Unable to read from serial port on command line

I tried reading from /dev/ttyS0 on 1 terminal as root with cat:

cat < /dev/ttyS0

and sending message to the serial port from another terminal as root with echo:

echo -ne "hello" > /dev/ttyS0

But I got nothing from the cat. Can someone enlighten me to the cause of the problem?

1 Answer

The device /dev/ttyS0 normally corresponds to your computer's first serial port. Nowadays, computers do not have serial ports, but some USB devices may appear at a /dev/ttySx port of your system. If the device was connected to your computer during boot, you can find the device name probably by running:

journalctl -b | grep ttyS

If the device is not connected yet, first run the command:

journalctl -f | grep ttyS

and then connect the device; you should see a message indicating the port number x.

If you are trying to send data to the virtual consoles of a desktop system, first press the key combination Ctrl+Alt+F4 and log in to the virtual console displayed in text mode. Then run the command

who am i

to verify that the device you have logged in is actually /dev/tty4 (the second word of the above command's output should be tty4 in this case).

On the main graphical screen (which can be reached by pressing the key combination Ctrl+Alt+F2) open a terminal and run the command:

echo "Hello" >/dev/tty4

Switch back to the console using Ctrl+Alt+F4 and see that message displayed.

Similarly run the following command on the main graphical screen:

cat </dev/tty4

Switch back to the console and try to enter some characters. Some of the characters will be echoed to the screen (by bash), some will not. The characters not echoed will be displayed on the main graphical screen as the output of the cat command. The reason for this is that the same device is being read by two processes (one on the text console which is bash and the other on the graphical screen which is the cat command).

Note:

For a non-root user to be able to read & write to a serial port, that user needs to have read & write permission for that device. In Ubuntu, this can easily be done by adding the current user to the dialout group using the

sudo addgroup _your_user_ dialout

You may have to log out and then log back in for this change to take effect.