Project 11: Status Bar and Mode Rendering

Render a status line with session info and a clock without disturbing pane content.

Quick Reference

Attribute Value
Difficulty Level 3: Advanced
Time Estimate 1 week
Main Programming Language C (Alternatives: Rust, Go)
Alternative Programming Languages Rust, Go
Coolness Level Level 3: UI Polish
Business Potential 1: The “Quality of Life”
Prerequisites screen buffers, timers
Key Topics status line, format tokens, timed updates

1. Learning Objectives

By completing this project, you will:

  1. Build a working implementation of status bar and mode rendering and verify it with deterministic outputs.
  2. Explain the underlying Unix and terminal primitives involved in the project.
  3. Diagnose common failure modes with logs and targeted tests.
  4. Extend the project with performance and usability improvements.

2. All Theory Needed (Per-Concept Breakdown)

Status Line Composition and Timed Redraw

  • Fundamentals Status Line Composition and Timed Redraw is the core contract that makes the project behave like a real terminal tool. It sits at the boundary between raw bytes and structured state, so you must treat it as both a protocol and a data model. The goal of the fundamentals is to understand what assumptions the system makes about ordering, buffering, and ownership, and how those assumptions surface as user-visible behavior. Key terms include: format tokens, truncate, timer, status row. In practice, the fastest way to gain intuition is to trace a single input through the pipeline and note where it can be delayed, reordered, or transformed. That exercise reveals why Status Line Composition and Timed Redraw needs explicit invariants and why even small mistakes can cascade into broken rendering or stuck input.

  • Deep Dive into the concept A deep understanding of Status Line Composition and Timed Redraw requires thinking in terms of state transitions and invariants. You are not just implementing functions; you are enforcing a contract between producers and consumers of bytes, and that contract persists across time. Most failures in this area are caused by violating ordering guarantees, dropping state updates, or misunderstanding how the operating system delivers events. This concept is built from the following pillars: format tokens, truncate, timer, status row. A reliable implementation follows a deterministic flow: Gather metadata -> Expand format tokens -> Pad or truncate to width -> Render single row. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness. When you design the project, treat each key term as a source of constraints. For example, if a term implies buffering, decide the buffer size and how overflow is handled. If a term implies state, decide how that state is initialized, updated, and reset. Finally, validate your assumptions with deterministic fixtures so you can reproduce bugs. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness.

  • How this fit on projects This concept is the backbone of the project because it defines how data and control flow move through the system.

  • Definitions & key terms

    • format tokens -> placeholders expanded into dynamic status values
    • truncate -> shorten a string to fit within a width
    • timer -> scheduled trigger that fires on an interval
    • status row -> the dedicated row where status text is rendered

  • Mental model diagram (ASCII)

[Input] -> [Status Line Composition and Timed Redraw] -> [State] -> [Output]

  • How it works (step-by-step, with invariants and failure modes)

    1. Gather metadata
    2. Expand format tokens
    3. Pad or truncate to width
    4. Render single row

  • Minimal concrete example

#{session_name} | #{pane_title} | %H:%M

  • Common misconceptions

    • “Status is just a string” -> it must not overwrite pane content.

  • Check-your-understanding questions

    • How do you update the clock without redrawing panes?
    • How do you handle long strings?

  • Check-your-understanding answers

    • Render only the status row.
    • Truncate or ellipsize at terminal width.

  • Real-world applications

    • tmux status line

  • Where you’ll apply it

  • References

    • tmux 3 Ch. 5

  • Key insights Status Line Composition and Timed Redraw works best when you treat it as a stateful contract with explicit invariants.

  • Summary You now have a concrete mental model for Status Line Composition and Timed Redraw and can explain how it affects correctness and usability.

  • Homework/Exercises to practice the concept

    • Implement a token expander for two tokens.

  • Solutions to the homework/exercises

    • Replace tokens with values in a buffer.

3. Project Specification

3.1 What You Will Build

A status bar renderer that formats session/window/pane metadata and updates on a timer without flicker.

3.2 Functional Requirements

  1. Requirement 1: Render a dedicated status row
  2. Requirement 2: Update clock on interval
  3. Requirement 3: Truncate long strings
  4. Requirement 4: Toggle status on/off

3.3 Non-Functional Requirements

  • Performance: Avoid blocking I/O; batch writes when possible.
  • Reliability: Handle partial reads/writes and cleanly recover from disconnects.
  • Usability: Provide clear CLI errors, deterministic output, and helpful logs.

3.4 Example Usage / Output

    $ ./mytmux
[status] [work] [pane:vim] [12:34]
[exit code: 0]

$ ./mytmux --status-format "#{very_long}" --cols 10
[error] status format exceeds terminal width
[exit code: 2]

3.5 Data Formats / Schemas / Protocols

    Status format tokens like #{session_name} and #{pane_title}.

3.6 Edge Cases

  • Terminal width too narrow
  • Long session names

3.7 Real World Outcome

This section defines a deterministic, repeatable outcome. Use fixed inputs and set TZ=UTC where time appears.

3.7.1 How to Run (Copy/Paste)

make
./mytmux

3.7.2 Golden Path Demo (Deterministic)

The “success” demo below is a fixed scenario with a known outcome. It should always match.

