Is there a way to connect to a serial terminal just as you would do with SSH? There must be a simpler way than tools such as Minicom, like this

$ serial /dev/ttyS0 

I know I can cat the output from /dev/ttyS0 but only one way communication is possible that way, from the port to the console. And echo out to the port is just the same but the other way around, to the port.

How can I realize two way communication with a serial port the simplest possible way on Unix/Linux?

  • 2
    Great answers people!. Unfortunately no one seem to fully fit my purpose when working with embedded systems with a limited set of commmands. I did however find another way using a shell scrip which I add as one of the answers to my question. – ihatetoregister Oct 18 '11 at 10:15

13 Answers 13

I find screen the most useful program for serial communication since I use it for other things anyway. It's usually just screen /dev/ttyS0 <speed>, although the default settings may be different for your device. It also allows you to pipe anything into the session by entering command mode and doing exec !! <run some program that generates output>.

Background

The main reason why you need any program like minicom to communicate over a serial port is that the port needs to be set up prior to initiating a connection. If it weren't set up appropriately, the cat and echo commands would not do for you what you might have expected. Notice that once you run a program like minicom, the port is left with the settings that minicom used. You can query the communication settings using the stty program like this:

stty < /dev/ttyS0

If you have done it right; after booting the computer and before running any other program like minicom, the communication settings will be at their default settings. These are probably different than what you will need to make your connection. In this situation, sending the commands cat or echo to the port will either produce garbage or not work at all.

Run stty again after using minicom, and you'll notice the settings are set to what the program was using.

Minimal serial communication

Basically, two things are needed to have two-way communication through a serial port: 1) configuring the serial port, and 2) opening the pseudo-tty read-write.

The most basic program that I know that does this is picocom. You can also use a tool like setserial to set up the port and then interact with it directly from the shell.

  • 4
    picocom also will let you connect to a serial port without reconfiguring it (--noinit) and will let you exit without restoring the serial port configuration (--noreset or use Ctrl-A/Ctrl-Q to quit picocom). I've found picocom to be much easier to use than minicom. For reasons I haven't figured out, minicom will sometime simply not send or receive data on a port that worked moments before or that picocom has no trouble with. It's probably some arcane configuration option, but whatever it is I can't figure it out (and this behavior has happened on more than one machine). – Michael Burr Oct 24 '13 at 21:11

I found a way using a shell script here that put cat as a background process and a while loop that read the user input and echo it out to the port. I modified it to be more general and it fitted my purpose perfectly.

#!/bin/sh

# connect.sh

# Usage:
# $ connect.sh <device> <port speed>
# Example: connect.sh /dev/ttyS0 9600

# Set up device
stty -F $1 $2

# Let cat read the device $1 in the background
cat $1 &

# Capture PID of background process so it is possible to terminate it when done
bgPid=$!

# Read commands from user, send them to device $1
while read cmd
do
   echo "$cmd" 
done > $1

# Terminate background read process
kill $bgPid
  • 1
    Also, I've made a slightly modified version below which can also send Ctrl+C and Ctrl+Z. – Fritz Sep 22 '16 at 17:06
  • @Fritz Nice find! So if I get this right, the background process would never be killed because $? doesn't seem to expand to anything right? – ihatetoregister Sep 26 '16 at 6:23
  • 1
    @ihatetoregister: Not quite correct. The background process is killed, but not for the reasons one might expect. $? expands to the exit code of the last non-background command (in this case stty -F $1 $2), so it expands to 0 when there was no error. Thus, the last line becomes kill 0 which in turn seems to kill the current shell and all its children (it behaves differently in interactive shells I believe). For all the details see the following explanation: unix.stackexchange.com/questions/67532/… – Fritz Sep 26 '16 at 9:03
  • 1
    Howerver, the real gotcha in your script is that the background process is only killed if you press Ctrl+D to end your script, because that ends the while loop cleanly. If you kill it with Ctrl+C or with the kill command, then the cat process stays alive. To fix that you would need to use the trap command to execute kill $bgPid when the shell exits, like my script down below does. Honestly, I wouldn't even mind if you just added my whole script to your post. I tried to do that, but the edit was rejected. – Fritz Sep 26 '16 at 11:08
  • Could you please clarify where the connect comes in, as this is commented out in your answer – Chris Halcrow Dec 9 '16 at 20:31

If UUCP is installed on the system, you may use the command cu, e.g.

