Ros2

ros2

The foundation tool skill for the robium ROS vertical. Everything that touches ROS 2 itself — workspaces, colcon, package anatomy, nodes, topics/services/ actions, QoS, launch files, parameters, TF2, rosdep, and gluing third-party ROS packages together — lives here. nav2, gazebo, and rviz2 all assume this skill's content and cross-reference it rather than re-explaining ROS 2 basics; load this skill alongside any of them. This skill is ROS 2 only — ROS 1 reached end-of-life with Noetic Ninjemys and is out of scope everywhere in robium.

When to use this skill

• Any ROS 2 development or debugging task: creating a package, building a workspace, writing nodes, wiring topics/services/actions, writing launch files, setting parameters, working with TF2, resolving dependencies.
• The trigger phrases in the description: 'ros2', 'colcon', 'launch file', 'package.xml', 'QoS mismatch', 'TF', 'node not receiving messages', 'rosdep'.
• A ROS 2 node isn't receiving messages it should be — start here (QoS is the first suspect), not in application logic.
• Cross-references — go to the sibling skill instead when the question is:
  • Autonomous navigation specifics (costmaps, planners, controllers, behavior trees, AMCL/localization) → nav2. This skill covers the ROS 2 substrate Nav2 runs on, not navigation algorithms.
  • Simulating a ROS 2 robot → gazebo.
  • Visualizing ROS 2 data → rviz2 (local display) or foxglove (remote/ headless).
  • Choosing uv vs Docker, or macOS/GPU environment setup → environments.
  • Module boundaries, non-ROS transports at a system boundary, Dockerfiles/ compose for a multi-module app → integration. This skill's "bridge two third-party packages" pattern below is intra-system (still ROS 2 topics); crossing to a non-ROS peer is integration's call.
  • The whole-stack decision this feeds into → architect (routes here).

Key directives

• Delegation posture: embed. No good upstream "how do I use ROS 2" skill exists to point to — this content lives here, in depth, not as a thin pointer to docs.ros.org. Link out only for exact version/tag facts that change release-to-release.
• Always rosdep install before building. A workspace that hasn't had rosdep install --from-paths src -y --ignore-src run against it is not ready to build — missing system/package dependencies produce confusing colcon failures that look like source bugs. Run rosdep first, every time a new package or a fresh clone enters the workspace, not just on first setup. See references/workspace-and-packages.md.
• QoS compatibility is the first suspect for silent topic failures. A node that runs cleanly, discovers its peer, and still exchanges zero messages is almost always a QoS mismatch (e.g. one side RELIABLE, the other BEST_EFFORT), not a code bug — DDS drops the connection silently with no error on either side. Check ros2 topic info -v before debugging anything else. See references/interfaces-and-qos.md and references/debugging.md.
• Prefer workspace overlays over editing third-party package source. When a third-party ROS package needs different behavior, put your changes in an overlay package (a new package, or a COLCON_IGNOREd fork built on top) rather than hand-editing files inside an installed or vendored package. Overlays survive rosdep update/reinstalls and keep the diff visible; in-place edits to third-party source silently rot. See references/workspace-and-packages.md.
• Never write distro, tag, or API-surface facts from memory. ROS 2's distro cadence, package availability, and even some API idioms (e.g. the rclpy.init() context-manager form) change release to release. Verify before repeating a claim in a real project — every example in this skill is marked status: unverified for exactly this reason.

Quick start

1. Confirm the workspace exists and rosdep is current:

mkdir -p ~/ros2_ws/src && cd ~/ros2_ws
source /opt/ros/lyrical/setup.bash
rosdep update
rosdep install --from-paths src -y --ignore-src

2. Build and source the overlay:

colcon build --symlink-install
source install/setup.bash

3. Run something. The examples/package-ament-python/ directory in this skill is a minimal, internally-consistent ament_python package (one parameterized publisher node, one launch file) — copy it into src/, rebuild, and run with ros2 run ros2_example_pkg talker or ros2 launch ros2_example_pkg talker.launch.py.

For any step beyond this, see the matching usage pattern below and the references it points to.

Usage patterns

Create a package → build → run. ros2 pkg create --build-type ament_python --license Apache-2.0 --node-name <node> <package> scaffolds package.xml, setup.py, setup.cfg, the resource marker, and a starter node. Fill in package.xml's <description>/<maintainer>, add real dependencies, colcon build --symlink-install from the workspace root, source install/setup.bash, then ros2 run <package> <executable>. See references/workspace-and-packages.md and examples/package-ament-python/.

