How to rostopic pub multiple line for std_msgs/String type topic

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How is this multiple lines? There are no CR or LF in the topic that is sent. This also doesn't answer how to do \r or \n in a message

Testing this on my ROS Melodic/Ubuntu Bionic install I see this output:

$ rostopic echo /send_BTSVR
data: "first line\nsecond line\nthird line "
---
data: "first line\nsecond line\nthird line "
---
data: "first line\nsecond line\nthird line "

etc.

Afaict, that is a multiline string encoded in a singe std_msgs/String. A Python node subscribing to this with print(msg.data) shows me this:

$ python ./sub.py
first line
second line
third line 
---
first line
second line
third line 
---

etc.

\n==line feed==LF, which is the standard Linux newline.

\r would be something else. I've not checked how to get that in a YAML string, but I would expect it to work the same way (ie: by adding the escape sequence to the string).

Did you expect rostopic echo to also print the multi-line string?