rostopic command-line tools - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding rostopic echo output

What will rostopic echo /chatter display when the topic /chatter publishes messages of type std_msgs/String with the data field set to "hello world"?

A It prints the message data as plain text lines: <br>data: "hello world"

B It prints only the topic name repeatedly without message content

C It shows a list of all topics but no message data

D It throws an error because <code>rostopic echo</code> cannot read string messages

💡 Hint

Think about what rostopic echo does when subscribed to a topic.

rostopic echo prints the content of messages published on a topic. For std_msgs/String, it shows the data field with the string value.

❓ component_behavior

intermediate

Behavior of rostopic hz command

What does the command rostopic hz /sensor_data display when the topic /sensor_data is publishing messages at 10 Hz?

A It lists all topics currently active in the ROS system

B It prints all message contents published on <code>/sensor_data</code>

C It shows the average publishing rate close to 10 Hz updated every second

D It causes a syntax error because <code>rostopic hz</code> requires a message type argument

💡 Hint

Consider what measuring frequency means for a topic.

rostopic hz subscribes to a topic and calculates the message publishing frequency, showing an average rate.

📝 Syntax

advanced

Correct syntax for publishing a message with rostopic pub

Which command correctly publishes a single message with data "42" to the topic /numbers of type std_msgs/Int32?

A rostopic pub /numbers std_msgs/Int32 data=42 --once

B rostopic pub /numbers std_msgs/Int32 --data 42 --once

C rostopic pub /numbers std_msgs/Int32 "42" --once

D rostopic pub /numbers std_msgs/Int32 "data: 42" --once

💡 Hint

Remember the YAML format for message data in rostopic pub.

The message data must be provided in YAML format inside quotes, specifying the field name and value.

🔧 Debug

advanced

Identifying error cause in rostopic pub

You run rostopic pub /cmd_vel geometry_msgs/Twist "linear: 1.0, angular: 0.5" --once but get a syntax error. What is the most likely cause?

A The topic name <code>/cmd_vel</code> is invalid and causes the error

B The message fields are incorrectly formatted; nested fields require indentation and proper YAML structure

C The message type <code>geometry_msgs/Twist</code> does not exist in ROS

D The <code>--once</code> flag is not supported with <code>rostopic pub</code>

💡 Hint

Think about how YAML represents nested data.

Nested message fields require proper indentation and YAML syntax. Writing them inline with commas causes syntax errors.

❓ state_output

expert

Effect of rostopic bw on topic bandwidth measurement

What does rostopic bw /camera/image_raw display when the topic publishes 5 messages per second, each about 2 MB in size?

A It shows the bandwidth as approximately 10 MB/s, updating live

B It shows the message count but not bandwidth

C It shows the bandwidth as 5 bytes per second due to a unit error

D It throws an error because <code>rostopic bw</code> only works with numeric message types

💡 Hint

Bandwidth is message size times message rate.

rostopic bw calculates the data rate by multiplying message size by frequency, showing bandwidth in bytes per second.

Practice

1. What is the main purpose of the rostopic command-line tools in ROS?

easy

A. To view and send messages between different parts of a robot

B. To compile ROS packages

C. To control robot motors directly

D. To create new ROS nodes automatically

Solution

1. Step 1: Understand the role of rostopic tools

   The rostopic tools are designed to interact with ROS topics, which are channels for messages between robot parts.

2. Step 2: Identify the correct purpose

   They allow users to view messages, publish test messages, and check topic information, not compile or control hardware directly.

3. Final Answer:

   To view and send messages between different parts of a robot -> Option A

4. Quick Check:

   rostopic = message viewing and sending [OK]

Hint: Remember: rostopic is for message communication, not building code [OK]

Common Mistakes:

• Confusing rostopic with build tools like catkin
• Thinking rostopic controls hardware directly
• Assuming rostopic creates nodes automatically

2. Which of the following is the correct syntax to list all active topics using rostopic?

easy

A. rostopic list

B. rostopic show

C. rostopic echo

D. rostopic pub

Solution

1. Step 1: Recall rostopic commands

   rostopic list shows all active topics, rostopic show shows message type, rostopic echo prints messages, and rostopic pub publishes messages.

