mjhjmtu

I regularly use cat to view debugging information in the console from my FPGA development board over the serial connection, but I never have had to tell linux what the baud rate is. How does cat know what the baud rate of the serial connection is?

The stty utility sets or reports on terminal I/O characteristics for the device that is its standard input. These characteristics are used when establishing a connection over that particular medium. cat doesn't know the baud rate as such, it rather prints on the screen information received from the particular connection.

As an example stty -F /dev/ttyACM0 gives the current baud rate for the ttyACM0 device.

cat just uses whatever settings the port is already configured for. With this little C snippet you can see the baud rate currently set for a particular serial port:

get-baud-rate.c

#include <termios.h>
#include <unistd.h>
#include <stdio.h>

int main() 
 struct termios tios;
 tcgetattr(0, &tios);
 speed_t ispeed = cfgetispeed(&tios);
 speed_t ospeed = cfgetospeed(&tios);
 printf("baud rate in: 0%on", ispeed);
 printf("baud rate out: 0%on", ospeed);
 return 0;

Run it:

./get-baud-rate < /dev/ttyS0 # or whatever your serial port is

The numbers you get can be looked up in /usr/include/asm-generic/termios.h, where there are #defines such as B9600 etc. Note that the numbers in the header file and in the get-baud-rate output are in octal.

Maybe you can experiment and see what these numbers are like on a fresh boot and whether they change later.