For my video chat sessions I use my Logitech E3500 USB web cam. On my Linux box this hardware device sort of works fine, but it does have it's own set of quirks I've developed workarounds for over the years. One of the things that bug me is that even though you can change it's configuration options trough GUI or CLI, this hardware device has it's defaults, and these defaults are back every time you plug it in. In this article I'll show you how to create script that changes web cam settings using CLI tools I've explained in one of my earlier articles, and how to make this script being executed every time web cam is plugged in.

Creating script to be executed

In this step you create script for whatever you need to be done after your device is plugged in. I'll describe my use case but yours will probably be different.

First I don't like my web cam using 60Hz setting for reducing artificial light sources induced flickering by default, because where I live we use 50Hz power sources. Secondly I would like exposure auto priority setting disabled to make my web cam image more natural. To achieve this I usually use v4l2-ctl command line program from v4l-utils package. Logitech E3500 web cam sometimes freeze due to snd-usb-audio module so my script also contains logic to work around this. I'll first install v4l-utils like this:

sudo apt-get install v4l-utils

Now I can place my v4l2-ctl commands inside /usr/bin/logitech_e3500.sh file. Logger command is used to write to syslog when script is triggered or when commands produce error:

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#!/bin/bash
 
logger "Logitech E3500 detected, adjusting configuration options..."
v4l2-ctl -c power_line_frequency=1 2>&1 | logger
v4l2-ctl -c exposure_auto_priority=0 2>&1 | logger
logger "Logitech E3500 bug #459445 detection..."
sleep 10
number_of_string_occurences=$(cat /var/log/kern.log | tail -30 | grep -iw "cannot set freq 16000 to ep 0x86" | wc -l)
if [ $number_of_string_occurences -ge 3 ]
        then
        logger "Logitech E3500 bug #459445 detected! Working around it!"
        sleep 5
        rmmod -w snd-usb-audio
        sleep 7
        modprobe snd-usb-audio
else
        logger "Logitech E3500 bug #459445 NOT detected!"
fi

I'll also make this script executable:

sudo chmod +x /usr/bin/logitech_e3500.sh

Creating udev rule to trigger script

First thing to do is to find out a few things about my web cam. I'll use lsusb command to do this. It's output might look like this:

...
Bus 001 Device 004: ID 046d:09a4 Logitech, Inc. QuickCam E 3500
...

I'll write down 046d:09a4 part because I'll need it to target my USB device while creating udev rule. Now it's time to create the udev rule. I place following inside /etc/udev/rules.d/85-my_e3500_rule.rules file:

ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="046d", ATTR{idProduct}=="09a4", RUN+="/usr/bin/logitech_e3500.sh"

Please note ATTR{idVendor}=="046d" and ATTR{idProduct}=="09a4" parts where I target the device this rule is created for.

Now it's time to put your rule to test. First let's restart udev service to pickup our new rule. Now you can use tail command to watch syslog while plugging your device in to monitor your script in action.

sudo service udev restart
tail -f /var/log/syslog

If you've configured all settings correctly and if you script is using logger command you should see your log status messages being printed after plugging in the device.

That's it. Enjoy!