Inspecting topics (ros2 topic)
ROS2 has a tool to help us inspect topics. This is used with considerable frequency in practice to troubleshoot and speed up the development of publishers and subscribers. As usual, we can get more information on this tool as follows.
ros2 topic -h
which outputs the detailed information of the tool, as shown below. In particular, the highlighted fields are used quite frequently in practice.
usage: ros2 topic [-h]
[--include-hidden-topics]
Call `ros2 topic <command>
-h` for more detailed usage.
...
Various topic related sub-commands
options:
-h, --help show this help message
and exit
--include-hidden-topics
Consider hidden topics
as well
Commands:
bw Display bandwidth used by topic
delay Display delay of topic from timestamp in header
echo Output messages from a topic
find Output a list of available topics of a given type
hz Print the average publishing rate to screen
info Print information about a topic
list Output a list of available topics
pub Publish a message to a topic
type Print a topic's type
Call `ros2 topic <command> -h` for more detailed usage.
Start a publisher
During the development of a publisher, it is extremely useful to be able to check if topics are being properly made before we venture into making the subscribers. To see some of the tools for this job, we start by running the publisher Node we wrote in the last section.
Warning
Be sure to terminate the Nodes we used in the past section before proceeding (e.g. with CTRL+C), otherwise, the output will look different from what is described here.
ros2 run python_package_that_uses_the_messages amazing_quote_publisher_node
Getting all topics with ros2 topic list
In particular, when there are many topics, it is difficult to remember every name. To see all currently active topics, we can run
ros2 topic list
which, in this case, outputs
/amazing_quote
/parameter_events
/rosout
showing, in particular, the /amazing_quote topic what we were looking for.
Hint
The ros2 topic info is one of the main tools to find out typos in the names of topics. For example, if there was a typo in our topic we might find, in fact, two topics being listed, when we only expected one. For instance,
/amazing_quote /amazing_quotes /parameter_events /rosout
grep is your new best friend
Note
If you want more information on grep, check the Ubuntu Manpage
When the list of topics is too large, we can use grep to help filter the output. E.g.
ros2 topic list | grep quote
which outputs only the lines that contain quote, that is
/amazing_quote
Getting quick info with ros2 topic info
To get some quick information on a topic, we can run
ros2 topic info /amazing_quote
which outputs the message type and the number of publishers and subscribers connected to that topic
Type: package_with_interfaces/msg/AmazingQuote
Publisher count: 1
Subscription count: 0
Checking topic contents with ros2 topic echo
The ros2 topic echo is the main tool that we can use to inspect topic activity. We can check all the options of ros2 topic echo with the command below. The output is quite long so it’s not replicated here.
ros2 topic echo -h
To inspect the topic whose name we already know, we run
ros2 topic echo /amazing_quote
which outputs the following
id: 6
quote: Use the force, Pikachu!
philosopher_name: Uncle Ben
---
id: 7
quote: Use the force, Pikachu!
philosopher_name: Uncle Ben
---
id: 8
quote: Use the force, Pikachu!
philosopher_name: Uncle Ben
---
id: 9
quote: Use the force, Pikachu!
philosopher_name: Uncle Ben
---
id: 10
quote: Use the force, Pikachu!
philosopher_name: Uncle Ben
---
id: 11
quote: Use the force, Pikachu!
philosopher_name: Uncle Ben
---
grep is still your best friend
Whenever the topic is too crowded or the messages too fast, it might be difficult to pinpoint a single field we are looking for. In that case, grep can also help.
For example let us say that we want to see only the id fields of the messages. We can do
ros2 topic echo /amazing_quote | grep id
which will output only the lines with that pattern, e.g.
id: 1550
id: 1551
id: 1552
id: 1553
Measuring publishing frequency with ros2 topic hz
There are situations in which we are interested in knowing if the topics are receiving messages at an expected rate, without particular interest in the contents of the messages. We can do so with
ros2 topic hz /amazing_quote
which will output, after some time,
WARNING: topic [/amazing_quote] does not appear to be published yet
average rate: 2.000
min: 0.500s max: 0.500s std dev: 0.00007s window: 4
average rate: 2.000
min: 0.500s max: 0.500s std dev: 0.00013s window: 7
average rate: 2.000
min: 0.500s max: 0.500s std dev: 0.00011s window: 9
We must wait for a while until messages are received so that the tool can measure the frequency properly. You probably have noticed that the frequency measured by ros2 topic hz is compatible with the period of the Timer in our publisher Node.
Stop the publisher
Now we have exhausted all relevant tools that can give us information related to the publisher. Let us close the publisher with CTRL+C so that we can evaluate how these tools can help us analyze a subscriber.
Start the subscriber and get basic info
ros2 run python_package_that_uses_the_messages amazing_quote_subscriber_node
When only the subscriber is running, we can still get the basic info on the topic, e.g.
ros2 topic list
which also outputs
/amazing_quote
/parameter_events
/rosout
and
ros2 topic info /amazing_quote
which, differently from before, outputs
Type: package_with_interfaces/msg/AmazingQuote
Publisher count: 0
Subscription count: 1
Testing your subscribers with ros2 topic pub
To somewhat quickly evaluate a subscriber, we can use the ros2 topic pub. It allows us to publish messages to check the behavior of our subscribers.
In our case, we can send an AmazingQuote using YAML (More info). You can also refer to the YAML Cheat Sheet at QuickRef.ME.
ros2 topic pub /amazing_quote \
package_with_interfaces/msg/AmazingQuote \
'{
id: 1994,
quote: So you’re telling me there’s a chance,
philosopher_name: Lloyd
}'
Note
To improve readability, the command above uses the escape character \. You can see more on this at the bash docs. You can also refer to the bash Cheat Sheet at QuickRef.ME.
which will result in our subscriber outputting
[INFO] [1684222464.960446589] [amazing_quote_subscriber_node]:
I have received the most amazing of quotes.
It says
'So you’re telling me there’s a chance'
And was though by the following genius
-- Lloyd
This latest quote had the id=1994.
[INFO] [1684222465.953452826] [amazing_quote_subscriber_node]:
I have received the most amazing of quotes.
It says
'So you’re telling me there’s a chance'
And was though by the following genius
-- Lloyd
This latest quote had the id=1994.
For complicated messages, properly writing the message on the terminal can be a handful. In that case, it might be better to make a minimal script to test the subscriber instead. Refer to Create the Node with a publisher.