dir > List.txt

dir > list.txt 2>&1

/*
  Tee utility by VladD
  Use it in whichever way you like
  Hopefully it doesn't do anything bad, but no warranty
  Compiles with MSVC 2012
*/

#include "stdafx.h" // comment this line out if compiling with non-Microsoft compiler

#include <iostream>
#include <fstream>
#include <vector>
#include <string>
#include <cstdlib>

using namespace std;

bool append = false;
char** curr_arg;
vector<ostream*> target_streams;
vector<string> stream_names;

bool have_open_error = false;
bool have_io_error = false;

void evaluate_args()
{
    for (; *curr_arg; curr_arg++)
    {
        string arg(*curr_arg);
        if (arg == "--")
        {
            curr_arg++;
            break;
        }
        else if (arg == "-a")
        {
            append = true;
        }
        // add more arguments here
        else if (arg.size() > 0 && arg[0] == '-')
        {
            cerr << "Unknown argument: " << arg << endl;
            exit(1);
        }
        else
        {
            break;
        }
    }
}

void create_streams()
{
    auto openmode = ios::out | (append ? ios::app : ios::trunc);

    target_streams.push_back(&cout);
    stream_names.push_back("-");

    for (; *curr_arg; curr_arg++)
    {
        string arg(*curr_arg);
        ostream* p;
        if (arg == "-")
        {
            p = &cout;
        }
        else
        {
            p = new ofstream(arg, openmode);
            if (!*p)
            {
                delete p;
                cerr << "Cannot open file `" << arg << "'" << endl;
                have_open_error = true;
                return;
            }
        }
        target_streams.push_back(p);
        stream_names.push_back(arg);
    }
}

void destroy_streams()
{
    for (auto p : target_streams)
    {
        if (p != &cout)
            delete p;
    }
}

void process()
{
    string line;
    while (getline(cin, line))
    {
        for (size_t i = 0; i < target_streams.size(); i++)
        {
            auto p = target_streams[i];
            if (!p)
                continue;
            (*p) << line << endl;
            if (!*p)
            {
                cerr << "Error occured writing to stream `" <<
                        stream_names[i] << "'" << endl;
                delete p;
                target_streams[i] = nullptr;
                have_io_error = true;
            }
        }
    }
}

int main(int argc, char* argv[])
{
    curr_arg = argv + 1;
    evaluate_args();

    create_streams();
    process();

    destroy_streams();
    return (have_open_error ? 0x2 : 0) | (have_io_error ? 0x4 : 0);
}

tee [ -ai ] [ File ... ]

-a      Append the output to the files rather than overwriting them.

 -i      Ignore the SIGINT signal.

dir > C:log.txt && dir

dir >list.txt >&2

dir >list.txt 1>&2