roslaunch if condition

edit

The documentation on the roslaunch page for this syntax is complete, i.e. you can't do comparisons or any operators for that matter. You can only test the value itself.

This is supported as of ROS Kinetic.

<group if="$(arg foo)">
  <!-- stuff that will only be evaluated if foo is true -->
</group>

See also http://wiki.ros.org/roslaunch/XML

Is this answer still current? I see that xacro got an upgrade: from http://wiki.ros.org/xacro#Conditional... "The more powerful evaluation capabilities in ROS Jade allow for much more complex expression. Virtually any python expression that evaluates to a Boolean is feasible."

EDIT 2015.6.30:

I found a related ticket suggesting that Ken's answer is probably still current, though I don't yet have jade to check for sure.

A small addendum from 2021. Embedding the if ...eval statement in a group gives a nice structure. As an example (taken from here):

<arg name="team" default="Red"/>
<arg name="type" default="painter"/>

<group if="$(eval team == 'Red')">
  <group if="$(eval type == 'painter')">
    <param name="robot_description" 
      command="$(find xacro)/xacro $(find robopaint)/urdf/red/paintbot_red.urdf.xacro" />
  </group>
  <group if="$(eval type == 'attacker')">
    <param name="robot_description" 
      command="$(find xacro)/xacro $(find robopaint)/urdf/red/attackbot_red.urdf.xacro" />
  </group>
</group>

<group if="$(eval team == 'Blue')">
  <group if="$(eval type == 'painter')">
    <param name="robot_description" 
      command="$(find xacro)/xacro $(find robopaint)/urdf/blue/paintbot_blue.urdf.xacro" />
  </group>
  <group if="$(eval type == 'attacker')">
    <param name="robot_description" 
      command="$(find xacro)/xacro $(find robopaint)/urdf/blue/attackbot_blue.urdf.xacro" />
  </group>
</group>

<arg name="mov_b" default="true"/>

<group if="$(eval arg('mov_b') == true)"> <include file="$(find diff_robot)/launch/includes/move_base.launch.xml"/> </group>