Migrating Launch Files

While launch files in ROS 1 are always specified using XML files, ROS 2 supports both XML and YAML files. ROS 2 also supports Python launch scripts to enable more flexibility (see launch package). However, for typical use cases, XML and YAML should be preferred over Python.

This guide describes how to write ROS 2 XML launch files for an easy migration from ROS 1.

Background 

A description of the ROS 2 launch system can be found in Launch System tutorial.

Migrating tags 

launch

Available in ROS 1.
launch is the root element of any ROS 2 launch XML file.

node

Available in ROS 1.
Launches a new node.
Differences from ROS 1:
- type attribute is now exec.
- ns attribute is now namespace.
- The following attributes aren’t available: machine, respawn_delay, clear_params.

Example

<launch>
   <node pkg="demo_nodes_cpp" exec="talker"/>
   <node pkg="demo_nodes_cpp" exec="listener"/>
</launch>

param

Available in ROS 1.
Used for passing a parameter to a node.
There’s no global parameter concept in ROS 2. For that reason, it can only be used nested in a node tag. Some attributes aren’t supported in ROS 2: type, textfile, binfile, executable.
The command attribute is now value="$(command '...' )".

Example

<launch>
   <node pkg="demo_nodes_cpp" exec="parameter_event">
      <param name="foo" value="5"/>
   </node>
</launch>

Type inference rules

Here are some examples of how to write parameters:

<node pkg="my_package" exec="my_executable" name="my_node">
   <!--A string parameter with value "1"-->
   <param name="a_string" value="'1'"/>
   <!--A integer parameter with value 1-->
   <param name="an_int" value="1"/>
   <!--A float parameter with value 1.0-->
   <param name="a_float" value="1.0"/>
   <!--A string parameter with value "asd"-->
   <param name="another_string" value="asd"/>
   <!--Another string parameter, with value "asd"-->
   <param name="string_with_same_value_as_above" value="'asd'"/>
   <!--Another string parameter, with value "'asd'"-->
   <param name="quoted_string" value="\'asd\'"/>
   <!--A list of strings, with value ["asd", "bsd", "csd"]-->
   <param name="list_of_strings" value="asd, bsd, csd" value-sep=", "/>
   <!--A list of ints, with value [1, 2, 3]-->
   <param name="list_of_ints" value="1,2,3" value-sep=","/>
   <!--Another list of strings, with value ["1", "2", "3"]-->
   <param name="another_list_of_strings" value="'1';'2';'3'" value-sep=";"/>
   <!--A list of strings using an strange separator, with value ["1", "2", "3"]-->
   <param name="strange_separator" value="'1'//'2'//'3'" value-sep="//"/>
</node>

Parameter grouping

In ROS 2, param tags are allowed to be nested. For example:

<node pkg="my_package" exec="my_executable" name="my_node" ns="/an_absoulute_ns">
   <param name="group1">
      <param name="group2">
         <param name="my_param" value="1"/>
      </param>
      <param name="another_param" value="2"/>
   </param>
</node>

That will create two parameters:

A group1.group2.my_param of value 1, hosted by node /an_absolute_ns/my_node.
A group1.another_param of value 2 hosted by node /an_absolute_ns/my_node.

It’s also possible to use full parameter names:

<node pkg="my_package" exec="my_executable" name="my_node" ns="/an_absoulute_ns">
   <param name="group1.group2.my_param" value="1"/>
   <param name="group1.another_param" value="2"/>
</node>

rosparam

Available in ROS 1.
Loads parameters from a yaml file.
It has been replaced with a from attribute in param tags.

Example

<node pkg="my_package" exec="my_executable" name="my_node" ns="/an_absoulute_ns">
   <param from="/path/to/file"/>
</node>

remap

Available in ROS 1.
Used to pass remapping rules to a node.
It can only be used within node tags.

