0. bash原理介绍

bash实际上就是一个负责解析输入字符串工具.

我们需要做的事是这些:

1. 手动分割出输入的字符串
2. 判断哪些变量是内建命令(自己执行),哪些命令是普通命令(创建子进程执行)
3. 实现的功能有: echo export cd 常规指令 输入、输出流重定向

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<string.h>
#include<sys/types.h>
#include<sys/wait.h>
#include<sys/stat.h>
#include<fcntl.h>#define HOST_NAME "hecs-225896"
#define LINE_SIZE 128
#define DELIM " "
#define SHEEL_COMMAND 0
#define NORMAL_COMMAND 1
#define IN_RESTREAM 0
#define OUT_RESTREAM 1
#define APPEND_RESTREAM 2 
#define NONE 0char pwd[LINE_SIZE];
char command[LINE_SIZE];
char* _argv[LINE_SIZE];
char myenv[LINE_SIZE];
int lastcode;
char * filename;
int restream = NONE;
int stream = 0;char * getpwd()
{return getcwd(pwd,sizeof(pwd));
}
char* getusr()
{return getenv("USER");
}
void interactive()
{char symbol;if(!strcmp(getenv("USER"),"root"))symbol='#';elsesymbol='$'; printf("%s@"HOST_NAME":""%s""%c ",getusr(),getpwd(),symbol);fgets(command,sizeof(command)-1,stdin);//消除'\n'command[strlen(command)-1]='\0';// printf("%s",command);
}
int split()
{for(int i=0;command[i];i++){if(command[i]=='>')  //写入重定向{command[i++]='\0';restream=OUT_RESTREAM;if(command[i]=='>') //追加重定向{restream=APPEND_RESTREAM;command[i++]='\0';while(command[i]==' ')i++;filename=command+i;printf("filename:%s\n",filename);stream=open(filename,O_CREAT|O_APPEND,0666);}else{while(command[i]==' ')i++;filename=command+i;printf("filename:%s\n",filename);stream=open(filename,O_CREAT|O_WRONLY|O_TRUNC,0666);}break;}else if(command[i]=='<'){restream=IN_RESTREAM;command[i++]='\0';while(command[i]==' ')i++;filename=command+i;printf("filename:%s\n",filename);stream=open(filename,O_RDONLY);break;}}int _argc=0;_argv[_argc++]=strtok(command,DELIM);while(_argv[_argc]=strtok(NULL,DELIM)){_argc++;}_argv[_argc]=NULL;return _argc;
}
int JudgeCommand()
{if(!strcmp(_argv[0],"cd")){return SHEEL_COMMAND;}else if(!strcmp(_argv[0],"echo")){return SHEEL_COMMAND;}else if(!strcmp(_argv[0],"export")){return SHEEL_COMMAND;}return NORMAL_COMMAND;
}
int execute_NormalCommand()
{pid_t id=fork();if(id<0){perror("fork faild\n");return 1;}else if(id==0){if(restream == OUT_RESTREAM || restream == APPEND_RESTREAM ){dup2(stream,1);}else if(restream == IN_RESTREAM){dup2(stream,0);}execvp(_argv[0],_argv);exit(2);}else if(id>0){int status=0;pid_t wid=waitpid(id,&status,0);if(wid==id)lastcode = WEXITSTATUS(status);}
}
int execute_ShellCommand(int argc)
{if(argc>=2&&!strcmp(_argv[0],"cd")){int rev=chdir(_argv[1]);if(rev!=0){perror("return faild");}}else if(argc>=2&&!strcmp(_argv[0],"echo")){if(_argv[1][0]=='$'){printf("%c",_argv[1][0]);char * env=getenv(_argv[1]+1);if(env)printf("%s\n",env);}else if(!strcmp(_argv[1],"?")){printf("%d\n",lastcode);lastcode=0;}elseprintf("%s\n",_argv[1]);}else if(argc>=2&&!strcmp(_argv[0],"export")){strcpy(myenv,_argv[1]);putenv(_argv[1]);}}
void BuildCommand(int *_argc)
{if(!strcmp(_argv[0],"ls")){_argv[(*_argc)++]="--color";_argv[*_argc]=NULL;}
}int main()
{while(1){interactive();int argc=split();if(argc==0)continue;// for(int i=0;i<argc;i++)printf("%s\n",_argv[i]);int RunFlag=JudgeCommand();BuildCommand(&argc);if(RunFlag==NORMAL_COMMAND){execute_NormalCommand();}else{execute_ShellCommand(argc);}}
}