linux的shell，第一次写，只是简单的一些功能

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>

extern char **getline();

main(int argc, char *argv[], char *envp[]) {

  char **args;
  static char p[39]="> ";
  pid_t  pid;
  FILE *fp;
  int BG = 0;
  while(1) {
           fprintf(stderr,"user@user-laptop:%s%s",getcwd(NULL,NULL),p);
    args = getline();
    int jj = 0;
    if(strcmp(*args,"cd") == 0){     //implement the "cd" function, so that you can enjoy your work at
         chdir(*(args + 1));         //different directories that you want.
    }
    while(*(args+jj) != NULL)
          jj++;
    jj--;                            //*(args+j)  indicating the last char of the line.
    int status;                     //a flag indicates wether it is a background process.
    switch(pid = fork()){           //fork a new child process.
               case -1:
                    printf("Unable to fork. Will now exit./n");
                    break;
               case 0:
                    if(strcmp(*args,"help")==0){
                         printf("Most of the linux command has been implemented.../n");
                         printf("Although it is not perfect.../n");
                    }
                    else if(strcmp(*args,"add")==0){//sum-result of the addition, i-iterator of the array "args".
                                                    //printf("--- argc= %d%d%d ,args= ",segs,start,end);
                                                    //the command of "add".
                    int sum=0;int i=1;
                    while(*(args+i)!=NULL){         //loop part.
                         if(i==1) printf("--- ");
                         if(i>=2) printf(" + ");
                         printf(*(args+i));
                         if((*(args+i))[0]=='0'&&(*(args+i))[1]=='x'){//check and convert heximal number to decimal.
                              int length=2;char temp=(*(args+i))[2];
                              while(temp!='/0'){length++;temp=(*(args+i))[length];}
                              int index=2;int hex=0;
                              while(index<length){
                                   double exp=length-index-1;
                                   if((*(args+i))[index]-48<=9)
                                        hex=hex+((*(args+i))[index]-48)*(int)pow((double)16,exp);
                                   else hex=hex+((*(args+i))[index]-96+9)*(int)pow((double)16,exp);
                                   index++;
                              }
                              sum=sum+hex;
                         }//end if.
                         else
                              sum=sum+atoi(*(args+i));
                              i++;
                    }
                    printf(" = %d ---/n",sum);
                    //end the command of "add"
                    }
                    else if(strcmp(*args,"arg")==0){            //the "arg" function.
                         int tag[20]={0};
                         int i=0;int segs=0;
                         while(*(args+i)!=NULL){
                              if(strcmp(*(args+i),"/"")==0)
                                   tag[i]=1;
                              i++;
                         }
                         printf("--- args = ");
                         int start=0;int end=0;int temp=0;int index=0;
                         while(index<i){
                              if(tag[index]==1){
                                   if(start==0) {start=index;}
                                   else{end=index;segs=segs-(end-start);start=0;}
                              }
                              if(index>0){
                                   printf(*(args+index));
                                   if(start==0) printf(", ");
                                   else printf(" ");
                              }
                              index++;segs++;
                         }
                         printf("      argc is %d ---/n",segs-1);
                    }
                    else{
                         int choice = 0;
                         int i = 0;
                         while(*(args+i) != NULL){                    //get data from files and do something.
                              if(strcmp(*(args+i),"<") == 0){
                                   if(strcmp(*(args + jj),"&")==0)
                                         *(args + jj) = '/0';
                                   execvp(*args,args + 1);
                              }
                              i ++;
                         }
                         i = 0;
                         while(*(args+i) != NULL){
                              if(strcmp(*(args+i),">") == 0){          //output data to files.
                                   if(strcmp(*(args + jj),"&")==0)
                                         *(args + jj) = '/0';
                                   args[i]='/0';
                                   choice=1;
                                   freopen(*(args+1+i),"w",stdout);
                                   execvp(*args,args);
                                   fclose(stdout);                     //return the standard outputstream to screen.
                                  }
                              i ++;
                             }
                         i = 0;
                         while(*(args+i) != NULL){                     //using pipe
                              if(strcmp(*(args+i),"|") == 0){
                                   choice = 2;
                                   args[i]='/0';
                                   int pp[2];                          //pp[2] are for the argments to pass signals.
                                   pipe(pp);
                                   pid_t pid2 = fork();
                                   if(pid2 == 0){
                                        close(pp[0]);
             
9732
                           close(1);
                                        dup(pp[1]);
                                        execvp(*args,args);
                                   }
                                   else{                              //waiting for child.
                                        waitpid(pid2,NULL,0);
                                        close(pp[1]);
                                   }
                                   pid_t pid3 = fork();
                                   if(pid3 == 0){                     //child 2 to do using information of child 1.
                                        close(pp[1]);
                                        close(0);
                                        dup(pp[0]);
                                        execvp(*(args+i+1),args+i+1);
                                        }
                                   else{
                                        waitpid(pid3,NULL,0);
                                        close(pp[0]);
                                   }
                              }
                              i++;
                         }//end while
                         if(choice == 0){
                              if(strcmp(*(args + jj), "&") == 0)
                                    *(args + jj)='/0';
                                   execvp(*args,args);}
                    }
                    return;
                    break;
               default:
                       //put it to the background to exec.
                        if(strcmp(*(args + jj), "&") != 0){
                             waitpid(pid,NULL,0);
                        }
                       //exit.
                       if(strcmp(*args,"exit")==0)
                            return;
      }//end switch
  }//end while
}
 

/*lex.c use flex to convert*/

%{
int _numargs = 100;
char *_args[100];
int _argcount = 0;
%}

WORD [a-zA-Z0-9///.-]+
SPECIAL [()><|&;*/"]

%%
  _argcount = 0;
  _args[0] = NULL;

{WORD}|{SPECIAL} {
if(_argcount < _numargs-1) {
_args[_argcount++] = (char *)strdup(yytext);
_args[_argcount] = NULL;
}
}

/n return (int)_args;

[ /t]+

.

%%

char **getline() {
return (char **)yylex();
}