A node publishes information using messages which are linked to topics. The message has a simple structure that uses standard types or types developed by the user.

Message types use the following standard ROS naming convention; the name of the package, then / and then the name of the .msg file. For example, std_msgs/ msg/String.msg has the std_msgs/String message type.

ROS has the rosmsg command-line tool to get information about messages.
The accepted parameters are as follows:

• rosmsg show: This displays the fields of a message
• rosmsg list: This lists all messages
• rosmsg package: This lists all of the messages in a package
• rosmsg packages: This lists all of the packages that have the message
• rosmsg users: This searches for code files that use the message type
• rosmsg md5: This displays the MD5 sum of a message