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Modern wiki: https://github.com/ros/xacro/wiki

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This package is most useful when working with large XML documents such as robot descriptions. It is heavily used in packages such as the urdf. See for example, this tutorial for how xacro is used to simplify urdf files. See here, for an example showing the use of the advanced features (python evaluation, yaml integration) introduced in Jade.

Use of new features on Indigo

Many of the features highlighted as "New in Jade" below are also accessible from Indigo. To use them, the --inorder flag is required. This flag is discussed below, and for some background information see this pull request. Put simply, this flag is used to trigger Jade-enabled xacro processing on Indigo.

Example

Consider the following Xacro XML snippet:

<xacro:macro name="pr2_arm" params="suffix parent reflect">
  <pr2_upperarm suffix="${suffix}" reflect="${reflect}" parent="${parent}" />
  <pr2_forearm suffix="${suffix}" reflect="${reflect}" parent="elbow_flex_${suffix}" />
</xacro:macro>

<xacro:pr2_arm suffix="left" reflect="1" parent="torso" />
<xacro:pr2_arm suffix="right" reflect="-1" parent="torso" />

This snippet expands to:

<pr2_upperarm suffix="left" reflect="1" parent="torso" />
<pr2_forearm suffix="left" reflect="1" parent="elbow_flex_left" />
<pr2_upperarm suffix="right" reflect="-1" parent="torso" />
<pr2_forearm suffix="right" reflect="-1" parent="elbow_flex_right" />

If we also define macros for pr2_upperarm and pr2_forearm, then this snippet could expand to describe an entire robotic arm.

The remainder of this document describes the features of xacro.

Property and Property Blocks

Properties are named values that can be inserted anywhere into the XML document. Property blocks are named snippets of XML that can be inserted anywhere that XML is allowed. Both use the property tag to define values.

New in Jade: Properties are local if defined inside of a xacro:macro. See Local properties.

The following example shows how to declare and use a property:

<xacro:property name="the_radius" value="2.1" />
<xacro:property name="the_length" value="4.5" />

<geometry type="cylinder" radius="${the_radius}" length="${the_length}" />

The two properties are inserted into the geometry expression by placing the names inside dollared-braces (${}). If you want a literal "${", you should escape it as "$${".

Here's an example of using a property block:

<xacro:property name="front_left_origin">
  <origin xyz="0.3 0 0" rpy="0 0 0" />
</xacro:property>

<pr2_wheel name="front_left_wheel">
  <xacro:insert_block name="front_left_origin" />
</pr2_wheel>

Math expressions

Within dollared-braces (${}), you can also write simple math expressions. Currently, basic arithmetic and variable substitution is supported. Here's an example:

<xacro:property name="radius" value="4.3" />
<circle diameter="${2 * radius}" />

New in Jade:

Since ROS Jade, Xacro employs python to evaluate expressions enclosed in dollared-braces, ${}. This allows for more complex arithmetic expressions. Functions and constants from the python math module (e.g. pi and trigonometric functions) are available for use. Examples:

<xacro:property name="R" value="2" />
<xacro:property name="alpha" value="${30/180*pi}" />
<circle circumference="${2 * pi * R}" pos="${sin(alpha)} ${cos(alpha)}" />
<limit lower="${radians(-90)}" upper="${radians(90)}" effort="0" velocity="${radians(75)}" />

Conditional Blocks

Xacro has conditional blocks similar to roslaunch. This is useful for things like configurable robots or loading different Gazebo plugins. It follows this syntax:

<xacro:if value="<expression>">
  <... some xml code here ...>
</xacro:if>
<xacro:unless value="<expression>">
  <... some xml code here ...>
</xacro:unless>

The expression needs to evaluate to "0", "1", "true", or "false", otherwise an error will be thrown.

