练习：

实现shell里面的umask命令

void get_umask(int argc,char * argv[])
{
    if(argc == 2)//get umask
    {
        mode_t mode = umask(0000);

        int user = 0,group = 0,other = 0;

        if(mode & S_IRUSR)
            user = user + 4;
        if(mode & S_IWUSR)
            user = user + 2;
        if(mode & S_IXUSR)
            user = user + 1;

        if(mode & S_IRGRP)
            group = group + 4;
        if(mode & S_IWGRP)
            group = group + 2;
        if(mode & S_IXGRP)
            group = group + 1;

        if(mode & S_IROTH)
            other = other + 4;
        if(mode & S_IWOTH)
            other = other + 2;
        if(mode & S_IXOTH)
            other = other + 1;

        umask(mode);
        printf("0%d%d%d\n",user,group,other);
    }
    else//set umask
    {
        int len = strlen(argv[2]);
        switch(len)
        {
            case 1:
            {
                int a;
                a = argv[2][0] - '0';
                mode_t mode = 0;
                if(a < 0 || a > 7)
                    {
                        printf("octal number out of range\n");
                        return ;
                    }
                if(a & 0x04)
                    mode = mode | S_IROTH;
                if(a & 0x02)
                    mode = mode | S_IWOTH;
                if(a & 0x01)
                    mode = mode | S_IXOTH;
                umask(mode);
                break;
            }
            case 2:
            {
                int a,b;
                mode_t mode = 0;
                a = argv[2][0] - '0';
                b = argv[2][1] - '0';

                if(a < 0 || a > 7 || b < 0 || b > 7)
                    {
                        printf("octal number out of range\n");
                        return ;
                    }

                if(b & 0x04)
                    mode = mode | S_IROTH;
                if(b & 0x02)
                    mode = mode | S_IWOTH;
                if(b & 0x01)
                    mode = mode | S_IXOTH;

                if(a & 0x04)
                    mode = mode | S_IRGRP;
                if(a & 0x02)
                    mode = mode | S_IWGRP;
                if(a & 0x01)
                    mode = mode | S_IXGRP;
                umask(mode);
                break;
            }
            case 3:
            {
                int a,b,c;
                mode_t mode = 0;
                a = argv[2][0];
                b = argv[2][1];
                c = argv[2][2];

                if(a < 0 || a > 7 || b < 0 || b > 7 || c < 0 || c > 7)
                    {
                        printf("octal number out of range\n");
                        return ;
                    }


                if(a & 0x04)
                    mode = mode | S_IRUSR;
                if(a & 0x02)
                    mode = mode | S_IWUSR;
                if(a & 0x01)
                    mode = mode | S_IXUSR;

                if(b & 0x04)
                    mode = mode | S_IRGRP;
                if(b & 0x02)
                    mode = mode | S_IWGRP;
                if(b & 0x01)
                    mode = mode | S_IXGRP;

                if(c & 0x04)
                    mode = mode | S_IROTH;
                if(c & 0x02)
                    mode = mode | S_IWOTH;
                if(c & 0x01)
                    mode = mode | S_IXOTH;
                umask(mode);
                break;
            }
            default:
            {
                int a,b,c,d;
                mode_t mode = 0;
                a = argv[2][len-4];
                b = argv[2][len-3];
                c = argv[2][len-2];
                d = argv[2][len-1];

                if(a != 0 || b < 0 || b > 7 || c < 0 || c > 7 || d < 0 || d >7)
                    {
                        printf("octal number out of range\n");
                        return ;
                    }

                if(b & 0x04)
                    mode = mode | S_IRUSR;
                if(b & 0x02)
                    mode = mode | S_IWUSR;
                if(b & 0x01)
                    mode = mode | S_IXUSR;

                if(c & 0x04)
                    mode = mode | S_IRGRP;
                if(c & 0x02)
                    mode = mode | S_IWGRP;
                if(c & 0x01)
                    mode = mode | S_IXGRP;

                if(d & 0x04)
                    mode = mode | S_IROTH;
                if(d & 0x02)
                    mode = mode | S_IWOTH;
                if(d & 0x01)
                    mode = mode | S_IXOTH;
                umask(mode);
                break;
            }
        }
    }

}

《UNIX高级环境编程》读书笔记之文件与目录（3）