This example program sends repeating sequence of characters "0123456789" on USART3 on the ST Nucleo F767ZI eval board.
The sending is done in a blocking way.
Note that the Nucleo board exports the serial port as an ttyACM device on Linux or a COM port on Windows. On Linux, if you can locate the correct serial port as
/dev/serial/by-id/usb-STMicroelectronics_STM32_STLink_066CFF515056805087171825-if02
(note there will be differences based on the serial number of your board) this may also show up as /dev/ttyACM0 if it was the first serial port enumerated, if you have additional serial devices attached to the USB port of your system they may show up as additional ttyACM1/ttyACM2/... etc.
Connect to the port using screen /dev/ttyACM0 115200 where you replace
the serial port with the one for your system.