New in Jade: 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:

<xacro:property name="var" value="useit"/>
<xacro:if value="${var == 'useit'}"/>
<xacro:if value="${var.startswith('use') and var.endswith('it')}"/>

<xacro:property name="allowed" value="${[1,2,3]}"/>
<xacro:if value="${1 in allowed}"/>

Rospack commands

Xacro allows you to use certain rospack commands with dollared-parentheses ($()).

<foo value="$(find xacro)" />
<foo value="$(arg myvar)" />

Xacro currently supports most of the rospack commands that roslaunch supports using substitution args (with the exception of eval, see here). Arguments need to be specified on the command line using the myvar:=true syntax.

New in Indigo: Since ROS Indigo, it is also possible to define defaults like so:

<xacro:arg name="myvar" default="false"/>

Using this you can run xacro like:

<param name="robot_description" command="xacro $(arg model) myvar:=true" />

In xacro files, args is a _global_ dictionary accessible from within any included file or macro.

Macros

The main feature of xacro is its support for macros. Define macros with the macro tag, and specify the macro name and the list of parameters. The list of parameters should be whitespace separated. They become macro-local properties.

<xacro:macro name="pr2_caster" params="suffix *origin **content **anothercontent">
  <joint name="caster_${suffix}_joint">
    <axis xyz="0 0 1" />
  </joint>
  <link name="caster_${suffix}">
    <xacro:insert_block name="origin" />
    <xacro:insert_block name="content" />
    <xacro:insert_block name="anothercontent" />
  </link>
</xacro:macro>

<xacro:pr2_caster suffix="front_left">
  <pose xyz="0 1 0" rpy="0 0 0" />
  <container>
    <color name="yellow"/>
    <mass>0.1</mass>
  </container>
  <another>
    <inertial>
      <origin xyz="0 0 0.5" rpy="0 0 0"/>
      <mass value="1"/>
      <inertia ixx="100"  ixy="0"  ixz="0" iyy="100" iyz="0" izz="100" />
    </inertial>
  </another>
</xacro:pr2_caster>

The example declares a macro "pr2_caster", which takes two parameters: suffix and origin. Note that "origin" is starred. This indicates that origin is a block parameter instead of a simple text parameter. Look ahead to the use of pr2_caster. The suffix property is defined in the pr2_caster tag as an attribute, but no origin property is defined. Instead, origin refers to the first element inside (the "pose" block, in this case). The double-starred version ("content", "anothercontent") allows to insert an arbitrary number of elements that are passed within elements subsequently available ("container", "another" respectively in the example above). This example expands to the following:

<joint name="caster_front_left_joint">
  <axis xyz="0 0 1" />
</joint>
<link name="caster_front_left">
  <pose xyz="0 1 0" rpy="0 0 0" />
  <color name="yellow" />
  <mass>0.1</mass>
  <inertial>
    <origin xyz="0 0 0.5" rpy="0 0 0"/>
    <mass value="1"/>
    <inertia ixx="100"  ixy="0"  ixz="0" iyy="100" iyz="0" izz="100" />
  </inertial>
</link>

Multiple block parameters will be processed in the specified order:

<xacro:macro name="reorder" params="*first *second">
  <xacro:insert_block name="second"/>
  <xacro:insert_block name="first"/>
</xacro:macro>
<reorder>
  <first/>
  <second/>
</reorder>

Macros may contain other macros. The outer macro will be expanded first, and then the inner macro will be expanded. For example:

<a>
  <xacro:macro name="foo" params="x">
    <in_foo the_x="${x}" />
  </xacro:macro>

  <xacro:macro name="bar" params="y">
    <in_bar>
      <xacro:foo x="${y}" />
    </in_bar>
  </xacro:macro>

  <xacro:bar y="12" />
</a>

becomes:

<a>
  <in_bar>
    <in_foo the_x="12.0"/>
  </in_bar>
</a>

New in JadeComments in front of macros will be removed during processing. They are considered to be related to the macro (e.g. explaining the macro). If you want to keep a comment in the final document, separate it with an empty line from the macro.

