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rosbagwriter

Create and write logs to rosbag log file

Since R2021b

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    Description

    Use the rosbagwriter object to create a rosbag log file and write logs to the bag file. Each log contains a topic, its corresponding timestamp, and a ROS message.

    Note

    The rosbagwriter object locks the created bag file for use, it is necessary to delete and clear the rosbagwriter object in order to use the bag file with a reader or perform other operations.

    Creation

    Syntax

    bagwriter = rosbagwriter(filepath)
    bagwriter = rosbagwriter(___,Name,Value)

    Description

    example

    bagwriter = rosbagwriter(filepath) creates a rosbag log file in the location specified by path and returns the corresponding rosbagwriter object.

    If you do not specify the name of the bag file in the filepath, the object assigns the current timestamp as the file name. If the folders you specify in filepath are not present in the directory, the object creates them and places the bag file accordingly. The filepath input argument sets the FilePath property.

    example

    bagwriter = rosbagwriter(___,Name,Value) sets the Compression and ChunkSize properties using name-value arguments. Use this syntax with the input argument in the previous syntax.

    Properties

    expand all

    This property is read-only.

    Path to the rosbag file, specified as a character vector.

    Data Types: char

    This property is read-only.

    Timestamp of the first message written to the bag file, specified as a scalar in seconds.

    Data Types: double

    This property is read-only.

    Timestamp of the last message written to the bag file, specified as a scalar in seconds.

    Data Types: double

    This property is read-only.

    Number of messages written to the bag file, specified as a scalar.

    Data Types: double

    Compression format of the message chunks, specified as "bz2", "lz4", or "uncompressed".

    Example: "Compression","lz4"

    Data Types: char | string

    Size of each message chunk, specified as a nonzero positive integer in bytes. The value specify the buffer within the bag file object. Reducing this value results in more writes to disk.

    Example: "ChunkSize",819200

    Data Types: double

    This property is read-only.

    Current bag file size, specified as a nonnegative integer in bytes.

    Data Types: double

    Object Functions

    writeWrite logs to rosbag log file
    deleteRemove rosbag writer object from memory

    Examples

    collapse all

    Open Live Script

    Retrieve all the information from the rosbag log file.

    rosbag('info','path_record.bag')
    Path:     /tmp/Bdoc23a_2213998_1471372/tp0bd177dc/ros-ex73035957/path_record.bag
Version:  2.0
Duration: 10.5s
Start:    Jul 05 2021 08:09:52.86 (1625486992.86)
End:      Jul 05 2021 08:10:03.40 (1625487003.40)
Size:     13.3 KB
Messages: 102
Types:    geometry_msgs/Point [4a842b65f413084dc2b10fb484ea7f17]
Topics:   /circle  51 msgs  : geometry_msgs/Point
          /line    51 msgs  : geometry_msgs/Point

    Create a rosbagreader object of all the messages in the rosbag log file.

    reader = rosbagreader('path_record.bag');

    Select all the messages related to the topic '/circle'.

    bagSelCircle = select(reader,'Topic','/circle');

    Retrieve the list of timestamps from the topic.

    timeStamps = bagSelCircle.MessageList.Time;

    Retrieve the messages in the selection as a cell array.

    messages = readMessages(bagSelCircle);

    Create a rosbagwriter object to write the messages to a new rosbag file.

    circleWriter = rosbagwriter('circular_path_record.bag');

    Write all the messages related to the topic '/circle' to the new rosbag file.

    write(circleWriter,'/circle',timeStamps,messages);

    Remove the rosbagwriter object from memory and clear the associated object.

    delete(circleWriter)
clear circleWriter

    Retrieve all the information from the new rosbag log file.

    rosbag('info','circular_path_record.bag')
    Path:     /tmp/Bdoc23a_2213998_1471372/tp0bd177dc/ros-ex73035957/circular_path_record.bag
Version:  2.0
Duration: 10.4s
Start:    Jul 05 2021 08:09:52.86 (1625486992.86)
End:      Jul 05 2021 08:10:03.29 (1625487003.29)
Size:     8.8 KB
Messages: 51
Types:    geometry_msgs/Point [4a842b65f413084dc2b10fb484ea7f17]
Topics:   /circle  51 msgs  : geometry_msgs/Point

    Load the new rosbag log file.

    readerCircle = rosbagreader('circular_path_record.bag');

    Create a time series for the coordinates.

    tsCircle = timeseries(readerCircle,'X','Y');

    Plot the coordinates.

    plot(tsCircle.Data(:,1),tsCircle.Data(:,2))
axis equal

    Figure contains an axes object. The axes object contains an object of type line.

    Open Live Script

    Create a rosbagwriter object and a rosbag file in the current working directory. Specify the compression format of the message chunks and the size of each message chunk.

    bagwriter = rosbagwriter("bagfile.bag", ...
    "Compression","lz4",...
    "ChunkSize",1500)
    bagwriter = 
  rosbagwriter with properties:

       FilePath: '/tmp/Bdoc23a_2213998_1460604/tp11cd810c/ros-ex26181333/bagfile.bag'         
      StartTime: 0                                                                            
        EndTime: 0                                                                            
    NumMessages: 0                                                                            
    Compression: 'lz4'                                                                        
      ChunkSize: 1500                                                                    Bytes
       FileSize: 4117                                                                    Bytes

    Start node and connect to ROS master.

    rosinit
    Launching ROS Core...
Done in 0.67932 seconds.
Initializing ROS master on http://172.30.180.30:54885.
Initializing global node /matlab_global_node_88420 with NodeURI http://dcc952465glnxa64:39269/ and MasterURI http://localhost:54885.

    Write a single log to the rosbag file.

    timeStamp = rostime("now");
rosMessage = rosmessage("nav_msgs/Odometry");
write(bagwriter,"/odom",timeStamp,rosMessage);
bagwriter
    bagwriter = 
  rosbagwriter with properties:

       FilePath: '/tmp/Bdoc23a_2213998_1460604/tp11cd810c/ros-ex26181333/bagfile.bag'         
      StartTime: 1.6779e+09                                                                   
        EndTime: 1.6779e+09                                                                   
    NumMessages: 1                                                                            
    Compression: 'lz4'                                                                        
      ChunkSize: 1500                                                                    Bytes
       FileSize: 4172                                                                    Bytes

    Shut down the ROS network.

    rosshutdown
    Shutting down global node /matlab_global_node_88420 with NodeURI http://dcc952465glnxa64:39269/ and MasterURI http://localhost:54885.
Shutting down ROS master on http://172.30.180.30:54885.

    Remove rosbag writer object from memory and clear the associated object.

    delete(bagwriter)
clear bagwriter

    Create a rosbagreader object and load all the messages in the rosbag log file. Verify the recently written log.

    bagreader = rosbagreader('bagfile.bag')
    bagreader = 
  rosbagreader with properties:

           FilePath: '/tmp/Bdoc23a_2213998_1460604/tp11cd810c/ros-ex26181333/bagfile.bag'
          StartTime: 1.6779e+09
            EndTime: 1.6779e+09
        NumMessages: 1
    AvailableTopics: [1x3 table]
    AvailableFrames: {0x1 cell}
        MessageList: [1x4 table]


    bagreader.AvailableTopics
    ans=1×3 table
             NumMessages       MessageType             MessageDefinition      
             ___________    _________________    _____________________________

    /odom         1         nav_msgs/Odometry    {'std_msgs/Header Header...'}

    Version History

    Introduced in R2021b

    See Also

    Objects

    Functions