Conditional launching in ROS - Step-by-Step Execution

Concept Flow - Conditional launching

Start launch file

↓

Evaluate condition

↓

Launch Node

↓

Continue launch

↓

End

The launch file starts, checks a condition, then launches a node only if the condition is true, otherwise it skips it and continues.

Execution Sample

ROS

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, LogInfo
from launch.conditions import IfCondition
from launch_ros.actions import Node
from launch.substitutions import LaunchConfiguration, PythonExpression

def generate_launch_description():
    return LaunchDescription([
        DeclareLaunchArgument('use_node', default_value='true'),
        Node(
            package='demo_pkg',
            executable='demo_node',
            condition=IfCondition(LaunchConfiguration('use_node'))
        ),
        LogInfo(condition=IfCondition(LaunchConfiguration('use_node')), text='Node launched'),
        LogInfo(condition=IfCondition(PythonExpression(["'", LaunchConfiguration('use_node'), "' == 'false'"])), text='Node skipped')
    ])

This launch file declares a boolean argument and launches a node only if the argument is true, logging accordingly.

Execution Table

Step	Action	Condition Evaluated	Result	Node Launched	Log Message
1	Start launch file	N/A	N/A	No	No
2	Evaluate 'use_node' argument	use_node == 'true'	True	No	No
3	Check IfCondition for Node	True	True	Yes	No
4	Launch Node 'demo_node'	N/A	N/A	Yes	No
5	Check IfCondition for LogInfo 'Node launched'	True	True	Yes	Node launched
6	Check IfCondition for LogInfo 'Node skipped'	False	False	No	No
7	End launch file	N/A	N/A	Yes	Node launched

💡 Launch ends after evaluating conditions and launching node if condition is true.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
use_node	undefined	'true'	'true'	'true'	'true'
Node Launched	No	No	Yes	Yes	Yes
Log Message	None	None	None	'Node launched'	'Node launched'

Key Moments - 2 Insights

Why does the node only launch when 'use_node' is true?

Because the Node action has an IfCondition that checks if 'use_node' is true (see execution_table step 3). If false, the node is skipped.

What happens to the log messages when the node is skipped?

The 'Node skipped' log only appears if 'use_node' is false (execution_table step 6). Since 'use_node' is true here, only 'Node launched' logs.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the node actually launched?

A Step 4

B Step 2

C Step 6

D Step 1

💡 Hint

Check the 'Node Launched' column in execution_table rows.

At which step does the condition 'use_node == true' get evaluated?

A Step 3

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the 'Condition Evaluated' column in execution_table.

If 'use_node' was set to 'false', what would happen at step 3?

A Node would launch

B Log 'Node launched' would print

C Node would be skipped

D Launch file would stop immediately

💡 Hint

Refer to the 'Result' and 'Node Launched' columns in execution_table for false condition.

Concept Snapshot

Conditional launching in ROS launch files:
- Use DeclareLaunchArgument to define a condition variable.
- Use IfCondition with Node or other actions.
- Node launches only if condition is true.
- Use LogInfo with conditions to log messages.
- Helps control which nodes start based on parameters.

Full Transcript

This example shows how a ROS launch file uses conditional launching. It starts by declaring a launch argument 'use_node' with default 'true'. The launch system evaluates this argument. If 'use_node' is true, the Node action launches the 'demo_node'. It also logs 'Node launched'. If 'use_node' were false, the node would be skipped and a different log would appear. The execution table traces each step, showing condition checks and actions. Variables like 'use_node' and 'Node Launched' track state changes. This helps beginners see how conditions control node launching in ROS.

Practice

1. What is the main purpose of using conditional launching in ROS launch files?

easy

A. To update ROS to the latest version

B. To speed up the robot's movement

C. To change the robot's hardware automatically

D. To start nodes only when certain conditions are met

Solution

1. Step 1: Understand conditional launching concept

   Conditional launching allows starting nodes only if specific conditions are true, saving resources.

2. Step 2: Identify the main purpose

   The main goal is to control which nodes run based on conditions, not to affect hardware or speed directly.

3. Final Answer:

   To start nodes only when certain conditions are met -> Option D

4. Quick Check:

   Conditional launching = start nodes conditionally [OK]

Hint: Think: conditional launching controls node start based on conditions [OK]

Common Mistakes:

• Confusing conditional launching with robot speed control
• Thinking it updates hardware automatically
• Assuming it updates ROS version

2. Which syntax correctly uses IfCondition in a ROS 2 Python launch file to conditionally start a node?

easy

A. Node(if='launch_arg == true', package='pkg', executable='node')

B. Node(condition=IfCondition(launch_arg == true), package='pkg', executable='node')

C. Node(condition=IfCondition('launch_arg == "true"'), package='pkg', executable='node')

D. Node(condition=IfCondition('launch_arg = true'), package='pkg', executable='node')

Solution

1. Step 1: Recall correct IfCondition usage

   IfCondition expects a string expression comparing launch arguments, e.g., 'launch_arg == "true"'.

2. Step 2: Check each option's syntax

   Node(condition=IfCondition('launch_arg == "true"'), package='pkg', executable='node') uses correct string comparison and quotes. Others have syntax errors or wrong operators.

