How to send line break with curl?
Your shell is passing \ followed by n rather than a newline to curl rather than "my message\n". Bash has support for another string syntax that supports escape sequences like \n and \t. To use it, start the string with $' and end the string with ':
curl -X PUT -d $'my message\n' http://localhost:8000/helloSee ANSI-C Quoting in the Bash Reference Manual
There's a much easier way!
curl -X PUT -d $'my message\n' http://localhost:8000/helloThis will use ANSI-C Quoting to insert the newline character.
No piping, no data files. See also Sending Newlines with cURL.