# rostopic echo option
# Display messages published to a topic.
$ rostopic echo /topic_name

# --offset
# Display time in messages as offset from current time (e.g. to calculate lag/latency).
$ rostopic echo --offset /topic_name

# --filter
# Display messages that match a specified Python expression.
$ rostopic echo --filter "m.data=='foo'"  /topic_name

# The Python expression can use any Python builtins plus the variable m (the message). For example, to filter based on the frame_id of the first transform in a tf/tfMessage:
$ rostopic echo --filter "m.transforms[0].child_frame_id == 'my_frame'" /tf

# -c
# Clear the screen after each message is published. Cannot be used with -p.
$ rostopic echo -c /topic_name

# -b
# Display messages in a bag file:
$ rostopic echo -b log_file.bag /topic_name

# -p
# Display messages in a matlab/octave-friendly plotting format. Cannot be used with -c.
$ rostopic echo -p /topic_name

# -w NUM_WIDTH New in Indigo
# Print all numeric values with a fixed width. 

# --nostr, --noarr
# Exclude string and array fields from the plotting output.
$ rostopic echo -p --nostr --noarr /topic_name

# -n COUNT New in C Turtle
# Echo COUNT messages and exit. 

# echo <topic-name/field>
# Display specific fields in a message.
$ rostopic echo /my_topic/field_name