Sunday, January 10, 2010

Automate gadget-related tasks by setting up a udev rule

If you're reading this post, odds are good that you own a computer.  Since it is specifically this post that you're reading, odds are good that you also have some kind of peripheral, such as a camera, printer, mp3 player, webcam, usb flash drive, usb hard drive, etc, and that there are certain tasks that you need to perform every time you plug it in.

For instance, you may want your computer to automatically import your pictures when you plug in your camera.  You may need to send your printer its firmware (like I do) every time you plug it into your computer.  You may want to back up your phone or send files to your flash drive automatically.  Or perhaps you just want Skype to launch automatically whenever you have your webcam connected.  Whatever your situation, odds are good that there is some way that you can automate tasks and make your life easier by saving time with udev rules.

Udev is the new device manager for the Linux kernel, replacing the older manager, hotplug.  In this post I will show you how to configure a rule so that udev runs a bash script (which I will also show you how to create) whenever you plug in your device.

Creating the udev rule

The nice thing about udev is that you can make rules for individual devices.  This means that you can have a different procedure run when you plug in your external hard drive than the one that runs when you plug in your USB flash drive.  To get started, plug in the device for which you would like the rule created.  Then run lsusb.

You will get a list of 5 or 6 items, and you should see your device listed by manufacturer.  On that same line, just before the manufacturer's name, you should see two 4-digit alphanumeric values seperated by a colon.  The first value is the Vendor ID and the second is the Product ID; you will need both for your udev rule.  As an example, when I created my rule for my mp3 player, this was the output of lsusb:
Bus 001 Device 012: ID 041e:4139 Creative Technology, Ltd Zen Nano Plus
Bus 001 Device 002: ID 0c45:62c0 Microdia Sonix USB 2.0 Camera
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

You can see my mp3 player listed on the first line, and that the Vendor ID is 041e and the Product ID is 4139.  On to the next step.

Go into /etc/udev/rules.d and, as root, create a new file named 85-my_rule.rules (the file must begin with a 2-digit number followed by a hyphen, and it must end with ".rules"; it doesn't really matter what you put in the middle) and open it in your favorite text editor.  Enter the following:

     ACTION=="add", ATTRS{idVendor}=="your-devices-vendor-id", ATTRS{idProduct}=="your-devices-product-id", RUN+="/path/to/your/bash-script" the rule I created for my Zen Nano looked like this:

     ACTION=="add", ATTRS{idVendor}=="041e", ATTRS{idProduct}=="4139", RUN+="/home/jizldrangs/bin/sync-zennano"

This basically tells udev that whenever you plug in a device with the vendor ID and product ID that you specified, it is to execute the bash script that you specified.  With this setup, the options for what you do when the device is plugged in is only limited by what can be scripted in Bash, so you can see how powerful this can be.

If for whatever reason you would like to manually mount or unmount the volume, you can tell udev to place a symlink in the /dev directory by using the SYMLINK keyword.  Just append SYMLINK+="my_symlink_name" to the end of the rule, and after it runs you can find a symlink to the device in /dev which you can use with the mount or umount commands.

When you are finished with that, restart udev:

     sudo service udev restart

Creating the script

The script that is run when the device is connected can do pretty much anything you want.  As long as it can be done from the command line, you can automate it with udev. 

As an example, and as a segue into my discussion on the various pitfalls of udev, here is what I ended up with:

#! /bin/sh

export HOME=/home/jizldrangs
xhost local:jizldrangs
export DISPLAY=:0.0

CURRENTLY=$( date +%s )
if [ -e $TIMEFILE ]; then
    echo "sync-zennano.time exists"
    echo $CURRENTLY
    echo $FIRST_RUN_TIME
    echo "num of seconds: ${NUMOFSECONDS}"
    if [ $NUMOFSECONDS -gt 60 ]; then
        echo "sync-zennano has not run in over 60 seconds"
        rm -f $TIMEFILE


if ! [ -e $TIMEFILE ]; then

# Send the following block of commands to the background
    sleep 10

    notify-send -u normal -t 2000 -i info "Zen Nano Sync" "Starting synchronization.  Please wait."

    sudo -u jizldrangs unison zennano -auto -batch -terse -ui text

    notify-send -u normal -t 2500 -i info "Zen Nano Sync" "Synchronization of Zen Nano is complete"
} &
    touch $TIMEFILE
    chmod 666 $TIMEFILE
    date +s% > $TIMEFILE


There are several udev "gotchas" that I'm working around here (I only really wanted to run 3 lines, but had to add a bunch of other stuff to get it working right).  It's worth going through each one...

Udev Gotchas

There are a relatively large number of things to watch out for:
  1.  For some reason, it is common for udev to run your script many times in quick succession.  Whenever I plug in my mp3 player, udev runs my script at least 10 times.  To get around that, I added a few lines to my script that would write the time to disk the first time it is run, then check for the existence of the file and not run the payload of the script if it had already been run within the last 60 seconds.  It's clunky but it works.
  2. Make sure you make the script executable with a "chmod 700 /path/to/script".  
  3. Keep in mind that your script will be run as root.  This means:
    1. For security reasons, keep it in a folder where it can't be accessed by anyone but root and remove read/write permissions from all other users, including yourself.
    2. Any environment variables or elements you may be relying on won't be available.  In my script, I am running a Unison profile located in my home directory, so in order to get it working I had to export the path to my home directory into the HOME variable in the first few lines of the script.  Then when it came time to run Unison, i added "sudo -u jizdrangs" at the beginning of the line, which causes that line to be run as me rather than as root.
  4. Udev will wait for your script to finish before mounting the device, so if you are doing any tasks that rely on the device being mounted, send that series of commands to the background by surrounding them with curly braces and adding a space and an ampersand after the close-curly-brace.  This will "detach" the commands, or cause udev to continue its work without waiting for those commands to finish.  Then add a "sleep 10" at the beginning of the commands being sent to the background, so that udev has time to mount the device before the rest of the commands are executed.
  5. Because udev runs the script in its own environment, it won't have information on your display.  I like to have notifications pop up, so I added the "export DISPLAY=:0.0" line towards the top.  The notifications I get using notify-send would not work without it.
It took me a while to get all this working, but it was worth the knowledge I gained along the way.  Now whenever I plug in my mp3 player, it automatically synchronizes my podcasts, and it displays notifications telling me when it begins the sync process and when it is finished.

See also:

No comments:

Post a Comment