Capturing input from usb device in Linux

Question

So, after searching a little bit... have found the following way to capture the input from my USB QR code scanner.

import sys
pipe = open('/dev/input/event3', 'r')
while 1:
        for character in pipe.read():
                print(character)

It works, but I still have 2 questions regarding the method above.

I'm on a Raspberry device with Debian Wheezy and a GUI, and can confirm Question 2 happening on a GUI... don't know yet the output on a system without GUI

Any better way of doing it?
The method above, when I click right the script cancels and outputs all the entries that have been scanned, at once. Why is that?

UPDATE

I think I have miss interpreted the result for the code above, as it finally it outputs ASCII characters, so I ended up doing the following:

sys.stdin = open('/dev/tty')
a = raw_input('Scan: ')

Eric · Accepted Answer · 2013-01-28 14:54:54Z

1

Question 1 really depends on your QR scanner. I think you are lucky this works like this, since much of the time, usb devices have complex protocols to communicate with hosts. For example, with a mouse, how would you differentiate between clicks and mouse movement? You need some kind of data format to exchange.

Question 2 is probably because your method blocks at pipe.read(), waiting for input. Somehow, only the right click ends the read() function and allows the print to work out

answered Jan 28, 2013 at 14:54

Eric

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6 Comments

Alex Over a year ago

well... the QR scanner, without any thing done by me, it outputted the result by its self to where it could (mostly terminal, as it's always opened)... so it was only a matter of grabbing that output

Eric Over a year ago

my guess is that your linux distro somehow loaded a driver for the scanner

Eric Over a year ago

you could try getting more information about it with either lsmod or lsusb, which should show which driver was loaded. This would allow you to find more documentation about it

Alex Over a year ago

this USB devices, gets identified as soon as plugged in. lsusb => Bus 001 Device 007: ID 05f9:2216 PSC Scanning, Inc. The complex thing it's done inside the scanner (converting the code) and then it's simple output... so no big mystery or heavy driver codding to get it to work

Eric Over a year ago

You need to understand that while the scanner probably does a good deal of processing, having a usb compatible device is a pretty complex task. usb.org/developers/devclass_docs/HID1_11.pdf. The driver will hide a lot of that complexity and present you with a nice /dev interface that you can use afterwards.

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