On Linux, I want to send a command string (i.e. some data) to a serial port (containing control characters), and listen to the response (which also usually might contain control characters).
How can I do this as simplest as possible on Linux? An example is appreciated!
All devices on Unix are mapped to a device file, the serial ports would be /dev/ttyS0 /dev/ttyS1 ... .
First have a look at the permissions on that file, lets assume you are using /dev/ttyS1.
ls -l /dev/ttyS1
You will want read.write access, if this is a shared system then you should consider the security consequences of opening it up for everyone.
chmod o+rw /dev/ttyS1 A very simple crude method to write to the file, would use the simple echo command.
echo -ne '\033[2J' > /dev/ttyS1 and to read
cat -v < /dev/ttyS1 You can have cat running in one terminal, and echo in a 2nd.
If everything is gibberish, then baud rate, bit settings might need setting before you start sending. stty will do that. !! NOTE stty will use stdin as default file descriptor to affect.
Equivalent commands.
stty 19200 < /dev/ttyS1 stty 19200 -F /dev/ttyS1 This might be enough for you to script something and log ? Not sure what you are trying to achieve.
For a more interactive, remembers your default settings approach would be to use minicom it is just a program which does everything I've mentioned so far. (similar to hyperterminal in Windows, you might be familiar).
An intermediate solution, would use a terminal program like screen which will work on a serial device.
screen /dev/ttyS1 man screen man minicom man stty for more information
All you have to do is open two terminals. In the first terminal you cat everything from the device, e.g.
cat /dev/ttyS0 in the other terminal, you can send arbitrary hex characters and text to the terminal e.g. as follows:
echo -e "\x7E\x03\xD0\xAF und normaler Text" > /dev/ttyS0 The echo -e command enables the interpretation of backslash escapes.
One has to make sure of course that (i) the serial settings (speed, word length, flow ctrl, etc) are correct and (ii) the serial device (on the other end) is not blocking.
This could be a better approach:
stty -F /dev/ttyUSB0 115200 raw -echo #CONFIGURE SERIAL PORT exec 3</dev/ttyUSB0 #REDIRECT SERIAL OUTPUT TO FD 3 cat <&3 > /tmp/ttyDump.dat & #REDIRECT SERIAL OUTPUT TO FILE PID=$! #SAVE PID TO KILL CAT echo "R" > /dev/ttyUSB0 #SEND COMMAND STRING TO SERIAL PORT sleep 0.2s #WAIT FOR RESPONSE kill $PID #KILL CAT PROCESS wait $PID 2>/dev/null #SUPRESS "Terminated" output exec 3<&- #FREE FD 3 cat /tmp/ttyDump.dat #DUMP CAPTURED DATA 
watch -n 0.2 ./send.sh I discover that after few calls or when send CTRL+C signal and re-execute it, my output data are printed in oddly way. The reason is about left cat-s process that have simulations access to my port. You can list that by lsof /dev/ttyUSB0 So I add lsof -t /dev/ttyUSB0 | xargs -r kill -9 at the beginning, to be sure that only one cat process exists in a given runtime of my function. exec 3< and then stty. Opening the device (with exec 3<) locks it into an “open” status, which should ensure that settings are persistent. If you do the stty first, the device will be closed and the settings might be reset. Programs that talk to serial devices:
picocom minicom socat or from shell you can do:
stty -speed 19200 < /dev/ttyS0 # sets the speed of the port exec 99<>/dev/ttyS0 (or /dev/ttyUSB0...etc) printf "AT\r" >&99 read answer <&99 # this reads just a CR read answer <&99 # this reads the answer OK exec 99>&- You can read and write to a device simulataneously like so:
cat /dev/cu.usbmodem411 & cat > /dev/cu.usbmodem411 Your message is sent to the second cat from stdin, and the first cat relays the response to stdout, turning your terminal into a chatroom.
To finish up, ctrl-c, then run fg then ctrl-c again.
Besides other answers we can use cutecom a GUI application to communicate with serial devices in Unix like systems.
It seems more practical for non terminal gurus.
Install on Debian based systems
$ sudo apt install cutecom 
For a more useful example usage we can connect PC to an ESP32 board with an USB-UART convertor interface.
After connection we can detect and use it under the name of /dev/ttyUSB1.
You can ensure the serial device name connected to PC with consulting dmesg.
$ sudo dmesg --follow-new [ 7859.806558] usb 1-6.4: ch341-uart converter now attached to ttyUSB1 So read what ESP32 write on serial port with cutecom.
