ROS 2 Commands

Linux terminal commands

Introduction

When working with ROS 2, the terminal is your primary interface. It is essential for launching nodes, inspecting topic lists, monitoring sensor data, configuring parameters, executing programs, and debugging errors.

While the terminal does not replace a code editor, it is indispensable for gaining real-time insights into your system. If your quadcopter stops receiving sensor data, a node fails to launch, or a required topic is missing, the terminal is your first line of defense.

This guide provides a collection of essential Linux and ROS 2 commands frequently used during development and debugging.

Basic Linux Commands

Linux commands are used to navigate directories, manage files, launch programs, and monitor system processes.

Navigating Directories

Command	Description
pwd	Displays the current working directory path
ls	Lists files and folders in the current directory
ls -la	Lists all files (including hidden ones) with detailed permissions and sizes
cd ~/clover2_ws	Changes the directory to ~/clover2_ws
cd ..	Moves up one directory level

Working with Files and Folders

Command	Description
mkdir test_folder	Creates a new folder named test_folder
touch notes.txt	Creates an empty file named notes.txt or updates the timestamp of an existing one
cp notes.txt notes_copy.txt	Copies notes.txt to a new file named notes_copy.txt
mv notes_copy.txt old_notes.txt	Renames or moves a file
rm old_notes.txt	Deletes the file old_notes.txt

Warning

The rm command deletes files permanently; they are not moved to a trash bin. Always double-check your file path and name before pressing enter. Use rm -r with extreme caution, as it deletes a folder and all of its contents recursivel.

Viewing and Searching

Command	Description
cat notes.txt	Displays the entire content of notes.txt in the terminal
less notes.txt	Opens notes.txt in a scrollable viewer
grep "error" log.txt	Searches for the string error within log.txt
find . -name "*.py"	Searches for all Python files in the current directory and its subdirectories

In the command find . -name "*.py" the dot (.) represents the current directory. The -name parameter specifies the search criteria, and the pattern "*.py" matches any filename ending in .py.

To exit less viewer, press q.

Installing Packages

Command	Description
sudo apt update	Updates the local list of available packages from the repositories
sudo apt upgrade	Upgrades all currently installed packages to their latest versions
sudo apt install <package>	Installs a specific package

Warning

sudo executes commands with administrative (root) privileges. Always ensure you fully understand a command before running it with sudo to avoid unintended system changes.

Managing Processes

Command	Description
ps aux | grep ros2	Lists all running processes and filters for those containing ros2
top	Displays real-time system processes and resource usage
htop	An interactive, more user-friendly version of top
kill 12345	Terminates the process with the ID (PID) 12345

Basic ROS 2 Commands

The ros2 command is organized into subcommands. For example, ros2 topic list is used for interacting with topics, while ros2 node is used for managing nodes.

Example of ros2 node and ros2 topic commands

Help

Command	Description
ros2 --help	Displays general help for the ros2 command
ros2 topic --help	Displays help for topic-related subcommands
ros2 topic echo --help	Displays help for the echo subcommand and its parameters

The --help flag is invaluable if you forget specific syntax or need to view available parameters. You can append --help to almost any ROS 2 command to access its specific documentation.

Nodes

Command	Description
ros2 node list	Lists all currently active nodes
ros2 node info /camera_node	Displays the topics, services, and parameters associated with the /camera_node

If a node does not appear in the list, it has not been discovered by the system. This typically indicates that the node is not running or is not communicating on the same ROS_DOMAIN_ID.

Topics

Command	Description
ros2 topic list	Lists all available topics in the system
ros2 topic info /scan	Displays the message type and the number of publishers and subscribers for the /scan topic
ros2 topic echo --once /scan	Prints a single message from the /scan topic and then exits
ros2 topic hz /scan	Displays the current frequency (rate) at which messages are being published on the /scan topic

To print only a specific field from a message, use:

Command	Description
ros2 topic echo --once /scan --field ranges	Prints only the ranges field from a single message on the /scan topic

Services

Command	Description
ros2 service list	Lists all available services
ros2 service type /reset	Displays the service type for /reset
ros2 service call /reset std_srvs/srv/Empty "{}"	Sends a request to the /reset service using the std_srvs/srv/Empty interface

Services are used to trigger specific actions via a request-response mechanism: a client sends a request, and the service provides a response. For example, a service might be used to reset a device’s state. Note that specific quadcopter controls, such as arming or disarming, will depend on the specific packages and interfaces installed on your system.

Parameters

Command	Description
ros2 param list	Lists all parameters available to the running nodes
ros2 param get /camera_node exposure	Retrieves the current value of the exposure parameter for the /camera_node
ros2 param set /camera_node exposure 100	Sets the exposure parameter of the /camera_node to 100

Parameters can be used to adjust the publishing rate, switch a device’s operating mode, or modify other settings defined by the node developer.

Parameters are scoped to specific nodes. Therefore, the ros2 param list command must always be accompanied by a node name to be effective.

Launching Programs

Command	Description
ros2 pkg executables <package>	Lists all registered executable files within the specified <package>
ros2 run <package> <executable>	Launches a specific <executable> from the <package>
ros2 launch clover2 clover2.launch.py	Executes the clover2.launch.py launch file from the clover2 package

Files ending in .launch.py must be executed using the ros2 launch command, rather than ros2 run. If a package or executable cannot be found, first ensure that your environment has been properly sourced. You can then verify the availability of your files using these commands:

Command	Description
ros2 pkg list | grep clover	Searches for installed packages containing the string clover
ros2 pkg executables clover2	Lists all executable files available in the clover2 package

While ros2 run is convenient for launching a single node, most real-world projects utilize ros2 launch. This is because complex systems typically require multiple nodes to be launched and configured simultaneously.

Recording Data

Command	Description
ros2 bag record /scan	Records messages from the /scan topic into a bag file
ros2 bag record -a	Records messages from all currently available topics
ros2 bag play rosbag2_2026_05_22-12_00_00	Replays the recorded data from the specified bag file

Recording data into bag files is essential when you need to save sensor data to replay later, allowing you to test algorithms without a physical robot or live sensors.

Practical Debugging Scenario

If you are expecting data on the /scan topic but are not receiving it, follow this systematic troubleshooting workflow:

1. Verify the required node is running:

   Command	Description
   ros2 node list	Lists all active nodes

2. Locate the topic:

   Command	Description
   ros2 topic list | grep scan	Filters the topic list to find any topic containing scan

3. Inspect the topic details:

   Command	Description
   ros2 topic info /scan	Checks the message type and verifies if there are active publishers and subscribers

4. Test the data stream:

   Command	Description
   ros2 topic echo --once /scan	Attempts to print a single message to verify data is actually flowing
   ros2 topic hz /scan	Monitors the frequency of incoming messages to check for stability

• If the topic is missing from the list. Check if the node responsible for that topic is actually running.

• If the topic exists but no messages appear. Check the physical device connection, verify the driver’s parameters, and inspect the terminal where the node was launched for error logs.