Echo to serial port usb

[mac johnsto <macjohnsto@xxxxxxxxx>]

Hi everyone I am facing a problem with echo command. The project says about
a client who send an echo command to a server sending a signal to an
arduino (connected to server).  The server runs one time the commands:
*stty - F /dev/ttyACM0 raw - echo*
*cat < /dev/ttyACM0 > /dev/null*

Using putty everything goes well. The problem is when I use my code
(chapter 4 of libssh documentation) the first echo command I send works but
all the others that follow not working and the command* cat < /dev/ttyACM0
> /dev/null*
terminates.