3. Final Answer:

   Node(condition=IfCondition('launch_arg == "true"'), package='pkg', executable='node') -> Option C

4. Quick Check:

   IfCondition needs string expression with == [OK]

Hint: Use quotes around condition string and double equals for comparison [OK]

Common Mistakes:

• Using single equals (=) instead of double equals (==)
• Not quoting the condition expression
• Passing boolean directly instead of string

3. Given this launch snippet, what nodes will start if the launch argument use_camera is set to false?

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.conditions import IfCondition
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        DeclareLaunchArgument('use_camera', default_value='false'),
        Node(
            package='camera_pkg',
            executable='camera_node',
            condition=IfCondition('use_camera == "true"')
        ),
        Node(
            package='lidar_pkg',
            executable='lidar_node'
        )
    ])

medium

A. Only lidar_node starts

B. Both camera_node and lidar_node start

C. Only camera_node starts

D. No nodes start

Solution

1. Step 1: Analyze the condition for camera_node

   camera_node starts only if 'use_camera' equals 'true'. Here, 'use_camera' is 'false', so it won't start.

2. Step 2: Check lidar_node launch condition

   lidar_node has no condition, so it always starts.

3. Final Answer:

   Only lidar_node starts -> Option A

4. Quick Check:

   Condition false skips camera_node, lidar_node always runs [OK]

Hint: Nodes without conditions always start; check condition expressions carefully [OK]

Common Mistakes:

• Assuming camera_node starts when use_camera is false
• Thinking no nodes start if condition fails
• Ignoring unconditional nodes

4. Identify the error in this ROS 2 launch file snippet that prevents conditional launching from working:

Node(
    package='my_pkg',
    executable='my_node',
    condition=IfCondition(use_sensor == 'true')
)

medium

A. IfCondition cannot be used with Node

B. use_sensor should be a string inside quotes

C. Missing package name

D. Executable name must be capitalized

Solution

1. Step 1: Check IfCondition argument type

   IfCondition expects a string expression, but here use_sensor is used as a variable without quotes.

2. Step 2: Confirm correct syntax

   The condition should be written as IfCondition('use_sensor == "true"') with quotes around the expression.

3. Final Answer:

   use_sensor should be a string inside quotes -> Option B

4. Quick Check:

   IfCondition needs string condition expression [OK]

Hint: Always quote condition expressions in IfCondition [OK]

Common Mistakes:

• Passing variable without quotes to IfCondition
• Thinking IfCondition is invalid with Node
• Ignoring case sensitivity in executable

5. You want to launch either sim_node or real_node based on a launch argument use_sim. Which launch snippet correctly implements this conditional launching in ROS 2 Python launch file?

hard

A. Node(package='robot_pkg', executable='sim_node', condition=IfCondition('use_sim == "true"'))
Node(package='robot_pkg', executable='real_node', condition=IfCondition('use_sim == "false"'))

B. Node(package='robot_pkg', executable='sim_node', condition=IfCondition(use_sim == true))
Node(package='robot_pkg', executable='real_node', condition=IfCondition(use_sim == false))

C. Node(package='robot_pkg', executable='sim_node')
Node(package='robot_pkg', executable='real_node')

D. Node(package='robot_pkg', executable='sim_node', condition=IfCondition('use_sim = true'))
Node(package='robot_pkg', executable='real_node', condition=IfCondition('use_sim = false'))

Solution

1. Step 1: Check condition syntax for sim_node

   Node(package='robot_pkg', executable='sim_node', condition=IfCondition('use_sim == "true"')) Node(package='robot_pkg', executable='real_node', condition=IfCondition('use_sim == "false"')) uses correct string expressions with double equals and quotes for 'use_sim == "true"'.

2. Step 2: Check condition syntax for real_node

   Node(package='robot_pkg', executable='sim_node', condition=IfCondition('use_sim == "true"')) Node(package='robot_pkg', executable='real_node', condition=IfCondition('use_sim == "false"')) also correctly uses 'use_sim == "false"' for real_node, ensuring only one node launches.

3. Step 3: Verify other options

   Node(package='robot_pkg', executable='sim_node', condition=IfCondition(use_sim == true)) Node(package='robot_pkg', executable='real_node', condition=IfCondition(use_sim == false)) lacks quotes and uses booleans incorrectly; Node(package='robot_pkg', executable='sim_node') Node(package='robot_pkg', executable='real_node') launches both nodes unconditionally; Node(package='robot_pkg', executable='sim_node', condition=IfCondition('use_sim = true')) Node(package='robot_pkg', executable='real_node', condition=IfCondition('use_sim = false')) uses single equals (=) which is invalid.

4. Final Answer:

   Option A correctly uses IfCondition with string expressions and double equals -> Option A

5. Quick Check:

   Use quotes and == for conditions to launch nodes selectively [OK]

Hint: Use '==' inside quotes for conditions; avoid single '=' [OK]

Common Mistakes:

• Using single equals (=) instead of double equals (==)
• Not quoting the condition expression
• Launching both nodes without conditions