in MIT 6.828 homework shell
https://pdos.csail.mit.edu/6.828/2018/homework/xv6-shell.html
I complement shell’s pipe part , which enable me to execute some pipe command such as ls > temp and it works well. But when I execute ./my_sh < t.sh
//t.sh

 1 ls > y                                                                                        
 2 cat < y | sort | uniq | wc > y1
 3 cat y1
 4 rm y1
 5 ls |  sort | uniq | wc
 6 rm y

It gets error like:

     10      10      62
     10      10      62
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory
cat: y1: No such file or directory
rm: cannot remove 'y1': No such file or directory
      9       9      60
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory
cat: y1: No such file or directory
rm: cannot remove 'y1': No such file or directory
      9       9      60
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory
cat: y1: No such file or directory
rm: cannot remove 'y1': No such file or directory
      9       9      60
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory
rm: cannot remove 'y': No such file or directory

Here is my code:

 86     case '|':
 87     pcmd = (struct pipecmd*)cmd;
 88     // Your code here ...
 89     //build pipe
 90     if(pipe(p)== -1){
 91       perror("pipe");
 92       _exit(-1);
 93     }
 94     //pipe read at right
 95     pid_t lpid,rpid;
 96     if((lpid = fork1()) == 0){
 97       close(1);//0-1-2|stdin-stdout-stderr
 98       dup(p[1]);//let fd=1 point to write-side pipe
 99       close(p[0]);
100       close(p[1]);
101       runcmd(pcmd->left);
102     }
103     wait(&r);
104     if((rpid = fork1()) == 0){                                                                
105       close(0);
106       dup(p[0]);//let fd=0 point to read-side pipe
107       close(p[0]);
108       close(p[1]);
109       runcmd(pcmd->right);
110     }
111     close(p[0]);
112     close(p[1]);
113     wait(&r);
114     break;

When I change way of fork , it works well

 86   case '|':
 87     pcmd = (struct pipecmd*)cmd;
 88     // Your code here ...
 89     //build pipe
 90     if(pipe(p)== -1){
 91       perror("pipe");
 92       _exit(-1);
 93     }
 94     //pipe read at right
 95     pid_t lpid;                                                                               
 96     lpid = fork1();
 97     if(lpid == 0){
 98       close(1);//0-1-2|stdin-stdout-stderr
 99       dup(p[1]);//let fd=1 point to write-side pipe
100       close(p[0]);
101       close(p[1]);
102       runcmd(pcmd->left);
103     }
104     else{
105       close(0);
106       dup(p[0]);//let fd=0 point to read-side pipe
107       close(p[0]);
108       close(p[1]);
109       wait(&r);
110       runcmd(pcmd->right);
111     }
112     break;

Why the first way gets error , and what is the difference between those two ways.