Todo CLI

This project is a small command-line todo application written in Go. It was created as a learning project to understand two basic Go topics:

• working with maps
• encoding and decoding JSON files

The application stores todos locally in todos.json and provides a simple interactive command loop. Todos can be marked as not started, in progress, or done.

Requirements

• Go 1.25 or newer

Running the application

From the project directory, run:

go run .

The program then waits for a command. The prompts and status messages in the current implementation are in German, while this documentation is in English.

Available commands

Command	Description
list	Displays all current todos grouped by status and their indexes.
add	Asks for a new todo and adds it with the Not started status.
working	Asks for an index and marks the todo as Working on.
done	Asks for an index and marks the todo as Done.
remove	Asks for an index and removes the corresponding todo.
edit	Asks for an index and replaces the todo text.
quit / exit	Saves the list to todos.json and exits.

Example session:

list
add
Read about Go maps
working
2
done
2
quit

How it works

1. Loading the todos

At startup, the program creates an empty map with integer keys and string values:

list := map[int]string{}

It then tries to read todos.json with os.ReadFile. If the file exists, json.Unmarshal converts the JSON data into the Go map. If the file does not exist, a small default list is created in memory.

The JSON file represents the map like this:

{
  "0": "TODO-STATUS-NONE!DATA-INFORMATION!Buy groceries",
  "1": "TODO-STATUS-DONE!DATA-INFORMATION!Rent a car"
}

JSON object keys are strings, but Go converts them to integer map keys when unmarshalling into map[int]string.

2. Reading commands

The program uses bufio.Scanner to read input from the terminal. Each input is trimmed with strings.TrimSpace and processed by a switch statement.

The loop continues until the user enters quit or exit. Invalid commands and input errors are reported in the terminal. Indexes shown to the user start at 1, while the map uses zero-based keys internally.

3. Adding a todo

When add is selected, the entered text is prefixed with the Not started status marker and stored in the map using the current map length as the key:

list[len(list)] = TodoStatusNone.key + "!DATA-INFORMATION!" + todo

This works for the normal add flow, but it is intentionally simple. After a todo is removed, map indexes can be rebuilt, so this is not a permanent unique ID system.

4. Updating and removing todos

The working and done commands update the status marker of the selected todo. The edit command replaces the selected map value with the entered text. remove deletes a todo by creating a new map without the selected item.

For all commands that require an index, the input is converted from a string to an integer with strconv.Atoi and checked against the current list length.

The removeTodoFromList function receives a pointer to the map. It creates a new map, copies every todo except the selected one, and assigns the new map back to the original variable. This also rebuilds the indexes from zero.

5. Saving the todos

The list is saved when quit or exit is entered. It is also saved when the process receives an interrupt or termination signal. json.MarshalIndent converts the Go map to readable JSON, and os.WriteFile writes the result to todos.json with file permissions 0644.

Project structure

.
├── main.go     # CLI logic, map operations, and JSON handling
├── todos.json  # Persisted todo data
├── go.mod      # Go module definition
└── README.md   # Project documentation

What this project demonstrates

• declaring and modifying a map[int]string
• iterating over a map with range
• passing a map to a function
• passing a pointer to a map when replacing the map itself
• reading and writing files
• converting between Go values and JSON with encoding/json
• validating and converting terminal input
• handling basic errors in Go

Current limitations

• A map is used as an indexed list, which is useful for learning but not ideal for a production todo application.
• The map iteration order is not guaranteed, so the order in which items are displayed or reindexed can vary.