Operating System

Publish Date: 2022-10-27

lab1：Shell

Question 1:

If you add a background command after the pipeline command, can the pipeline work?

Solution

由上图可知，background command会导致pipeline不能正常工作，shell提前打印出了ccs-lab@字样，并且在执行完background command后依然在等待用户输入，我实现的background如下

case CMD_BACK: {
	struct Cmd_Back *t = (struct Cmd_Back *) cmd;
	Cmd_t back = t->back;

	if(fork() == 0){
      	Cmd_run(back);
     }
  	//wait(0);  //no care child
	 break;
}

而在pipe的实现机制下

case CMD_PIPE: {
	struct Cmd_Pipe *t = (struct Cmd_Pipe *) cmd;
	Cmd_t left = t->left;
	Cmd_t right = t->right;
	int fd[2];
	pipe(fd);

    if(fork() == 0) {
       	close(1);
       	dup(fd[1]);
        close(fd[0]);
       	close(fd[1]);
       	Cmd_run(left);
	}
    wait(0);

    if(fork() == 0){
      	close(0);
       	dup(fd[0]);
        close(fd[0]);
        close(fd[1]);
     	Cmd_run(right);
   	}
    close(fd[0]);
    close(fd[1]);
    wait(0);
    break;
}

分析其原因，执行right节点命令时，fork出子进程来执行grep，由于此时执行的是background command，父进程并未等待子进程退出(grep程序还在运行)，可以直接返回到父进程，最终返回到main函数中，此时进入新一轮循环，css-lab@被打印出来了，但此时cpu又调度了grep进程，grep执行完后，又返回到main里，这时shell会等待用户输入命令，所以出现了一个空行，等待命令输入

Question 2:

Why fork and exec are not combined in a single call?

Solution

fork和exec分离，可以使得在fork后exec前，设置一些必要的参数，如设置argv[]参数、打开或关闭文件描述符

例如在实现redirect过程中，执行left节点命令之前，先关闭stdout/stdin，再打开要重定向的文件，然后直接left节点命令，即可实现重定向

Question 3:

What are the advantages of pipes over temporary files in this situation?

use pipe

echo hello world | wc

without pipe

echo hello world > /tmp/xyz ; wc < /tmp/xyz

Solution

代码简洁明了，不需要暂时文件来存储中间信息，便于编写组合命令，实现复杂命令

Exercise 1:

Please complete the corresponding part in:

Cmd_t Cmd_Back_new(Cmd_t back);
Cmd_t Cmd_Pipe_new(Cmd_t left, Cmd_t right);
Cmd_t Cmd_Redir_new(Cmd_t left, Cmd_t right, int fd);

Hint: you can refer to the data structure defined in the ast.h.

Soution

Cmd_t Cmd_Back_new(Cmd_t back){
    Cmd_Back cmd;
    NEW(cmd);
    cmd->type = CMD_BACK;
    cmd->back = back;
    return (Cmd_t)cmd;
}

Cmd_t Cmd_Pipe_new(Cmd_t left, Cmd_t right){
    Cmd_Pipe cmd;
    NEW(cmd);
    cmd->type = CMD_PIPE;
    cmd->left = left;
    cmd->right = right;
    return (Cmd_t)cmd;
}

Cmd_t Cmd_Redir_new(Cmd_t left, Cmd_t right, int fd){
    Cmd_Redir cmd;
    NEW(cmd);
    cmd->type = CMD_REDIR;
    cmd->left = left;
    cmd->right = right;
    cmd->fd = fd;
    return (Cmd_t)cmd;
}

Exercise 2:

Please complete the void Cmd_print(Cmd_t cmd). Then call void Cmd_print(Cmd_t cmd) in main.c and comment out Cmd_run(root).

Finally, compile and run the project in Clion and use the following test cases to check whether your answer is correct.

cat hello.txt | grep -n hello > c.txt ; ls -l

The result will be as shown in the figure below.

sleep 10 &

The result will be as shown in the figure below.

Solution

case CMD_BACK: {
    struct Cmd_Back *t = (struct Cmd_Back *) cmd;
    Cmd_t back = t->back;

    Cmd_print(back);
    printf("& ");
    break;
}

case CMD_PIPE: {
    struct Cmd_Pipe *t = (struct Cmd_Pipe *) cmd;
    Cmd_t left = t->left;
    Cmd_t right = t->right;

    Cmd_print(left);
    printf("| ");
    Cmd_print(right);
    break;
}

case CMD_REDIR: {
    struct Cmd_Redir *t = (struct Cmd_Redir *) cmd;
    Cmd_t left = t->left;
    Cmd_t right = t->right;

    Cmd_print(left);
    printf("> ");
    Cmd_print(right);
    break;
}

运行效果如下：

Exercise 3:

Please complete the void Cmd_run(Cmd_t cmd). Regarding redirection commands, you only need to implement output redirection. Commands like this:

cat hello.txt > c.txt

Then call void Cmd_run(Cmd_t cmd) in main.c and comment out Cmd_print(root).

