(SO answer repost)

You can use kafkacat to send messages to Kafka that include line breaks. To do this, use its -D operator to specify a custom message delimiter (in this example /):

kafkacat -b kafka:29092 \
        -t test_topic_01 \
        -D/ \
        -P <<EOF
this is a string message 
with a line break/this is 
another message with two 
line breaks!
EOF

Note that the delimiter must be a single byte - multi-byte chars will end up getting included in the resulting message See issue #140