Add a dependency. Declare it in package.xml as <exec_depend> (runtime) or <build_depend> (build-time C++), then re-run rosdep install --from-paths src -y --ignore-src before rebuilding — adding the tag alone does not install the underlying apt/pip package. See references/workspace-and-packages.md.

Write a launch file. Python launch files are the default choice (XML/YAML exist but are thinner and less composable): generate_launch_description() returning a LaunchDescription of Node actions, with DeclareLaunchArgument/LaunchConfiguration for anything that should be overridable from the command line, and the launch directory registered in setup.py's data_files plus <exec_depend>launch</exec_depend> / <exec_depend>launch_ros</exec_depend> in package.xml. See references/launch-patterns.md and examples/package-ament-python/launch/talker.launch.py.

Bridge two third-party packages (remap + relay). When two existing packages almost line up but use different topic names or message shapes, prefer wiring them at the launch/CLI layer over patching either package's source: remappings=[('from_topic', 'to_topic')] on a Node action (or <remap> in XML) for a straight rename, and ros2 run topic_tools relay <in> <out> (or relay_field for a field-level republish) when the fix needs to live as its own running node rather than a launch-time rename. This stays inside one ROS 2 system — crossing to a non-ROS peer is integration's call, not this pattern. See references/launch-patterns.md.

Parameterize a node. Call self.declare_parameter('name', default) in the node's __init__, read it with self.get_parameter('name').value (or the typed .get_parameter_value() accessors), and feed it from a launch file's parameters=[{...}] list, a YAML params file, or --ros-args -p name:=value on the CLI — don't hardcode values a launch file should own. See references/interfaces-and-qos.md and examples/package-ament-python/ros2_example_pkg/talker_node.py.

Platform gotchas

• macOS has no native ROS 2 — Docker only. There is no supported native ROS 2 install on macOS/Apple Silicon; every ROS 2 workflow on a Mac dev machine runs inside Docker, even for local iteration. Don't try to pip install/homebrew a native ROS 2 as a shortcut. See the environments skill's Docker patterns and its ROS 2 base-image guidance.
• ROS_DOMAIN_ID collisions are silent. All ROS 2 nodes default to domain ID 0; two unrelated ROS 2 systems on the same network segment with the same domain ID will discover and cross-talk with each other with no error. Export a project-unique ROS_DOMAIN_ID (export ROS_DOMAIN_ID=<n>) in every shell/container that runs this project's nodes, the same way you'd pick a non-default port.
• Shell sourcing order matters. Source the underlay (/opt/ros/<distro>/ setup.bash) before the workspace overlay (install/setup.bash) — each setup.bash only extends the environment the previous one built, so sourcing the overlay alone (or in the wrong order) silently drops the underlay's paths. A fresh shell that skips sourcing entirely is the most common "package not found" / "command not found: ros2" report — check this before anything else.
• set -u before sourcing a ROS setup script kills the script. ROS's own setup.bash reads unset variables, so a strict-mode wrapper (set -euo pipefail at the top, then source /opt/ros/<distro>/ setup.bash) aborts with an unbound-variable error from inside ROS's script — an alarming failure that has nothing to do with your code. Order it the other way: source first, then set -u. Verified 2026-07-11 (nav-trial).
• ros2 launch as container PID 1 ignores SIGTERM. The kernel drops unhandled signals to PID 1, and launch installs no SIGTERM handler — so the normal teardown path (docker stop, or an in-container os.kill(1, SIGTERM)) is a silent no-op and the container sits there until the 10 s timeout escalates to SIGKILL, taking the sim down hard. SIGINT hits launch's real shutdown path (clean exit 0, nodes shut down in order — verified in-container 2026-07-12, nav-trial demo). Either send SIGINT, or don't run launch as PID 1: an init shim (docker run --init, tini) reaps and forwards signals properly and is the better default for any launch-as-entrypoint image.
• TurtleBot 4: the Create 3 base is invisible to the Pi until you drop the stock CYCLONEDDS_URI interface restriction. /etc/turtlebot4/setup.bash exports CYCLONEDDS_URI=/etc/turtlebot4/cyclonedds_rpi.xml, whose <Interfaces> block pins CycloneDDS to wlan0, so it never discovers the base over usb0 — ros2 node list shows no /motion_control, no /battery_state, and /cmd_vel reaches nothing (the robot won't drive). /scan still streams because the RPLIDAR is a Pi-local node, which masks the break completely — seeing the scan proves nothing about the base link. Fix: comment out that export (CycloneDDS then defaults to all interfaces), sudo systemctl restart turtlebot4.service, and restart any foxglove_bridge so it inherits the env. Dead-ends that did NOT work: uncommenting usb0 in the xml, a Create 3 reboot (POST http://192.168.186.2/api/reboot), ntpd resync, a full Pi reboot, and matching the Pi/base clocks (the clock is a red herring — the Pi has no RTC and reboots to a stale date, and the base syncs NTP from the Pi, so don't manually jump the Pi clock or reboot it). Verified 2026-07-24 (tb4-teleop): base nodes appeared, /battery_state → 0.99, robot drove.
• Inspecting Create 3 (best-effort) topics from the CLI. ros2 topic echo defaults to RELIABLE QoS and silently receives nothing from the base's best-effort publishers — pass --qos-reliability best_effort. And ros2 topic pub --once /cmd_vel … HANGS on "Waiting for at least 1 matching subscription" because the base subscribes in a separate DDS realm invisible to the publisher (so ros2 topic info /cmd_vel also shows 0 subscribers even when working) — add -w 0 to publish without waiting. Verified 2026-07-24 (tb4-teleop).

