# Using colcon to build packages¶

This is a brief tutorial of how to create and build a ROS 2 workspace with colcon. It is a practical tutorial and not designed to replace the core documentation.

## Background¶

colcon is an iteration on the ROS build tools catkin_make, catkin_make_isolated, catkin_tools and ament_tools. For more information on the design of colcon see this document.

The source code can be found in the colcon GitHub organization.

## Prerequisites¶

### Install colcon¶

sudo apt install python3-colcon-common-extensions

python3 -m pip install colcon-common-extensions

pip install -U colcon-common-extensions


### Install ROS 2¶

To build the samples, you will need to install ROS 2.

Attention

If installing from Debian packages, this tutorial requires the desktop installation.

## Basics¶

A ROS workspace is a directory with a particular structure. Commonly there is a src subdirectory. Inside that subdirectory is where the source code of ROS packages will be located. Typically the directory starts otherwise empty.

colcon does out of source builds. By default it will create the following directories as peers of the src directory:

• The build directory will be where intermediate files are stored. For each package a subfolder will be created in which e.g. CMake is being invoked.

• The install directory is where each package will be installed to. By default each package will be installed into a separate subdirectory.

• The log directory contains various logging information about each colcon invocation.

Note

Compared to catkin there is no devel directory.

### Create a workspace¶

First, create a directory (ros2_example_ws) to contain our workspace:

mkdir -p ~/ros2_example_ws/src
cd ~/ros2_example_ws

mkdir -p ~/ros2_example_ws/src
cd ~/ros2_example_ws

md \dev\ros2_example_ws\src
cd \dev\ros2_example_ws


At this point the workspace contains a single empty directory src:

.
└── src

1 directory, 0 files


Let’s clone the examples repository into the src directory of the workspace:

git clone https://github.com/ros2/examples src/examples


Attention

It is recommended to checkout a branch that is compatible with the version of ROS that was installed (e.g. crystal).

cd ~/ros2_example_ws/src/examples/
git checkout \$ROS_DISTRO
cd ~/ros2_example_ws


Now the workspace should have the source code to the ROS 2 examples:

.
└── src
└── examples
├── CONTRIBUTING.md
├── rclcpp
├── rclpy

4 directories, 3 files


### Source an underlay¶

It is important that we have sourced the environment for an existing ROS 2 installation that will provide our workspace with the necessary build dependencies for the example packages. This is achieved by sourcing the setup script provided by a binary installation or a source installation, ie. another colcon workspace (see Installation). We call this environment an underlay.

Our workspace, ros2_examples_ws, will be an overlay on top of the existing ROS 2 installation. In general, it is recommended to use an overlay when you plan to iterate on a small number of packages, rather than putting all of your packages into the same workspace.

### Build the workspace¶

Attention

To build packages on Windows you need to be in a Visual Studio environment, see Building the ROS 2 Code for more details.

In the root of the workspace, run colcon build. Since build types such as ament_cmake do not support the concept of the devel space and require the package to be installed, colcon supports the option --symlink-install. This allows the installed files to be changed by changing the files in the source space (e.g. Python files or other not compiled resourced) for faster iteration.

colcon build --symlink-install


After the build is finished, we should see the build, install, and log directories:

.
├── build
├── install
├── log
└── src

4 directories, 0 files


### Run tests¶

To run tests for the packages we just built, run the following:

colcon test


### Source the environment¶

When colcon has completed building successfully, the output will be in the install directory. Before you can use any of the installed executables or libraries, you will need to add them to your path and library paths. colcon will have generated bash/bat files in the install directory to help setup the environment. These files will add all of the required elements to your path and library paths as well as provide any bash or shell commands exported by packages.

. install/setup.bash

. install/setup.bash

call install\setup.bat


### Try a demo¶

With the environment sourced we can run executables built by colcon. Let’s run a subscriber node from the examples:

ros2 run examples_rclcpp_minimal_subscriber subscriber_member_function


In another terminal, let’s run a publisher node (don’t forget to source the setup script):

ros2 run examples_rclcpp_minimal_publisher publisher_member_function


You should see messages from the publisher and subscriber with numbers incrementing.

colcon uses the package.xml specification defined in REP 149 (format 2 is also supported).

colcon supports multiple build types. The recommended build types are ament_cmake and ament_python. Also supported are pure cmake packages.

An example of an ament_python build is the ament_index_python package , where the setup.py is the primary entry point for building.

A package such as demo_nodes_cpp uses the ament_cmake build type, and uses CMake as the build tool.

For convenience, you can use the tool ros2 pkg create to create a new package based on a template.

Note

For catkin users, this is the equivalent of catkin_create_package.

## Tips¶

• If you do not want to build a specific package place an empty file named COLCON_IGNORE in the directory and it will not be indexed.

• If you want to avoid configuring and building tests in CMake packages you can pass: --cmake-args -DBUILD_TESTING=0.

• If you want to run a single particular test from a package:

colcon test --packages-select YOUR_PKG_NAME --ctest-args -R YOUR_TEST_IN_PKG