shell.c

一个linux下的简单shell程序

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

#define MAXLINE 4096
#define MAXPIPE 16
#define MAXARG 8

struct { 
     char *argv[MAXARG];
     char *in, *out;
} cmd[MAXPIPE+1];

int parse(char *buf, int cmdnum)
{
     int n = 0;
     char *p = buf;

     cmd[cmdnum].in = cmd[cmdnum].out = NULL;
     while (*p != '\0')
     {
          if (*p == ' ')
          {
               *p++ = '\0';
               continue;
          }
          if (*p == '<')
          {
               *p = '\0';
               while (*(++p) == ' ');
               cmd[cmdnum].in = p;
               if (*p++ == '\0')
                    return -1;
               continue;
          }
          if (*p == '>')
          {
               *p = '\0';
               while (*(++p) == ' ');
               cmd[cmdnum].out = p;
               if (*p++ == '\0')
                    return -1;
               continue;
          }
          if (*p != ' ' && ((p == buf) || *(p-1) == '\0'))
          {
               if (n < MAXARG - 1) {
                    cmd[cmdnum].argv[n++] = p++;
                    continue;
               } else
                    return -1;
          }
          p++;
     }
     if (n == 0)
          return -1;
     cmd[cmdnum].argv[n] = NULL;
     return 0;
}

int main()
{
     char buf[MAXLINE];
     pid_t pid;
     int fd, i, j, pfd[MAXPIPE][2], pipe_num, cmd_num;
     char* curcmd, *nextcmd;

     while (1) {
	  printf("mysh%% ");
	  if (!fgets(buf, MAXLINE, stdin))
	       exit(0);
	  if (buf[strlen(buf)-1]=='\n')
	       buf[strlen(buf)-1]='\0';
	  cmd_num = 0;
	  nextcmd = buf;
	  while (curcmd = strsep(&nextcmd, "|")) {
	       if (parse(curcmd, cmd_num++)<0) {
		    cmd_num--;
		    break;
	       }
	       if (cmd_num == MAXPIPE + 1)
		    break;
	  }
	  if (!cmd_num)
	       continue;
          pipe_num = cmd_num - 1;
	  for (i=0; i<pipe_num; i++)
	       if (pipe(pfd[i])) {
		    perror("pipe");
		    exit(1);
	       }
	  for (i=0; i<cmd_num; i++)
	       if ((pid=fork())==0)
		    break;

	  if (pid==0) {
	       if (pipe_num) {
		    if (i==0) {
			 dup2(pfd[0][1], STDOUT_FILENO);
			 close(pfd[0][0]);
			 for (j=1; j<pipe_num; j++) {
			      close(pfd[j][0]);
			      close(pfd[j][1]);
			 }
		    } else if (i==pipe_num) {
			 dup2(pfd[i-1][0], STDIN_FILENO);
			 close(pfd[i-1][1]);
			 for (j=0; j<pipe_num-1; j++) {
			      close(pfd[j][0]);
			      close(pfd[j][1]);
			 }
		    } else {
			 dup2(pfd[i-1][0], STDIN_FILENO);
			 close(pfd[i-1][1]);
			 dup2(pfd[i][1], STDOUT_FILENO);
			 close(pfd[i][0]);
			 for (j=0; j<pipe_num; j++) 
			      if (j!=i || j!=i-1) {
				   close(pfd[j][0]);
				   close(pfd[j][1]);
			      }
		    }
	       }
	       if (cmd[i].in) {
		    fd = open(cmd[i].in, O_RDONLY);
		    if (fd != -1)
			 dup2(fd, STDIN_FILENO);
	       }
	       if (cmd[i].out) {
		    fd = open(cmd[i].out, O_WRONLY|O_CREAT|O_TRUNC, 0644);
		    if (fd != -1)
			 dup2(fd, STDOUT_FILENO);
	       }
	       execvp(cmd[i].argv[0], cmd[i].argv);
	       fprintf(stderr, "executing %s error.\n", cmd[i].argv[0]);
	       exit(127);
	  }

	  for (i=0; i<pipe_num; i++) {
	       close(pfd[i][0]);
	       close(pfd[i][1]);
	  }
	  for (i=0; i<cmd_num; i++)
	       wait(NULL);
     }
}