Customization

• Different ROS 2 distro: this skill's commands are written against Lyrical Luth, the current LTS. Swap lyrical for another distro name in /opt/ros/<distro>/setup.bash and in the example package's dependency versions; re-verify package availability for that distro first. The ROS 2 + Nav2 + Gazebo navigation vertical currently defaults to Jazzy Jalisco instead, because Nav2 has not yet shipped binaries for Lyrical — see nav2's and architect's Platform gotchas for the current status before picking a distro for that path.
• ament_cmake instead of ament_python: the workspace/colcon/rosdep/launch mechanics in this skill are build-type-agnostic; only package internals differ (a CMakeLists.txt build/install pipeline instead of setup.py/setup.cfg, with find_package/ament_target_dependencies instead of Python imports). See references/workspace-and-packages.md for both anatomies side by side.
• Different node/topic names in the example package: package.xml's <name>, setup.py's package_name/console-script entry point, and the launch file's package=/executable= must all agree — rename all four places together (see examples/package-ament-python/) rather than drifting one and hitting a confusing ros2 run/ros2 launch "not found".

References

• references/workspace-and-packages.md — workspace layout, colcon build mechanics, underlay/overlay sourcing, ament_python vs ament_cmake package anatomy, rosdep workflow, adding dependencies.
• references/launch-patterns.md — Python launch files in depth: Node actions, launch arguments, parameter files, remapping, includes, and the remap+relay bridging pattern.
• references/interfaces-and-qos.md — topics/services/actions overview, the full QoS policy set, compatibility rules, preset profiles, and TF2 basics.
• references/debugging.md — the ros2 doctor/ros2 topic/ros2 node introspection toolkit, common failure signatures (unsourced shell, domain ID collision, QoS mismatch, missing rosdep install) and how to tell them apart.
• examples/package-ament-python/ — a minimal, internally-consistent ament_python package: package.xml, setup.py, setup.cfg, one parameterized publisher node, one launch file (status: unverified — each file links its own upstream source).
• Upstream: ROS 2 documentation (blocked for direct fetch on 2026-07-10 — verified via the ros2/ros2_documentation GitHub repo through ctx7 instead; re-check docs.ros.org directly when it's reachable), colcon documentation, ros2/launch, ros-tooling/topic_tools. Sibling skills: nav2, gazebo, rviz2 (load alongside), environments (macOS/Docker setup), integration (cross-boundary comms, compose), architect (routes here).

Changelog

• 1.2.0 (2026-07-24): tb4-teleop absorption — two TurtleBot 4 / Create 3 gotchas that cost ~7 failed resets: the stock CYCLONEDDS_URI interface restriction blocks base discovery over usb0 (robot won't drive; /scan masks it — disable the export), and Create 3 base topics need --qos-reliability best_effort to echo plus -w 0 to ros2 topic pub (split-DDS hides the subscriber). Clock-skew ruled out as a red herring.

• 1.1.0 (2026-07-13): nav-trial absorption — two container/shell gotchas that cost real debugging time: set -u before sourcing a ROS setup script aborts on ROS's own unset vars (source first, then set -u), and ros2 launch as PID 1 ignores SIGTERM (kernel drops unhandled signals to PID 1) so teardown must use SIGINT or an init shim. Both were previously captured only in demo-specific notes.

• 1.0.1 (2026-07-12): skill-refiner run 1 — provenance claims date-stamped ('this session' → 2026-07-10, the authoring session) so the staleness sweep can age them.