Very simple and minimal todo list CLI.

todo.cpp

#include <iostream>
#include <fstream>
#include <string>
#include <cstring>

/* Installation:
 * Compile from source: g++ todo.cpp -o todo -O3 
 * Put it into path: sudo mv ./todo /usr/local/bin
 *
 * Usage:
 * todo add <some task> (Multiple word tasks supported, no quotes needed)
 * todo list (List the tasks)
 * todo done <num> (Complete the task number shown in "todo list") 
 *
 * Tasks are stored in a plaintext file at $HOME/.todo
 *
 */ 

const string HOME = getenv("HOME");

void ensure_file_exists(){
    std::ifstream file(HOME + "/.todo");
    if (!file){
        file.close();
        std::ofstream file(HOME + "/.todo");
        file.close();
    }
}

void list_tasks(){
    std::ifstream file(HOME + "/.todo");
    std::string task;
    int num = 1;
    while (std::getline(file, task)){
        std::cout << '[' << num << "] " << task << '\n';
        ++num;
    }
    file.close();
}

void add_task(int argc, char** argv){
    std::ofstream file(HOME + "/.todo", std::ios::app);
    for (int i=2; i<argc; ++i){
        file.write(argv[i], strlen(argv[i]));
        file << ' ';
    }
    file << '\n';
    file.close();
}

bool remove_task(int task){
    if (task < 1)
        return false;
    std::ifstream file(HOME + "/.todo");
    std::ofstream tmp(HOME + "/.todotmp");
    int i = 0;
    std::string line;
    while (std::getline(file, line)){
        ++i;
        if (i != task)
            tmp << line << '\n';
    }
    file.close();
    tmp.close();
    std::remove((HOME + "/.todo").c_str());
    std::rename((HOME + "/.todotmp").c_str(), (HOME + "/.todo").c_str());
    return i >= task;
}

int main(int argc, char** argv){
    ensure_file_exists();
    if (argc==1){
        std::cout << "todo: Command requires an action\n";
    } else {
        if (!strcmp(argv[1], "add")){
            add_task(argc, argv);
            std::cout << "todo: Add successful\n";
        } else if (!strcmp(argv[1], "list")){
            std::cout << "todo:\n";
            list_tasks();
        } else if (!strcmp(argv[1], "done")){
            if (remove_task(atoi(argv[2]))){
                std::cout << "todo: Task completion successful\n";
            } else {
                std::cout << "todo: Task completion failed (Task # does not exist)\n";
            }
        } else {
            std::cout << "todo: Invalid action\n";
        }
    }
}