Bash Serial Scripting

Question

I am connecting to a modem over a serial port and trying to figure out how to send an AT command, and add conditionals depending on the output. I can connect using screen or minicom to /dev/ttyAMA0 and send the AT command and receive the response OK, but when I use

echo -en "AT" >/dev/ttyAMA0 && cat /dev/ttyAMA0

I only see what I am echoing, not what the response is. I need to be able to send the AT command, check to see if the output is OK, or ERROR, then based off that response, do something different. Why am I not getting any response from the serial device?

I am trying to create a bash script that can connect to the modem and send a text message, but need to know if there are errors rather than just blasting things through assuming it is working. Is there a better way to accomplish this?

antlersoft · Accepted Answer · 2017-10-30 15:42:21Z

1

Scripting a dialog through a terminal-like port is a complex enough problem that people have written special tools to do it; the classic is the Expect/Tcl library. I think Tcl is simple to learn, but there are Expect bindings for other scripting languages.

I found this script that uses Expect to communicate over a serial port.

answered Oct 30, 2017 at 15:42

antlersoft

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Phantom Lord · Accepted Answer · 2017-10-30 15:38:28Z

0

After echoing you cannot simply cat the device file. You need to start a listening loop:

while read -r str < /dev/ttyAMA0; do
  # $str will contain a line of text returned from modem.
done

answered Oct 30, 2017 at 15:38

Phantom Lord

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2 Comments

Alan Over a year ago

I don't understand why I can't just cat the device? When I have it open in two terminals, I see all the AT commands I put into terminal 1 on terminal 2 that has cat /dev/ttyAMA0 running. With this loop, I didn't see anything at all returned to the read variable.

Phantom Lord Over a year ago

I used this method and it worked fine. Sorry if this is not universal in all cases. And btw you need to check $str variable

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