rosservice command-line tools - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to list all available ROS services.

ROS

rosservice [1]

Drag options to blanks, or click blank then click option'

Alist

Bcall

Ctype

Dfind

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'call' instead of 'list' to show services.

Using 'type' which shows service type, not list.

The rosservice list command shows all active ROS services.

2fill in blank

medium

Complete the code to call a ROS service named '/add_two_ints'.

ROS

rosservice [1] /add_two_ints

Drag options to blanks, or click blank then click option'

Acall

Blist

Ctype

Dfind

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'list' which only shows services.

Using 'type' which shows the service type.

The rosservice call command is used to invoke a service.

3fill in blank

hard

Fix the error in the command to show the type of the service '/add_two_ints'.

ROS

rosservice [1] /add_two_ints

Drag options to blanks, or click blank then click option'

Afind

Bcall

Clist

Dtype

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'call' which tries to invoke the service.

Using 'list' which shows all services.

The rosservice type command shows the service message type.

4fill in blank

hard

Fill both blanks to find the node providing the service '/add_two_ints'.

ROS

rosservice [1] /add_two_ints | xargs rosnode [2]

Drag options to blanks, or click blank then click option'

Afind

Binfo

Clist

Dcall

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'list' instead of 'find' to locate the node.

Using 'call' instead of 'info' to get node details.

rosservice find locates the node providing the service, and rosnode info shows details about that node.

5fill in blank

hard

Fill all three blanks to call the service '/add_two_ints' with arguments a=5 and b=3.

ROS

rosservice [1] /add_two_ints '[2]: [3]'

Drag options to blanks, or click blank then click option'

Acall

Ba

C5

Db

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'list' instead of 'call' to invoke the service.

Mixing argument names or values.

Use rosservice call to invoke the service. Arguments are passed as key-value pairs like a: 5.

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