Shell Programming and Scripting

I'm new to Linux (Ubuntu 16.04), and very new to BASH scripting. I have a Numato 8-channel USB GPIO device, which is a DAQ that appears in the system as a serial port. In Linux it appears as ttyACM0. I can easily manipulate a GPO with, for example:

Code:

echo "gpio set 7" > /dev/ttyACM0
...followed by a carriage return:
echo -e '\r' > /dev/ttyACM0

But if I want to read a GPI, or one of the ADCs, I need to be able to read lines of data, (separated by carriage returns), sent back by the device...and save the information as a variable. So, for example I can send a request:

Code:

echo "adc read 0" > /dev/ttyACM0
echo -e '\r' > /dev/ttyACM0

I would then need to do two "read lines". The first would return an echo of the command I sent. The second would return the value of the ADC. I've not been able to figure out how to do these line reads. Does anyone know a simple command or series of commands to retrieve this serial information?

[I'm successfully doing automation with these devices with PowerShell under Windows 7, but I'd love to do the same from Linux machines]

Moderator's Comments:

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Did you try a

Code:

read INPUT </dev/ttyACM0
echo $INPUT

You may need to redefine the delimiter for your reads (man bash):

Yes, I've tried that read command. All I get is a blinking cursor...have to use cntrl-c to bail out and get back to prompt. Can you elaborate more on defining delimiter?

The cursor blinks as the read didn't stop as it didn't receive a <newline> char. I guess your device sends <carriage return>s? Try

Code:

read -d $'\r' INPUT </dev/ttyACM0

For testing, you might want to experiment with other read options:

Thanks for this information. After more experimentation I've discovered something that may be a clue. I've noticed that I can send one successful command to the device from BASH. Then I can make various unsuccessful attempts to read, which produce no viewable data...but can be forced to time out. I can send no more successful commands from BASH unless I open a Gnu Screen session and send some carriage returns to the device. So it appears that the device's buffer has data sitting in it that must be cleared before it will interact further. Bottom line, I'm not successfully reading it with BASH commands so far.

What if you read just a few chars like read -n2 (or 3,4,...)? Will any chars show up in your input variable?

No. I tested that variation also. The read won't end unless I use a timeout. Then the variable is blank. And, of course, the device becomes unresponsive until I intervene with Gnu Screen. I must be overlooking something simple. There must be a way to duplicate what I'm doing with this device in PowerShell.

I set up a serial connection between a Windows 7 PC and a Ubuntu 16 laptop. Using TeraTerm on the Windows machine, I can send a string...then handily read it from BASH on the laptop with:

read whatever < /dev/ttyUSB0

No problem. So...there's something different about the Numato device...something I'm missing.