Reference Errors with vectors

here is my code:

#include <iostream>
#include <string>
#include <vector>

using namespace std;

class command {
    public:
        static void tokenizer(string str, vector<string>& words, string
delimiter){
            string::size_type pos1 = 0;
            string::size_type pos2 = str.find_first_of(delimiter);
            words.push_back(str.substr(pos1 ,pos2 - pos1));
            pos1 = pos2 + 1;
            while(pos2 <= str.length()){
                pos2 = str.find_first_of(delimiter, pos1);
                if(pos2 > str.length()){
                    pos2 = str.length();
                }
                words.push_back(str.substr(pos1 ,pos2 - pos1));
                pos1 = pos2 + 1;
                pos2 += 1;
            }
        }
        static void getCommand(){
            string c;
            cin >> c;
            tokenizer(c, com, " ");
            if(com.size() == 1){
                cout << com[1] << "\n";
            } else if(com.size() == 2){
                cout << com[1] << "\n" << com[2] << "\n";
            } else { cout << "invalid command!\n"; }
        }
    private:
        static vector<string> com;
};

int main()
{
    command::getCommand();
    return 0;
}

Here is the error I get when I compile:

/tmp/ccNSmcoY.o: In function `command::getCommand()':
command.cpp:(.gnu.linkonce.t._ZN7command10getCommandEv+0x70): undefined
reference to `command::com'
command.cpp:(.gnu.linkonce.t._ZN7command10getCommandEv+0xeb): undefined
reference to `command::com'
command.cpp:(.gnu.linkonce.t._ZN7command10getCommandEv+0x104):
undefined reference to `command::com'
command.cpp:(.gnu.linkonce.t._ZN7command10getCommandEv+0x135):
undefined reference to `command::com'
command.cpp:(.gnu.linkonce.t._ZN7command10getCommandEv+0x14e):
undefined reference to `command::com'
/tmp/ccNSmcoY.o:command.cpp:(.gnu.linkonce.t._ZN7command10getCommandEv+0x165):
more undefined references to `command::com' follow
collect2: ld returned 1 exit status

If anyone can find something please let me know. thanks in advance.