XV6_SHELLパイプとリダイレクト

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
// Simplifed xv6 shell.

#define MAXARGS 10

// All commands have at least a type. Have looked at the type, the code
// typically casts the *cmd to some specific cmd type.
struct cmd {
  int type;          //  ' ' (exec), | (pipe), '' for redirection
};

struct execcmd {//     
  int type;              // ' '
  char *argv[MAXARGS];   // arguments to the command to be exec-ed
};

struct redircmd {
  int type;          // < or > 
  struct cmd *cmd;   // the command to be run (e.g., an execcmd)
  char *file;        // the input/output file
  int mode;          // the mode to open the file with
  int fd;            // the file descriptor number to use for the file
};

struct pipecmd {
  int type;          // |
  struct cmd *left;  // left side of pipe，  
  struct cmd *right; // right side of pipe，  
};

int fork1(void);  // Fork but exits on failure.
struct cmd *parsecmd(char*);

// Execute cmd.  Never returns.
void runcmd(struct cmd *cmd)
{
  int p[2], r;
  struct execcmd *ecmd;
  struct pipecmd *pcmd;
  struct redircmd *rcmd;

  if(cmd == 0)
    exit(0);
  
  switch(cmd->type){
  default:
    fprintf(stderr, "unknown runcmd
");
    exit(-1);

  case ' '://     
    ecmd = (struct execcmd*)cmd;
    if(ecmd->argv[0] == 0)
      exit(0);
//    fprintf(stderr, "exec not implemented
");
    // Your code here ..
    if (access(ecmd->argv[0], F_OK) == 0)//access                 ，mode_>R_OK(    ），W_OK(    ），X_OK(     ），F_OK(      ）    0，    -1,execv       ，  pathname              ID   ,execv(pathname,argv[])
    {
        execv(ecmd->argv[0], ecmd->argv);//pathname    ,argv           ，            ，     NULL
    }
    else
    {
	const char *binPath = "/bin/";
	int pathLen = strlen(binPath) + strlen(ecmd->argv[0]);
	char *abs_path = (char *)malloc((pathLen+1)*sizeof(char));
	strcpy(abs_path, binPath);//           
	strcat(abs_path, ecmd->argv[0]);//           
	if(access(abs_path, F_OK)==0)
	{
		execv(abs_path, ecmd->argv);
	}
	else
		fprintf(stderr, "%s: Command not found
", ecmd->argv[0]);
	
    }
    break;

  case '>':
  case 'fd);//         ，fd      
    if(open(rcmd->file, rcmd->mode, 0644) < 0)//                
    {
	    fprintf(stderr, "Unable to open file: %s
", rcmd->file);
	    exit(0);
    }
    runcmd(rcmd->cmd);
    break;

  case '|':
    pcmd = (struct pipecmd*)cmd;
    //fprintf(stderr, "pipe not implemented
");
    // Your code here ...
    // pipe       ，       fd       ，  p[0]          ，p[1]          
    // dup（old_fd)    fd，  old-fd     ,     fd
    if(pipe(p) < 0)
    {
        fprintf(stderr, "pipe failed
");
        //exit(0);
    }
    if(fork1() == 0)
    {
        close(1);//      
        dup(p[1]);//        p[1]    ，      
	//          
        close(p[0]);
        close(p[1]);
	//left       ，        
        runcmd(pcmd->left);
    }

    if(fork1() == 0)
    {
        close(0);
        dup(p[0]);
        close(p[0]);
        close(p[1]);
        runcmd(pcmd->right);
    }

    close(p[0]);
    close(p[1]);
    wait(&r);
    wait(&r);
    break;
  }    
  exit(0);
}

int
getcmd(char *buf, int nbuf)//    
{
  
  if (isatty(fileno(stdin)))//           
    fprintf(stdout, "$ ");//         
  memset(buf, 0, nbuf);
  fgets(buf, nbuf, stdin);//       nbuf    buf 
  if(buf[0] == 0) // EOF
    return -1;
  return 0;
}

int
main(void)
{
  static char buf[100];
  int fd, r;

