rosservice command-line tools - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding rosservice list output

What does the command rosservice list output when run in a ROS environment?

A A list of all available ROS services currently registered with the ROS master.

B A list of all parameters stored on the ROS parameter server.

C A list of all nodes currently running in the ROS system.

D A list of all active ROS topics currently publishing messages.

💡 Hint

Think about what rosservice commands are used for.

The rosservice list command shows all services registered with the ROS master. Services are like functions that nodes can call. Topics and parameters are different concepts in ROS.

❓ component_behavior

intermediate

Behavior of rosservice call with incorrect arguments

What happens when you run rosservice call /add_two_ints 1 but the service expects two integers as arguments?

A The service call ignores the missing argument and returns a default value.

B The service call succeeds and returns the sum of 1 and 0.

C The service call blocks indefinitely waiting for the second argument.

D The command returns an error indicating incorrect number of arguments.

💡 Hint

Consider how ROS services validate input arguments.

ROS services require the exact number and type of arguments. If arguments are missing, rosservice call returns an error about argument mismatch.

📝 Syntax

advanced

Correct syntax for calling a service with multiple arguments

Which of the following rosservice call commands correctly calls the service /set_pose with arguments x=1.0, y=2.0, and theta=0.5?

A rosservice call /set_pose {x:1.0, y:2.0, theta:0.5}

B rosservice call /set_pose "x: 1.0 y: 2.0 theta: 0.5"

C rosservice call /set_pose 1.0 2.0 0.5

D rosservice call /set_pose x=1.0 y=2.0 theta=0.5

💡 Hint

Check the official rosservice call argument format for positional arguments.

The rosservice call command expects positional arguments separated by spaces. Quoting or using dictionary-like syntax is invalid here.

🔧 Debug

advanced

Diagnosing rosservice call failure due to service unavailability

You run rosservice call /compute_path but get the error: ERROR: Service [/compute_path] not available. What is the most likely cause?

A The service name is misspelled or does not exist on the ROS master.

B The ROS master is not running, so no services are available.

C The service exists but the node providing it crashed after registration.

D The service call syntax is incorrect causing the error.

💡 Hint

Check the service name carefully and whether it is registered.

The error means the service name is not found on the ROS master. This usually happens if the name is wrong or the service was never started.

❓ state_output

expert

Output of rosservice type and args commands

Given a service named /toggle_light with type std_srvs/SetBool, what is the exact output of the commands rosservice type /toggle_light and rosservice args /toggle_light?

A <code>rosservice type /toggle_light</code> outputs <code>SetBool</code> and <code>rosservice args /toggle_light</code> outputs <code>bool value</code>.

B <code>rosservice type /toggle_light</code> outputs <code>std_srvs/SetBool</code> and <code>rosservice args /toggle_light</code> outputs <code>bool data</code>.

C <code>rosservice type /toggle_light</code> outputs <code>std_srvs/SetBool</code> and <code>rosservice args /toggle_light</code> outputs <code>bool value</code>.

D <code>rosservice type /toggle_light</code> outputs <code>SetBool</code> and <code>rosservice args /toggle_light</code> outputs <code>bool data</code>.

💡 Hint

Check the exact field names in the std_srvs/SetBool service definition.

The rosservice type command outputs the full service type including package name. The rosservice args command outputs the exact argument names and types as defined in the service. For std_srvs/SetBool, the argument is bool data.

Practice

1. What does the rosservice list command do in ROS?

easy

A. It shows all available services currently running in the ROS system.

B. It starts a new ROS service.

C. It deletes a specified ROS service.

D. It shows the status of ROS nodes.

Solution

1. Step 1: Understand the purpose of rosservice list

   This command is designed to display all active services in the ROS environment.

2. Step 2: Compare with other options

   Starting or deleting services and showing node status are done by other commands, not rosservice list.

3. Final Answer:

   It shows all available services currently running in the ROS system. -> Option A

4. Quick Check:

   List services = rosservice list [OK]

Hint: Remember: 'list' always shows all current services [OK]

Common Mistakes:

• Confusing service listing with node status
• Thinking it starts or deletes services
• Mixing up with rosnode commands

2. Which of the following is the correct syntax to call a ROS service named /add_two_ints with arguments a=2 and b=3 using rosservice?

easy

A. rosservice call /add_two_ints 2 3

B. rosservice call /add_two_ints {a: 2, b: 3}

C. rosservice call /add_two_ints --a 2 --b 3

D. rosservice call /add_two_ints a:=2 b:=3

Solution

1. Step 1: Recall rosservice call argument format

   Named arguments like a=2 and b=3 are passed using YAML dictionary syntax {a: 2, b: 3}.

2. Step 2: Check each option

   rosservice call /add_two_ints {a: 2, b: 3} uses the correct YAML format specifying arguments by name. A is positional (no names), B uses invalid flags, D uses unsupported :=.

3. Final Answer:

   rosservice call /add_two_ints {a: 2, b: 3} -> Option B

4. Quick Check:

   YAML {name: value} for named args [OK]

Hint: Use YAML '{name: value}' for named service arguments [OK]

Common Mistakes:

• Using positional args without naming a and b
• Using invalid flags like --a
• Using := instead of : in YAML

3. What will be the output of the command rosservice type /clear if the service /clear exists and is of type std_srvs/Empty?

medium

A. Error: Service type unknown

B. /clear is not a valid service

C. Service /clear has no type

D. std_srvs/Empty

Solution

1. Step 1: Understand rosservice type command

   This command returns the type of the specified service if it exists.

2. Step 2: Given service /clear exists and is std_srvs/Empty

   The command will output exactly the service type string.

3. Final Answer:

   std_srvs/Empty -> Option D

4. Quick Check:

   rosservice type returns service type string [OK]

Hint: rosservice type shows the service's message type [OK]

Common Mistakes:

• Assuming it shows service status instead of type
• Thinking it returns an error if service exists
• Confusing with rosnode commands

4. You run rosservice call /add_two_ints a:=5 b:=7 but get an error: ERROR: Service [/add_two_ints] not available. What is the most likely cause?

medium

A. The service /add_two_ints is not currently running or advertised.

B. The arguments a and b are incorrectly formatted.

C. The ROS master is not running.

D. The service /add_two_ints does not accept integer arguments.

Solution

1. Step 1: Analyze the error message

   The error says the service is not available, meaning it is not running or advertised.

2. Step 2: Check other options

   Argument format errors cause different errors; ROS master not running causes broader failures; service argument types mismatch cause different errors.

3. Final Answer:

   The service /add_two_ints is not currently running or advertised. -> Option A

4. Quick Check:

   Service not available means not running [OK]

Hint: Service not available means it's not running or advertised [OK]

Common Mistakes:

• Assuming argument format is wrong from this error
• Not checking if ROS master is running
• Thinking service argument types cause this error

5. You want to check the detailed request and response format of a service named /compute_path. Which command should you use to see the service definition?

hard

A. rosservice info /compute_path

B. rosservice type /compute_path

C. rosservice show /compute_path

D. rosservice find /compute_path

Solution

1. Step 1: Understand commands for service details

   rosservice show displays the full request and response message format of a service.

2. Step 2: Compare with other commands

   rosservice info shows service provider info, rosservice type shows only the type name, and rosservice find searches for services by type.

3. Final Answer:

   rosservice show /compute_path -> Option C

4. Quick Check:

   Use rosservice show to see request/response format [OK]

Hint: Use rosservice show to view full service message format [OK]

Common Mistakes:

• Using rosservice info expecting message format
• Confusing rosservice type with detailed format
• Trying rosservice find which searches by type