Example

<launch>
   <node pkg="demo_nodes_cpp" exec="talker">
      <remap from="chatter" to="my_topic"/>
   </node>
   <node pkg="demo_nodes_cpp" exec="listener">
      <remap from="chatter" to="my_topic"/>
   </node>
</launch>

include

Available in ROS 1.
Allows including another launch file.
Differences from ROS 1:
- Available in ROS 1, included content was scoped. In ROS 2, it’s not. Nest includes in group tags to scope them.
- ns attribute is not supported. See example of push_ros_namespace tag for a workaround.
- arg tags nested in an include tag don’t support conditionals (if, unless) or the description attribute.
- There is no support for nested env tags. set_env and unset_env can be used instead.
- Both clear_params and pass_all_args attributes aren’t supported.

Examples

See Replacing an include tag.

arg

Available in ROS 1.
arg is used for declaring a launch argument, or to pass an argument when using include tags.
Differences from ROS 1:
- value attribute is not allowed. Use let tag for this.
- doc is now description.
- When nested within an include tag, if, unless, and description attributes aren’t allowed.

Example

<launch>
   <arg name="topic_name" default="chatter"/>
   <node pkg="demo_nodes_cpp" exec="talker">
      <remap from="chatter" to="$(var topic_name)"/>
   </node>
   <node pkg="demo_nodes_cpp" exec="listener">
      <remap from="chatter" to="$(var topic_name)"/>
   </node>
</launch>

Passing an argument to the launch file

In the XML launch file above, the topic_name defaults to the name chatter, but can be configured on the command-line. Assuming the above launch configuration is in a file named mylaunch.xml, a different topic name can be used by launching it with the following:

ros2 launch mylaunch.xml topic_name:=custom_topic_name

There is some additional information about passing command-line arguments in Using Substitutions.

env

Available in ROS 1.
Sets an environment variable.
It has been replaced with env, set_env and unset_env:
- env can only be used nested in a node or executable tag. if and unless tags aren’t supported.
- set_env can be nested within the root tag launch or in group tags. It accepts the same attributes as env, and also if and unless tags.
- unset_env unsets an environment variable. It accepts a name attribute and conditionals.

Example

<launch>
   <set_env name="MY_ENV_VAR" value="MY_VALUE" if="CONDITION_A"/>
   <set_env name="ANOTHER_ENV_VAR" value="ANOTHER_VALUE" unless="CONDITION_B"/>
   <set_env name="SOME_ENV_VAR" value="SOME_VALUE"/>
   <node pkg="MY_PACKAGE" exec="MY_EXECUTABLE" name="MY_NODE">
      <env name="NODE_ENV_VAR" value="SOME_VALUE"/>
   </node>
   <unset_env name="MY_ENV_VAR" if="CONDITION_A"/>
   <node pkg="ANOTHER_PACKAGE" exec="ANOTHER_EXECUTABLE" name="ANOTHER_NODE"/>
   <unset_env name="ANOTHER_ENV_VAR" unless="CONDITION_B"/>
   <unset_env name="SOME_ENV_VAR"/>
</launch>

group

Available in ROS 1.
Allows limiting the scope of launch configurations. Usually used together with let, include and push_ros_namespace tags.
Differences from ROS 1:
- There is no ns attribute. See the new push_ros_namespace tag as a workaround.
- clear_params attribute isn’t available.
- It doesn’t accept remap nor param tags as children.

Example

launch-prefix configuration affects both executable and node tags’ actions. This example will use time as a prefix if use_time_prefix_in_talker argument is 1, only for the talker.

<launch>
   <arg name="use_time_prefix_in_talker" default="0"/>
   <group>
      <let name="launch-prefix" value="time" if="$(var use_time_prefix_in_talker)"/>
      <node pkg="demo_nodes_cpp" exec="talker"/>
   </group>
   <node pkg="demo_nodes_cpp" exec="listener"/>
</launch>

machine

It is not supported at the moment.

test

It is not supported at the moment.