Default parameters

New in Indigo Macro parameters can have default values:

<xacro:macro name="foo" params="x:=${x} y:=${2*y} z:=0"/>

If the defaults contain evaluation expressions, they will be evaluated at instantiation time.

New in JadeOften, you need to pass external variables into local macro params (as above for x). To ease this task, you can employ the ^ syntax:

<xacro:macro name="foo" params="p1 p2:=expr_a p3:=^ p4:=^|expr_b">

The caret ^ indicates to use the outer-scope property (with same name). The pipe | indicates to use the given fallback if the property is not defined in outer scope.

Local properties

New in JadeProperties and macros defined within a macro are local to this macro, i.e. not visible outside. Using the optional attribute scope="parent | global", a property definition can be exported to the parent scope of a macro (or the global scope).

Including other xacro files

You can include other xacro files using the xacro:include tag:

<xacro:include filename="$(find package)/other_file.xacro" />
<xacro:include filename="other_file.xacro" />
<xacro:include filename="$(cwd)/other_file.xacro" />

The file "other_file.xacro", will be included and expanded by xacro. New in Jade: Relative filenames are interpreted relative to the currently processed file. Note: When including files within a macro, not the macro-defining but the macro-calling file is the one that processes the include! $(cwd) explicitly allows to access files in the current working directory.

To avoid name clashes between properties and macros of various included files, you can specify a namespace for the included file - providing the attribute ns:

<xacro:include filename="other_file.xacro" ns="namespace"/>

Access to namespaced macros and properties is achieved by prepending the namespace, separated by a dot:

${namespace.property}

YAML support

New in Jade Properties can be dictionaries or lists too - manually declared with python syntax, like so:

<xacro:property name="props" value="${dict(a=1, b=2, c=3)}"/>
<xacro:property name="props_alt" value="${dict([('1a',1), ('2b',2), ('3c',3)])}"/>
<xacro:property name="numbers" value="${[1,2,3,4]}"/>

or loaded from YAML files like so:

<xacro:property name="yaml_file" value="$(find package)/config/props.yaml" />
<xacro:property name="props" value="${load_yaml(yaml_file)}"/>

Note, the evaluation brackets ${} distinguishing evaluation from plain text definition. Calibration data is an ideal candidate to be loaded from YAML.

The xacro property loaded from a YAML file is treated as dictionary. So if props.yaml is loaded into the props xacro property (as above) and contains

val1: 10
val2: 20

you can access val1 using

<xacro:property name="val1" value="${props['val1']}" />

Building from CMakeLists.txt

The following snippet shows how to use xacro during a package's make call:

# Generate .world files from .world.xacro files
find_package(xacro REQUIRED)
# You can also add xacro to the list of catkin packages:
#   find_package(catkin REQUIRED COMPONENTS ... xacro)

# Xacro files
file(GLOB xacro_files ${CMAKE_CURRENT_SOURCE_DIR}/worlds/*.world.xacro)

foreach(it ${xacro_files})
  # remove .xacro extension
  string(REGEX MATCH "(.*)[.]xacro$" unused ${it})
  set(output_filename ${CMAKE_MATCH_1})

  # create a rule to generate ${output_filename} from {it}
  xacro_add_xacro_file(${it} ${output_filename})

  list(APPEND world_files ${output_filename})
endforeach(it)

# add an abstract target to actually trigger the builds
add_custom_target(media_files ALL DEPENDS ${world_files})

New in Jade: While this cmake code provides full control over the target name and build order, there is a conveniency macro too:

file(GLOB xacro_files worlds/*.world.xacro)
xacro_add_files(${xacro_files} TARGET media_files)

If you want the generated files to have a .urdf extension it is possible to provide input files terminating with .urdf.xacro, the .xacro suffix will be removed by the CMake function. This results in files with a .urdf extension.

