todo.txt

http://todotxt.org/ (incl. list of apps which support 'todo.txt', all OS)
- Very simple todo list manager (GTD: getting things done).
- Syntax: https://github.com/todotxt/todo.txt
Share the file 'todo.txt' via drive.math.uzh.ch. Configure all apps to use shared file.

Thinlinc

GUI: Applications > Office > QTodoTxt

CLI: todo.sh

Setup: create a file ~/.todo/config

export TODO_DIR=<full path. E.g. '/home/a/crose/Nextcloud/todo'>
export TODO_FILE="$TODO_DIR/todo.txt"
export DONE_FILE="$TODO_DIR/done.txt"
export REPORT_FILE="$TODO_DIR/report.txt"

Use: open a terminal and type todo.sh -h

Thunderbird Extension 'Todo.txt': https://addons.thunderbird.net/en-US/thunderbird/addon/todotxt-extension/

Android: SimpleTask