Finally, compile and run the project in Clion and use the following test cases to check whether your answer is correct.

cat hello.txt | grep -n hello > c.txt ; cat c.txt ; ls

Before entering the command, create a file called hello.txt in the current working directory. The content of the file is as follows:

The result will be as shown in the figure below.

sleep 10 &

If you enter the command sleep 10, you cannot use the shell for ten seconds. However, when you add an extra & to your command, you'll be able to use the shell immediately. The result will be as shown in the figure below.

Thinking

BACK设计思路：

父进程不等候子进程的退出

PIPE设计思想：

先fork一个子进程用来运行left，使得文件描述符1指向管道的写端，当left执行时会输出到管道里，第二次fork，子进程也会指向这个管道，让文件描述符0指向管道的读端，当right执行时会从管道中读入，最后父进程关闭管道

REDIR设计思想：

获取right的文件名，关闭文件描述符0/1，然后再打开right文件，运行left即可

Solution

case CMD_BACK: {
    struct Cmd_Back *t = (struct Cmd_Back *) cmd;
    Cmd_t back = t->back;
    if(fork() == 0){
        Cmd_run(back);
    }
    //wait(0);		// don't care child
    break;
}

case CMD_PIPE: {
    struct Cmd_Pipe *t = (struct Cmd_Pipe *) cmd;
    Cmd_t left = t->left;
    Cmd_t right = t->right;
    int fd[2];
    pipe(fd);

    if(fork() == 0) {	// run left
        close(1);
        dup(fd[1]);
        close(fd[0]);
        close(fd[1]);
        Cmd_run(left);
    }
    wait(0);

    if(fork() == 0){	// run right
        close(0);
        dup(fd[0]);
        close(fd[0]);
        close(fd[1]);
        Cmd_run(right);
    }
    close(fd[0]);
    close(fd[1]);
    wait(0);
    break;
}

case CMD_REDIR: {
    struct Cmd_Redir *t = (struct Cmd_Redir *) cmd;
    Cmd_t left = t->left;
    Cmd_t right = t->right;

    Cmd_Atom redir = (Cmd_Atom) right; 	// get redir filename
    char *filename;
    struct node *node = redir->node;
    filename = node->data;

    close(t->fd);  // close fd 0/1
    open(filename, O_CREAT | O_RDWR, 0777); // new file obtain fd 0/1
    Cmd_run(left);
    break;
}

运行结果如下：

Challenge:

Implement the input redirection.

Solution

修改parser.y

%left '|' ';'
%left '>'
%left '&'
%left '<'

%%

line            :  command '\n'	    		{ root = $1; $$ = 0; return 0; }
		        | '\n'       		{ root = 0; $$ = 0; return 0; }
                ;
                
command	        :  basic_command		    { $$ = $1;}
		        |  command ';' command  	{ $$ = Cmd_Seq_new($1, $3);}
		        |  command '&'			    { $$ = Cmd_Back_new($1);}
		        |  command '|' command 		{ $$ = Cmd_Pipe_new($1, $3);}
		        |  command '>' command		{ $$ = Cmd_Redir_new($1, $3, 1);}
		        |  command '<' command		{ $$ = Cmd_Redir_new($1, $3, 0);}
		        ;

修改scanner.l

<<EOF>>                                 {return 0;}
"&"|";"|"|"             			    {return yytext[0];}
">"                                     {return yytext[0];}
"<"                                     {return yytext[0];}
"\n"						            {return yytext[0]; }
[ ]*					                {}
{ID}+				    		        {yylval = strdup(yytext); return T_ARG; }
.						                {printf("lex err:[%s]", yytext); return 0; }

原Redir代码无需修改，因为yyparse会根据<和>调用不同的构造函数，区别在于传入的fd不同

(yyval.cmd) = Cmd_Redir_new((yyvsp[-2].cmd), (yyvsp[0].cmd), 1);
(yyval.cmd) = Cmd_Redir_new((yyvsp[-2].cmd), (yyvsp[0].cmd), 0);

编译运行即可，效果如下：

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Sysprog：1. Shell

lab1：Shell

Question 1:

Solution

Question 2:

Solution

Question 3:

Solution

Exercise 1:

Soution

Exercise 2:

Solution

Exercise 3:

Thinking

Solution

Challenge:

Solution

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