Elements and Attributes

New in Jade: In order to add elements or attributes with a dynamically defined name, you can use the special xacro tags <xacro:element> and <xacro:attribute>:

<xacro:element xacro:name="${element_name}" [other attributes]>
 [content]
</xacro:element>
<xacro:attribute name="${attribute_name}" value="${attribute_value}"/>

Usage example for <xacro:attribute>:

<xacro:property name="foo" value="my_attribute_name"/>
<tag>
    <xacro:attribute name="${foo}" value="my_attribute_value"/>
</tag>

gives:

<tag my_attribute_name="my_attribute_value" />

Processing Order

Classicly, xacro first loads all includes, then processes all property and macro definitions and finally instantiates macros and evaluates expressions. Thus, later property or macro definitions will override previous ones. Additionally, the conditional tags, <if> and <unless>, have no effect on macro or property definitions nor the inclusion of additional files.

New in Jade:

Since ROS Jade, xacro provides the command-line option --inorder, that allows to process the whole document in read order. Hence the latest definition of a property or macro seen so far, will be used. This is a much more intuitive evaluation process that allows for some nice new features as well:

• Inclusion of files can be postponed or completely suppressed if the <include> tag is placed within a macro or within conditional tags respectively.

• Include filenames can be specified via properties or macro parameters.
• By changing properties at the global scope, the instantiation of a macro can yield different results if these properties are used in the macro.
• Property definitions can be conditional.
• Macros can define properties within a local scope without affecting outside stuff.

Because --inorder processing is much more powerful, in future versions beyond Jade, the new processing style will become default and you should check the compatibility of your xacro files. Usually, both processing styles should give identical results. You can easily check this like so:

rosrun xacro xacro file.xacro > /tmp/old.xml
rosrun xacro xacro --inorder file.xacro > /tmp/new.xml
diff /tmp/old.xml /tmp/new.xml

If there are any differences shown, you should check and adapt your xacro file. A common reason will be the late loading of calibration data (as properties). In this case, simply move them up front, i.e. before usage. To facilitate search for wrongly placed property definitions, you can run xacro with option --check-order. If there are any problematic properties, they will be listed on stderr:

Document is incompatible to --inorder processing.
The following properties were redefined after usage:
foo redefined in issues.xacro

Using the command-line option -vv or -vvv one can increase verbosity level to log all defintions of properties.

Unrolling loops

By recursively calling a macro, it is also possible to unroll a loop and generate multiple similar XML snippets for a list of items:

<robot name="loop example" xmlns:xacro="http://wiki/xacro">
        <xacro:macro name="loop" params="items:=^">
                <xacro:if value="${items}">
                        <!-- pop first item from list -->
                        <xacro:property name="item" value="${items.pop(0)}"/>

                        <item>${item}</item>

                        <!-- recursively call myself -->
                        <xacro:loop/>
                </xacro:if>
        </xacro:macro>

        <!-- define the list of items to iterate -->
        <xacro:property name="items" value="${[1,2,3,4,5]}" />

        <xacro:loop items="${list(items)}"/>
        Passing a list copy, the original list is untouched: ${items}

        <xacro:loop items="${items}" />
        Passing the list directly, it is emptied: ${items}
</robot>

Deprecated Syntax

New in Jade:

While in previous versions, xacro tags without the namespace prefix were accepted, this sloppy syntax is strongly discouraged as it prevents the use of those tags in the final XML. Since Jade, this syntax is deprecated and you should update your files accordingly. The following script will update your files for you:

find . -iname "*.xacro" | xargs sed -i 's#<\([/]\?\)\(if\|unless\|include\|arg\|property\|macro\|insert_block\)#<\1xacro:\2#g'

To suppress the legacy interpretation of sloppy xacro tags and allow their usage in the target XML, you can use the command line option --xacro-ns.

Debugging Syntax Errors

To get more verbose syntax error output run following command which generates urdf from xacro and checks for errors in syntax:

cd <path_to_xacro_file>
check_urdf <(xacro model.xacro)

Or:

xacro model.urdf.xacro > tmp.urdf && check_urdf tmp.urdf && rm tmp.urdf