New tags in ROS 2 

set_env and unset_env

See env tag description.

push_ros_namespace

include and group tags don’t accept an ns attribute. This action can be used as a workaround:

<!-Other tags-->
<group>
   <push_ros_namespace namespace="my_ns"/>
   <!--Nodes here are namespaced with "my_ns".-->
   <!--If there is an include action here, its nodes will also be namespaced.-->
   <push_ros_namespace namespace="another_ns"/>
   <!--Nodes here are namespaced with "another_ns/my_ns".-->
   <push_ros_namespace namespace="/absolute_ns"/>
   <!--Nodes here are namespaced with "/absolute_ns".-->
   <!--The following node receives an absolute namespace, so it will ignore the others previously pushed.-->
   <!--The full path of the node will be /asd/my_node.-->
   <node pkg="my_pkg" exec="my_executable" name="my_node" ns="/asd"/>
</group>
<!--Nodes outside the group action won't be namespaced.-->
<!-Other tags-->

let

It’s a replacement of arg tag with a value attribute.

<let name="foo" value="asd"/>

executable

It allows running any executable.

Example

<executable cmd="ls -las" cwd="/var/log" name="my_exec" launch-prefix="something" output="screen" shell="true">
   <env name="LD_LIBRARY" value="/lib/some.so"/>
</executable>

Replacing an include tag 

In order to include a launch file under a namespace as in ROS 1 then the include tags must be nested in a group tag.

<group>
   <include file="another_launch_file"/>
</group>

Then, instead of using the ns attribute, add the push_ros_namespace action tag to specify the namespace:

<group>
   <push_ros_namespace namespace="my_ns"/>
   <include file="another_launch_file"/>
</group>

Nesting include tags under a group tag is only required when specifying a namespace

Substitutions 

Documentation about ROS 1’s substitutions can be found in roslaunch XML wiki. Substitutions syntax hasn’t changed, i.e. it still follows the $(substitution-name arg1 arg2 ...) pattern. There are, however, some changes w.r.t. ROS 1:

env and optenv tags have been replaced by the env tag. $(env <NAME>) will fail if the environment variable doesn’t exist. $(env <NAME> '') does the same as ROS 1’s $(optenv <NAME>). $(env <NAME> <DEFAULT>) does the same as ROS 1’s $(env <NAME> <DEFAULT>) or $(optenv <NAME> <DEFAULT>).
find has been replaced with find-pkg-share (substituting the share directory of an installed package). Alternatively find-pkg-prefix will return the root of an installed package.
There is a new exec-in-pkg substitution. e.g.: $(exec-in-pkg <package_name> <exec_name>).
There is a new find-exec substitution.
arg has been replaced with var. It looks at configurations defined either with arg or let tag.
eval and dirname substitutions require escape characters for string values, e.g. if="$(eval '\'$(var variable)\' == \'val1\'')".

Type inference rules 

The rules that were shown in Type inference rules subsection of param tag applies to any attribute. For example:

<!--Setting a string value to an attribute expecting an int will raise an error.-->
<tag1 attr-expecting-an-int="'1'"/>
<!--Correct version.-->
<tag1 attr-expecting-an-int="1"/>
<!--Setting an integer in an attribute expecting a string will raise an error.-->
<tag2 attr-expecting-a-str="1"/>
<!--Correct version.-->
<tag2 attr-expecting-a-str="'1'"/>
<!--Setting a list of strings in an attribute expecting a string will raise an error.-->
<tag3 attr-expecting-a-str="asd, bsd" str-attr-sep=", "/>
<!--Correct version.-->
<tag3 attr-expecting-a-str="don't use a separator"/>

Some attributes accept more than a single type, for example value attribute of param tag. It’s usual that parameters that are of type int (or float) also accept an str, that will be later substituted and tried to convert to an int (or float) by the action.

Migrating Launch Files

Background

Migrating tags

launch

node

Example

param

Example

Type inference rules

Parameter grouping

rosparam

Example

remap

Example

include

Examples

arg

Example

Passing an argument to the launch file

env

Example

group

Example

machine

test

New tags in ROS 2

set_env and unset_env

push_ros_namespace

let

executable

Example

Replacing an include tag

Substitutions

Type inference rules

Background 

Migrating tags 

New tags in ROS 2 

Replacing an include tag 

Substitutions 

Type inference rules 