2. Step 2: Match command to listing topics

   Only rostopic list lists all active topics.

3. Final Answer:

   rostopic list -> Option A

4. Quick Check:

   List topics = rostopic list [OK]

Hint: List topics? Use 'rostopic list' command [OK]

Common Mistakes:

• Using 'rostopic show' to list topics
• Confusing 'rostopic echo' with listing
• Trying 'rostopic pub' to list topics

3. What will the command rostopic echo /chatter do when executed?

medium

A. Show the message type of /chatter topic

B. Publish messages to the /chatter topic

C. List all topics including /chatter

D. Display messages being published on the /chatter topic

Solution

1. Step 1: Understand the 'rostopic echo' command

   rostopic echo prints messages from a topic to the console in real-time.

2. Step 2: Apply to /chatter topic

   Running rostopic echo /chatter will display all messages currently being published on the /chatter topic.

3. Final Answer:

   Display messages being published on the /chatter topic -> Option D

4. Quick Check:

   Echo = show messages live [OK]

Hint: Echo means show messages live on a topic [OK]

Common Mistakes:

• Thinking echo publishes messages
• Confusing echo with listing topics
• Assuming echo shows message types

4. You run rostopic pub /chatter std_msgs/String "data: 'hello'" but get an error. What is the likely cause?

medium

A. You forgot to source your ROS environment

B. The topic /chatter does not exist

C. Incorrect message type syntax; quotes are misplaced

D. The std_msgs package is not installed

Solution

1. Step 1: Check the syntax of the publish command

   The correct syntax for publishing a string message requires proper YAML formatting and quotes around the entire message argument.

2. Step 2: Identify the error cause

   Using double quotes inside double quotes without escaping causes syntax errors. The message should be wrapped properly, e.g., single quotes outside or escaped quotes inside.

3. Final Answer:

   Incorrect message type syntax; quotes are misplaced -> Option C

4. Quick Check:

   Fix quotes in message syntax [OK]

Hint: Check quotes carefully when publishing messages [OK]

Common Mistakes:

• Assuming topic must exist before publishing
• Forgetting to source ROS setup
• Not installing required message packages

5. You want to monitor messages on multiple topics at once and save them to a file for later analysis. Which combination of rostopic commands and shell tools is best?

hard

A. Use rostopic echo /topic1 /topic2 > output.txt to save messages from both topics

B. Use rostopic echo -b bagfile.bag -p /topic1 /topic2 > output.csv after recording a bag file

C. Run rostopic echo /topic1 and rostopic echo /topic2 in separate terminals and manually copy output

D. Use rostopic pub /topic1 and rostopic pub /topic2 to save messages

Solution

1. Step 1: Understand monitoring multiple topics

   rostopic echo can show messages from one topic at a time; to handle multiple topics, recording a bag file is better.

2. Step 2: Use bag files and echo with -b option

   Recording with rosbag record /topic1 /topic2 saves messages, then rostopic echo -b bagfile.bag -p /topic1 /topic2 exports them to CSV for analysis.

3. Step 3: Evaluate options

   Use rostopic echo /topic1 /topic2 > output.txt to save messages from both topics is invalid syntax, C is manual and inefficient, D publishes messages instead of saving.

4. Final Answer:

   Use rostopic echo -b bagfile.bag -p /topic1 /topic2 > output.csv after recording a bag file -> Option B

5. Quick Check:

   Use bag files and rostopic echo -b for multiple topics [OK]

Hint: Record bag files to handle multiple topics easily [OK]

Common Mistakes:

• Trying to echo multiple topics directly without bag files
• Copy-pasting from multiple terminals manually
• Confusing publishing with saving messages