Hack 45. Randomize Your GNOME Wallpaper

Like many Linux enthusiasts, you spend hours each day staring at your computer screen. You're lucky to have nice graphical interfaces sprinkled with pretty pictures to make all the work you do seem more interesting, but after a time, things can still get dull. To spice up your daily working grind, it would be nice to have the GNOME desktop periodically change to a random image. With a collection of suitable wallpaper this could add a little more sparkle to your desktop.

Unfortunately, a default option for performing this kind of randomized wallpaper adjustment does not exist. Therefore, you will write a small script in this hack to accomplish this. After a little time preparing the script, not only will you have a randomized wallpaper set up, but also you will have a deeper understanding of how GNOME can be scripted.

6.2.1. Selecting a Random Image

With a small bash script, it is possible to randomly select an image from a directory and change the current GNOME wallpaper to that image. It's easy to forget just how powerful bash can be; more than just a simple command shell, bash has a whole host of features that make it well-suited for even complex programming tasks.

To begin this hack, you need to have a directory full of wallpapers somewhere. Assume this directory is located at /home/foo/Images/Wallpapers/. This script will take an image from that directory and set it as the current wallpaper. Here's the first part of the script:

#!/bin/bash
export DIR='/home/foo/Images/Wallpapers/'
export NUMBER=$RANDOM
export TOTAL=0

The first line is a standard piece of code that says which program should be used to run the script (in this case bash). After this line is the location of the directory containing your images, stored in the $DIR variable for future use. Next, you store a random number, which is generated by the built-in variable named $RANDOM. You also set $TOTAL to 0 to begin with; this variable stores the total number of images in the directory.

If you get an error when running this script that reports a "Bad Interpreter" or something similar, you should check the documentation for your distribution and make sure the path to the bash binary is correct (/bin/bash should be correct on most distributions).

After this initial code, you need to create a loop that counts the number of images in the directory using the output of the ls command. Because this script doesn't check file types, it is important that you store only images in this directory. Here is the loop:

for f in `ls $DIR`
do
    let "TOTAL += 1"
done
let "NUMBER %= TOTAL"

The line let "NUMBER %= TOTAL" is the part that actually selects which image will be used. This line divides the randomly generated number by the number of images in the directory and stores the remainder of this division in the $NUMBER variable. If you're wondering how this works, the remainder must be between 0 and 1, minus the number of images, and because in the next part the script starts counting from 0, it is possible for any image to be selected with this method.

The final part of the script simply counts through each image to see if it is the one that was selected. When it finds the correct image, it modifies the GConf setting that stores the filename of the wallpaper using the gconftool command (this might be gconftool-2 on some systems). Nautilus notices this change immediately and updates the wallpaper. So, here's the final script:

#!/bin/bash
export DIR='/home/adam/Images/Wallpapers/'
export NUMBER=$RANDOM
export TOTAL=0
for f in `ls $DIR`
do
    let "TOTAL += 1"
done
let "NUMBER %= TOTAL"
export CURRENT=0
for f in `ls $DIR`
do
    if [ $CURRENT = $NUMBER ]
    then
        /usr/bin/gconftool-2 -t string -s /desktop/gnome/background/picture_filename 
$DIR/$f
        break
    fi
    let "CURRENT += 1"
done

Save the script somewhere convenient such as /home/foo/setbg.sh. Also make it executable by running the following in a terminal:

foo@bar:~$  chmod +x setbg.sh

Now when you run this script your wallpaper will be changed.

6.2.2. Automating the Task

Using cron you can run this script automatically at set times. [Hack #70] discusses how to use cron. Another option is to use GNOME's session manager to have the script run when you log in. You can even add a launcher to your panel so that you can change the wallpaper with one click. Just enter /home/foo/setbg.sh as the program name.

Adam McMaster