temp[SIZE * 2]; 	getcwd(temp, sizeof(temp)); 	snprintf(cwd, sizeof(cwd), "PWD=%s", temp); 	putenv(cwd); }  const char* GetUserName() { 	const char* name = getenv("USER"); 	if (name == NULL) 	{ 		return "None"; 	} 	return name; }  const char* GetHostName() { 	const char* hostname = getenv("HOSTNAME"); 	if (hostname == NULL) 	{ 		return "None"; 	} 	return hostname; }  const char* GetCwd() { 	const char* cwd = getenv("PWD"); 	if (cwd == NULL) 	{ 		return "None"; 	} 	return cwd; }  void MakeCommandLine() { 	char line[SIZE]; 	const char* username = GetUserName(); 	const char* hostname = GetHostName(); 	const char* cwd = GetCwd();  	SkipPath(cwd); 	snprintf(line, sizeof(line), "[%s@%s %s]#", username, hostname, strlen(cwd) == 1 ? "/" : cwd + 1); 	printf("%s", line); 	fflush(stdout); }  int GetUserCommand(char command[], size_t n) { 	char* s = fgets(command, n, stdin); 	if (s == NULL) 	{ 		return -1; 	} 	command[strlen(command) - 1] = ZERO; 	return strlen(command); }   void splitCommand(char command[], size_t n) { 	gArgv[0] = strtok(command, SEP); 	int index = 1; 	while ((gArgv[index++] = strtok(NULL, SEP))); }  void ExecuteCommand() { 	pid_t id = fork(); 	if (id < 0) 	{ 		Die(); 	} 	else if (id == 0) 	{ 		execvp(gArgv[0], gArgv); 		exit(errno); 	} 	else 	{ 		int status = 0; 		pid_t rid = waitpid(id, &status, 0); 		if (rid > 0) 		{  		} 	} }  int CheckBuildin() { 	int yes = 0; 	const char* enter_cmd = gArgv[0]; 	if (strcmp(enter_cmd, "cd") == 0) 	{ 		yes = 1; 		Cd(); 	} 	return yes; }  int main() { 	int quit = 0; 	while (!quit) 	{ 		MakeCommandLine();  		char usercommand[SIZE]; 		int n = GetUserCommand(usercommand, sizeof(usercommand)); 		if (n < 0) 		{ 			return 1; 		}  		splitCommand(usercommand, sizeof(usercommand));  		n = CheckBuildin(); 		if (n) 		{ 			continue; 		}  		ExecuteCommand(); 	} 	return 0; }

判断echo命令

 这是记录最近一次的返回值（echo $?）啊麻烦死了自己实现shell

忙活半天只能做个青春版，还不能查历史输入的命令

#include #include #include #include #include #include #include  #define SkipPath(p) do{ p += (strlen(p) - 1); while (*p != '/')p--;}while(0)  #define SIZE 512 #define ZERO '\0' #define SEP " " #define NUM 32  int lastcode = 1; char cwd[SIZE * 2]; char* gArgv[NUM];  void Die() { 	exit(1); }  const char* Home() { 	const char* home = getenv("HOME"); 	if (home == NULL) 	{ 		return "/"; 	} 	return home; }  void Cd() { 	const char* path = gArgv[1]; 	if (path == NULL) 	{ 		path = Home(); 	} 	chdir(path); 	char temp[SIZE * 2]; 	getcwd(temp, sizeof(temp)); 	snprintf(cwd, sizeof(cwd), "PWD=%s", temp); 	putenv(cwd); }  const char* GetUserName() { 	const char* name = getenv("USER"); 	if (name == NULL) 	{ 		return "None"; 	} 	return name; }  const char* GetHostName() { 	const char* hostname = getenv("HOSTNAME"); 	if (hostname == NULL) 	{ 		return "None"; 	} 	return hostname; }  const char* GetCwd() { 	const char* cwd = getenv("PWD"); 	if (cwd == NULL) 	{ 		return "None"; 	} 	return cwd; }  void MakeCommandLine() { 	char line[SIZE]; 	const char* username = GetUserName(); 	const char* hostname = GetHostName(); 	const char* cwd = GetCwd();  	SkipPath(cwd); 	snprintf(line, sizeof(line), "[%s@%s %s]#", username, hostname, strlen(cwd) == 1 ? "/" : cwd + 1); 	printf("%s", line); 	fflush(stdout); }  int GetUserCommand(char command[], size_t n) { 	char* s = fgets(command, n, stdin); 	if (s == NULL) 	{ 		return -1; 	} 	command[strlen(command) - 1] = ZERO; 	return strlen(command); }   void splitCommand(char command[], size_t n) { 	gArgv[0] = strtok(command, SEP); 	int index = 1; 	while ((gArgv[index++] = strtok(NULL, SEP))); }  void ExecuteCommand() { 	pid_t id = fork(); 	if (id < 0) 	{ 		Die(); 	} 	else if (id == 0) 	{ 		execvp(gArgv[0], gArgv); 		exit(errno); 	} 	else 	{ 		int status = 0; 		pid_t rid = waitpid(id, &status, 0); 		if (rid > 0) 		{ 			lastcode = WEXITSTATUS(status); 			if (lastcode != 0) 			{ 				printf("%s:%s:%d\n", gArgv[0], strerror(lastcode), lastcode); 			} 		} 	} }  int CheckBuildin() { 	int yes = 0; 	const char* enter_cmd = gArgv[0]; 	if (strcmp(enter_cmd, "cd") == 0) 	{ 		yes = 1; 		Cd(); 	} 	else if (strcmp(enter_cmd, "echo") == 0 && strcmp(gArgv[1], "$?") == 0) 	{ 		yes = 1; 		printf("%d\n", lastcode); 		lastcode = 0; 	} 	return yes; }  int main() { 	int quit = 0; 	while (!quit) 	{ 		MakeCommandLine();  		char usercommand[SIZE]; 		int n = GetUserCommand(usercommand, sizeof(usercommand)); 		if (n < 0) 		{ 			return 1; 		}  		splitCommand(usercommand, sizeof(usercommand));  		n = CheckBuildin(); 		if (n) 		{ 			continue; 		}  		ExecuteCommand(); 	} 	return 0; }

 在shell中建立环境变量表也是默认创建指针

是一个缓冲区，可以自己申请，环境变量表是char* env[32]，而定义开个空间就直接自己设置就好：

char** env = (char*)malloc(sizeof(char*)*32);

 自己写个shell就对它怎样执行命令认识的更加深刻了