Setup Recovery: Diagnose Before Reinstalling

When a beginner ROS install fails, the fastest emotional reaction is often:

uninstall everything and start over.

That destroys evidence.

Use a failure ladder instead.

Failure 1: ros2: command not found

Check:

ls /opt/ros/jazzy
source /opt/ros/jazzy/setup.bash
which ros2
printenv | grep -i ROS

Reasoning:

• if /opt/ros/jazzy is missing, the installation layer is suspect;
• if it exists but ros2 appears only after sourcing, the installation is probably fine and the shell environment was the problem.

Failure 2: apt cannot find ros-jazzy-desktop

Inspect the layer before ROS package installation:

sudo apt update

Look for repository/signature errors. Revisit the ros2-apt-source installation instead of repeatedly requesting a package apt cannot discover.

Failure 3: ROS_DISTRO is not jazzy

Run:

printenv | grep -i ROS

A machine can contain multiple ROS distributions or previously sourced workspaces. Open a clean terminal and source only the intended Jazzy setup.

Failure 4: talker runs, listener hears nothing

First prove both terminals were sourced and both nodes are visible:

ros2 node list
ros2 topic list

Do not jump to advanced middleware tuning in a beginner setup exercise.

Failure record

Use:

Symptom:
Expected:
Observed:
Layer inspected first:
Command/evidence:
What the evidence ruled out:
Next action:

Unit gate

From a clean terminal, complete this sequence without copying unexplained commands:

source /opt/ros/jazzy/setup.bash
printenv | grep -i ROS
ros2 --help

Then run the talker/listener verification in two terminals.

Explain the path from Ubuntu package repositories to a communicating pair of nodes.

If you can do that, your Jazzy workstation is ready for the rest of the course.