What is the purpose of environment variables in launch files and how can we use them correctly (optenv in husky)?

Question

Hello! I have trouble grasping the necessity as long as the way of usage of the three environment variables in the husky_gazebo/launch/spawn_husky.launch file as shown in the code below.

What would be the difference of using simply false as default?

 <arg name="laser_enabled" default="$(optenv HUSKY_LMS1XX_ENABLED false)"/>
 <arg name="kinect_enabled" default="$(optenv HUSKY_UR5_ENABLED false)"/>
 <arg name="urdf_extras" default="$(optenv HUSKY_URDF_EXTRAS)"/>

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Originally posted by smarn on ROS Answers with karma: 54 on 2020-03-06

Post score: 0

Answer 1

The difference is that you are able to change the argument value with your environment variables, meaning you could avoid typing roslaunch husky_gazebo spawn_husky.launch laser_enabled:="VALUE" to set the argument, instead you would do export HUSKY_LMS1XX_ENABLED="VALUE".

Now there isn't a necessity of using them, it can simply be handy for some situations (essentially for reusabiltiy). In your example the arguments are related to the sensors of your robot, if you don't have a kinect then it should be normal to have the argument kinect_enabled to false. But if you do have one, it's not something that would change everyday so you wouldn't want to call the launch files related with specifying the arg (i.e. always call roslaunch husky_gazebo spawn_husky.launch kinect_enabled:="true"). You could indeed modify the launch file to have this value to false or true by default but if someday you reuse this launch file with a different robot you would have to be aware of this modification.

This launch file is from the husky_gazebo package which is provided for users with different configurations, instead of requiring each users to change the launch file depending on their configurations, each user can use the same launch file by simply changing their environment variables. The fact that the substitution args are optenv also suggests this intention, the user doesn't need to have those variables set to use the robot because by default the husky doesn't have those sensors but you can still use it. You can find some packages (like for the turtlebot3_gazebo) using env isntead of optenv for the robot model. Using env forces the user to have this variable set (roslaunch would fail if the variable is missing) which seems normal because there isn't a default turtlebot3 model.

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Originally posted by Delb with karma: 3907 on 2020-03-09

This answer was ACCEPTED on the original site

Post score: 3

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Original comments

Comment by smarn on 2020-03-10:
Thanks Delb you were really helpful, I think I get it! I will try some tests in order to grasp it completely

Comment by Delb on 2020-03-10:
Glad it helped you, if you feel that your question has been fully answered can you please mark it as correct (using the chek mark icon) please ?