3.7.3 If CLI: provide an exact terminal transcript

    $ ./mytmux
[status] [work] [pane:vim] [12:34]
[exit code: 0]

Failure Demo (Deterministic)

    $ ./mytmux --status-format "#{very_long}" --cols 10
[error] status format exceeds terminal width
[exit code: 2]

3.7.8 If TUI

At least one ASCII layout for the UI:

    +------------------------------+
    | Status Bar and Mode Rendering           |
    | [content area]               |
    | [status / hints]             |
    +------------------------------+

4. Solution Architecture

4.1 High-Level Design

    +-----------+     +-----------+     +-----------+
    |  Client   | <-> |  Server   | <-> |  PTYs     |
    +-----------+     +-----------+     +-----------+

4.2 Key Components

| Component | Responsibility | Key Decisions | |-----------|----------------|---------------| | Formatter | Expands tokens into strings. | Keep a small token table. | | Status renderer | Writes a single row buffer. | Separate from pane composition. | | Timer | Triggers redraws. | Use event loop timers. |

4.4 Data Structures (No Full Code)

    struct Status { char fmt[128]; char line[256]; };

4.4 Algorithm Overview

Key Algorithm: Timed status update

  1. Format string
  2. Render status row
  3. Schedule next tick

Complexity Analysis:

  • O(n) in string length

5. Implementation Guide

5.1 Development Environment Setup

    cc --version
make --version

5.2 Project Structure

    status-bar/
|-- src/
|   |-- status.c
|   `-- format.c
`-- Makefile

5.3 The Core Question You’re Answering

“How do you render UI chrome without breaking pane content?”

5.4 Concepts You Must Understand First

  1. status line
    • Why it matters and how it impacts correctness.
  2. format tokens
    • Why it matters and how it impacts correctness.
  3. timed updates
    • Why it matters and how it impacts correctness.

5.5 Questions to Guide Your Design

  • Where does the status line live?
  • How often should it update?

    5.6 Thinking Exercise

Design a formatter for #{session_name} tokens.

5.7 The Interview Questions They’ll Ask

  • How do you update the clock without flicker?

    5.8 Hints in Layers

  • Render status into a separate buffer row.

5.9 Books That Will Help

| Topic | Book | Chapter | |——-|——|———| | tmux usage | tmux 3 | Ch. 5 |

5.10 Implementation Phases

Phase 1: Foundation (1 week)

Goals:

  • Establish the core data structures and loop.
  • Prove basic I/O or rendering works.

Tasks:

  1. Implement the core structs and minimal main loop.
  2. Add logging for key events and errors.

Checkpoint: You can run the tool and see deterministic output.

Phase 2: Core Functionality (1 week)

Goals:

  • Implement the main requirements and pass basic tests.
  • Integrate with OS primitives.

Tasks:

  1. Implement remaining functional requirements.
  2. Add error handling and deterministic test fixtures.

Checkpoint: All functional requirements are met for the golden path.

Phase 3: Polish & Edge Cases (1 week)

Goals:

  • Handle edge cases and improve UX.
  • Optimize rendering or I/O.

Tasks:

  1. Add edge-case handling and exit codes.
  2. Improve logs and documentation.

Checkpoint: Failure demos behave exactly as specified.

5.11 Key Implementation Decisions

Decision Options Recommendation Rationale
I/O model blocking vs non-blocking non-blocking avoids stalls in multiplexed loops
Logging text vs binary text for v1 easier to inspect and debug

6. Testing Strategy

6.1 Test Categories

Category Purpose Examples
Unit Tests Validate components parser, buffer, protocol
Integration Tests Validate interactions end-to-end CLI flow
Edge Case Tests Handle boundary conditions resize, invalid input

6.2 Critical Test Cases

  1. Status line truncates properly
  2. Timer updates clock

    6.3 Test Data

text Format string with long session name; expect truncation.

7. Common Pitfalls & Debugging

7.1 Frequent Mistakes

| Pitfall | Symptom | Solution | |———|———|———-| | Status overlaps panes | Layout not reserving row | Subtract status height from pane layout. |

7.2 Debugging Strategies

  • Render status row in isolation for tests.

    7.3 Performance Traps

  • Redrawing full screen for status updates.

8. Extensions & Challenges

8.1 Beginner Extensions

  • Add color to status line.
  • Add left/right sections.

    8.2 Intermediate Extensions

  • Add user-defined format string.
  • Add conditional tokens.

    8.3 Advanced Extensions

  • Add plugins that contribute status segments.

9. Real-World Connections

9.1 Industry Applications

  • CLI dashboards
  • tmux

    9.3 Interview Relevance

  • Event loops, terminal I/O, and state machines are common interview topics.

10. Resources

10.1 Essential Reading

11. Self-Assessment Checklist

11.1 Understanding

  • I can explain the core concept without notes
  • I can explain how input becomes output in this tool
  • I can explain the main failure modes

11.2 Implementation

  • All functional requirements are met
  • All test cases pass
  • Code is clean and well-documented
  • Edge cases are handled

11.3 Growth

  • I can identify one thing I’d do differently next time
  • I’ve documented lessons learned
  • I can explain this project in a job interview

12. Submission / Completion Criteria

Minimum Viable Completion:

  • Tool runs and passes the golden-path demo
  • Deterministic output matches expected snapshot
  • Failure demo returns the correct exit code

Full Completion:

  • All minimum criteria plus:
  • Edge cases handled and tested
  • Documentation covers usage and troubleshooting

Excellence (Going Above & Beyond):

  • Add at least one advanced extension
  • Provide a performance profile and improvement notes