Taskwarrior

Taskwarrior is an open-source, cross platform time and task management tool, used to keep track of and handle tasks. It uses a command-line interface, although since its inception, graphical user interface wrappers have also been created... According to its author, Taskwarrior was created "to address layout and feature issues"[4] in the Todo.txt applications popularized by Gina Trapani. https://en.wikipedia.org/wiki/Taskwarrior (single-user to-do list)

https://taskwarrior.org/

to sync, use a cloud-storage backend