 $ cu -l /dev/ttyS0 -s 9600

This script is based on another answer, but sends everything over the serial port (except Ctrl+Q), not just single commands followed by Enter. This enables you to use Ctrl+C or Ctrl+Z on the remote host, and to use interactive "GUI" programs like aptitude or alsamixer. It can be quit by pressing Ctrl+Q.

#!/bin/bash

if [[ $# -lt 1 ]]; then
    echo "Usage:"
    echo "  femtocom <serial-port> [ <speed> [ <stty-options> ... ] ]"
    echo "  Example: $0 /dev/ttyS0 9600"
    echo "  Press Ctrl+Q to quit"
fi

# Exit when any command fails
set -e

# Save settings of current terminal to restore later
original_settings="$(stty -g)"

# Kill background process and restore terminal when this shell exits
trap 'set +e; kill "$bgPid"; stty "$original_settings"' EXIT

# Remove serial port from parameter list, so only stty settings remain
port="$1"; shift

# Set up serial port, append all remaining parameters from command line
stty -F "$port" raw -echo "$@"

# Set current terminal to pass through everything except Ctrl+Q
# * "quit undef susp undef" will disable Ctrl+\ and Ctrl+Z handling
# * "isig intr ^Q" will make Ctrl+Q send SIGINT to this script
stty raw -echo isig intr ^Q quit undef susp undef

# Let cat read the serial port to the screen in the background
# Capture PID of background process so it is possible to terminate it
cat "$port" & bgPid=$!

# Redirect all keyboard input to serial port
cat >"$port"

Try http://tio.github.io

"tio" is a simple TTY terminal application which features a straightforward commandline interface to easily connect to TTY devices for basic input/output.

Typical use is without options. For example:

tio /dev/ttyS0

Which corresponds to the commonly used options:

tio --baudrate 115200 --databits 8 --flow none --stopbits 1 --parity none /dev/ttyS0

It comes with full shell auto completion support for all options.

  • 2
    I'm using msys2 on Windows and you can install tio with pacman -S tio because it's among the available packages by default. screen, picocom, etc. are not. Thanks! – Kohányi Róbert May 23 at 13:11

BTW, the putty package (which does run on Linux) does include serial support.

It depends on what you want to do. Do you want to run a shell or applicaiton interactively from the terminal, connect out to another computer over the serial line, automate communication with a device over a serial port?

If you want bidirectional communication then I presume you want something interactive with a human on the terminal. You can configure the system to allow logins from a terminal over a serial port by seting up a getty(1) session on the serial port - getty is the tool for setting up a terminal and allowing logins onto it. Put an entry in your inittab(5) file to run it on the appropriate serial port on a respawn basis.

If you want to connect to a device and initiate automated two way conversations then you could see if expect will get you what you want. Use stty(1) to configure the port to the right parity, baud rate and other relevant settings.

If you want to communicate interactively with another computer over the serial port then you will need terminal emulation software. This does quite a lot - it sets up the port, interprets ANSI or other terminal command sequences (ANSI was far from being the only standard supported by serial terminals). Many terminal emulators also support file transfer protocols such as kermit or zmodem.

The ins and outs of serial communications and terminal I/O are fairly complex; you can read more than you ever wanted to know on the subject in the serial howto.

Putty works well on Linux and offers some convenience, especially for serial communications. It has one drawback I haven't been able to directly solve: no copy-paste from the Putty window itself. The windows version has a lovely auto-copy to clipboard on highlight, right-click to paste behaviour (and there are excellent plugins for both chrome and firefox to enable the same behavior), but on Linux, no copy love AFAIK.

If the lack of copy is a problem (it is for me) then turn on logging in putty and open a standard terminal window and # tail -f putty.log and bidirectional text is available for standard copypasta action.