  // Read and run input commands.
  while(getcmd(buf, sizeof(buf)) >= 0){
    if(buf[0] == 'c' && buf[1] == 'd' && buf[2] == ' '){//   cd  ，           
      // Clumsy but will have to do for now.
      // Chdir has no effect on the parent if run in the child.
      buf[strlen(buf)-1] = 0;  // chop 
，     0
      if(chdir(buf+3) < 0)//chdir is the same as cd,when sucess, it will return 0, if not, it will return -1 
        fprintf(stderr, "cannot cd %s
", buf+3);//print the message to the file sterr(stream)
      continue;
    }
    if(fork1() == 0)//the conmad is not cd, then fork
      runcmd(parsecmd(buf));//child process，parsecmd  buf，    runcmd
    wait(&r);//        
  }
  exit(0);
}

int
fork1(void)
{
  int pid;
  
  pid = fork();
  if(pid == -1)
    perror("fork");
  return pid;
}

struct cmd*
execcmd(void)
{
  struct execcmd *cmd;

  cmd = malloc(sizeof(*cmd));
  memset(cmd, 0, sizeof(*cmd));
  cmd->type = ' ';
  return (struct cmd*)cmd;
}

struct cmd*
redircmd(struct cmd *subcmd, char *file, int type)
{
  struct redircmd *cmd;

  cmd = malloc(sizeof(*cmd));
  memset(cmd, 0, sizeof(*cmd));
  cmd->type = type;
  cmd->cmd = subcmd;
  cmd->file = file;
  cmd->mode = (type == 'fd = (type == 'type = '|';
  cmd->left = left;
  cmd->right = right;
  return (struct cmd*)cmd;
}

// Parsing

char whitespace[] = " \t\r
\v";
char symbols[] = "";

int
gettoken(char **ps, char *es, char **q, char **eq)//        
{
  char *s;
  int ret;
  
  s = *ps;
  while(s < es && strchr(whitespace, *s))//    ，           
    s++;
  if(q)
    *q = s;
  ret = *s;//              
  switch(*s){
  case 0://    
    break;
  case '|':
  case ''://     
    s++;
    break;
  default:
    ret = 'a';
    while(s < es && !strchr(whitespace, *s) && !strchr(symbols, *s))
      s++;//s     symbols  
    break;
  }
  if(eq)
    *eq = s;
//q eq       symbols        
  while(s < es && strchr(whitespace, *s))
    s++;//         
  *ps = s;
  return ret;
}

int
peek(char **ps, char *es, char *toks)//              ， toks    1
{
  char *s;
  
  s = *ps;
  while(s < es && strchr(whitespace, *s))//            
    s++;
  *ps = s;
  return *s && strchr(toks, *s);//        tock    
}

struct cmd *parseline(char**, char*);
struct cmd *parsepipe(char**, char*);
struct cmd *parseexec(char**, char*);

// make a copy of the characters in the input buffer, starting from s through es.
// null-terminate the copy to make it a string.
char 
*mkcopy(char *s, char *es)
{
  int n = es - s;
  char *c = malloc(n+1);
  assert(c);
  strncpy(c, s, n);
  c[n] = 0;
  return c;
}

struct cmd*
parsecmd(char *s)
{//    
  char *es;
  struct cmd *cmd;

  es = s + strlen(s);//es points to the last blank char
  cmd = parseline(&s, es);
  peek(&s, es, "");//s         
  if(s != es){
    fprintf(stderr, "leftovers: %s
", s);
    exit(-1);
  }
  return cmd;
}

struct cmd*
parseline(char **ps, char *es)//              
{//         
  struct cmd *cmd;
  cmd = parsepipe(ps, es);
  return cmd;
}

struct cmd*
parsepipe(char **ps, char *es)//      
{
  struct cmd *cmd;

  cmd = parseexec(ps, es);//  ？
  if(peek(ps, es, "|")){
    gettoken(ps, es, 0, 0);
    cmd = pipecmd(cmd, parsepipe(ps, es));
  }
  return cmd;
}

struct cmd*
parseredirs(struct cmd *cmd, char **ps, char *es)//     
{
  int tok;
  char *q, *eq;

  while(peek(ps, es, "<>")){
    tok = gettoken(ps, es, 0, 0);//         
    if(gettoken(ps, es, &q, &eq) != 'a') {//            
      fprintf(stderr, "missing file for redirection
");
      exit(-1);
    }
    switch(tok){
    case '':
      cmd = redircmd(cmd, mkcopy(q, eq), '>');
      break;
    }
  }
  return cmd;
}

struct cmd*
parseexec(char **ps, char *es)//          
{//
  char *q, *eq;
  int tok, argc;
  struct execcmd *cmd;
  struct cmd *ret;
  
  ret = execcmd();
  cmd = (struct execcmd*)ret;

  argc = 0;
  ret = parseredirs(ret, ps, es);
  while(!peek(ps, es, "|")){
    if((tok=gettoken(ps, es, &q, &eq)) == 0)
      break;
    if(tok != 'a') {
      fprintf(stderr, "syntax error
");
      exit(-1);
    }
    cmd->argv[argc] = mkcopy(q, eq);
    argc++;
    if(argc >= MAXARGS) {
      fprintf(stderr, "too many args
");
      exit(-1);
    }
    ret = parseredirs(ret, ps, es);
  }
  cmd->argv[argc] = 0;
  return ret;
}