  • 1
    I find that Putty under Linux doesn't paste "the clipboard" (what you copy with control-C), but it will insert "the primary selection" (what you have currently selected in some program) with middle-mouse. Likewise you can select characters in Putty's screen to define the primary selection. But if I want text from a Putty screen to transfer to some VM, I need it to be the clipboard, so I have to use an intermediary program to receive the text from the main selection and then copy it to the clipboard. – cardiff space man Feb 28 '17 at 19:58

As mentioned before you can try picocom. The latest release (2.0) can also be used (safely) to set-up a "terminal server" since it no longer permits shell command injection. See:

https://github.com/npat-efault/picocom/releases

You might want to take a look at

http://serialconsole.sourceforge.net

Pro: doesn't have obvious security problems like minicom or picocom (if you don't have a problem giving the users shell access, no problem, but you most likely do have one if you want to set up a terminal server...)

You need to be sure to have the correct read write permits on the device, you could see it with:

$ls -l /dev/[serial device]

I rely on the script you found and made some modifications.

For the development systems I've used by now, they used to need:

  • None parity and
  • One stop bit

Those values are the default ones in the script.

So in order to connect, you can use it as simple as follows:

./connect.sh /dev/[serial device] [baud speed]

Example:

$./connect.sh /dev/ttyUSB0 19200

Script:

#!/bin/bash

# connect.sh


#Taken from example modified by: ihatetoregister
# On stack exchange, thread:
# http://unix.stackexchange.com/questions/22545/how-to-connect-to-a-serial-port-as-simple-as-using-ssh
# Modified by Rafael Karosuo <rafaelkarosuo@gmail.com>
#   - parity enabling and amount of stop bits
#   - no execution without minimum params
#   - exit code for stty
#   - bgPid fix, used $! instead of $? to take the PID of cat proc in background.
#   - exit command to end the program
#   - CR termination and strip of NL added by READ command, in order to make $cmd\r\n format instead of \n$cmd\n


# Usage:
# $./connect.sh <device> <port speed> [# Stop bits] [parity]

# Stop bits 1|2
# Parity even | odd

# If no last two params, then default values stopbits=1, parity=disab

# Example: 
# connect.sh /dev/ttyS0 9600 1 even, this will use 1 stop bit and even parity
# connect.sh /dev/ttyS0 9600, this will take default values for parity and stopbit


#Check if at least port and baud params provided
if [ -z "$1" ] || [ -z "$2" ]; then
    printf "\nusage: ./connect.sh <device> <port speed> [# Stop bits 1|2] [parity even|odd]\n\tNeed to provide at least port and baud speed parameters.\n\texample:connect.sh /dev/ttyS0 9600\n\n"
    exit 1;
else
    case "$3"   in
        2) stopb="cstopb";;
        *) stopb="-cstopb";;
    esac

    if [ "$4" = "even" ]; then
        par="-parodd"
    elif [ "$4" = "odd" ]; then
        par="parodd"
    else
        par="-parity"
    fi
    printf "\nThen stty -F $1 $2 $stopb $par\n";
fi

# Set up device
stty -F "$1" "$2" "$stopb" "$par" -icrnl

# Check if error ocurred
if [ "$?" -ne 0 ]; then
    printf "\n\nError ocurred, stty exited $?\n\n"
    exit 1;
fi

# Let cat read the device $1 in the background
cat -v "$1" &

# Capture PID of background process so it is possible to terminate it when done
bgPid="$!"

# Read commands from user, send them to device $1
while [ "$cmd" != "exit" ]
do
   read cmd
   echo -e "\x08$cmd\x0D" > "$1" #strip off the \n that read puts and adds \r for windows like LF

done

# Terminate background read process
kill "$bgPid"

P.S.: You need to know which kind of line feed is using your receiver system since this will determine how you'll need to send the commands in my case I needed a Windows like LF, means that I need to send

command\r

ASCII values for:

  • LF : 0Ah, line feed "\n"
  • CR : 0Dh, carrige return "\r"
  • BS : 08h, back space "<-"
  • (1) #!/bin/sh is ignored if it isn’t the first line of the file.  (2) Seriously?  You’re using 1 to specify even parity and 2 to specify odd?  (3) It’s conventional to have a “usage” or “help” message that documents all the parameters, not just the mandatory ones.  (4)  You should always quote your shell variable references (e.g., "$1", "$2", "$3", "$4", "$stopb", "$par", "$bgPid", and even "$?" and "$!") unless you have a good reason not to, and you’re sure you know what you’re doing. – Scott Jul 9 '16 at 7:15

Another issue that can occur is that your user account may need to set to the "dialout" group to access the serial port.

sudo usermod -a -G dialout $USER

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.