I want to have my wallpaper be seasonal (summer, fall, winter, spring), but also update daily with a seasonal themed wallpaper.

So essentially, I am thinking of having 4 directories (summer, fall, winter, spring). During summer, my wallpaper background would rotate through the images in the summer directory on a daily basis. Then on Sept. 21, the wallpaper directory would change to fall, and the wallpaper would then cycle through those images on a daily basis, etc.

I am comfortable writing a script, but where would I start?

How this question is unique

Edit: To clarify further about what makes this question unique. While there are many methods to create a slideshow, they all depend on setting the images directory. What I am asking is how to dynamically change the images directory. So slide show today comes out of the /images/winter/ directory, and slide show in spring comes out of the /images/spring/ directory. I could manually do this by just changing the directory in the appearance settings every season, but I don't want to have to that when I can tell the computer to do it for me.

The basic question is how to do something at the start of Spring, Summer, Fall and Winter. For this I would create a bash script that runs on boot, rather than clogging up cron with entries.

I've approached this answer using the OP's question "How do I develop a script?". So I've deviated from usual method of simply posting a bash script and enhanced the answer with:

• References are included within the code. They link to Stack Exchange answers for solving specific problems. For example: How to copy files, How to get day of year, etc.


• A section on "Testing" is provided as it is something we all need to do


• A section on "Enhancements" is provided because software is usually developed in versions where each is incrementally better than the previous version.

When do seasons start?

From the Farmer's Almanac:

Seasons of 2018

 Season     Astronomical Start                     Meteorological Start

 ======     =====================================  =====================

 SPRING     Tuesday, March 20, 12:15 P.M. EDT      Thursday, March 1 

 SUMMER     Thursday, June 21, 6:07 A.M. EDT       Friday, June 1

 FALL       Saturday, September 22, 9:54 P.M. EDT  Saturday, September 1

 WINTER     Friday, December 21, 5:23 P.M. EST     Saturday, December 1

Convert season start date to Day of Year

For our bash script to work we need to know what day of the year each seasons start.

$ echo $(date --date="March 20" '+%j')

079

$ echo $(date --date="June 21" '+%j')

172

$ echo $(date --date="Sep 22" '+%j')

265

$ echo $(date --date="Dec 21" '+%j')

355

# Reference: https://unix.stackexchange.com/questions/352176/take-input-arguments-and-pass-them-to-date

Create bash script: season.sh

Open the terminal using: Ctrl+Alt+T

Create the directory if it doesn't exist: mkdir -p ~/bin

Edit the script using: gedit ~/bin/season.sh

• 
  Note: Lubuntu user's need to use leafpad instead of gedit
  

Copy and paste the following lines into gedit:

#!/bin/bash

# NAME: season.sh

# PATH: ~/bin

# DATE: December 15, 2018



# NOTE: Written for: https://askubuntu.com/questions/1100934/change-dynamic-wallpaper-directory-every-season/1102084#1102084



# User defined variables, change to suit your needs

# Our directory names, lines indented for cosmetic reasons only

SlideShowDir="~/Season Slide Show"

   SpringDir="~/Pictures/Spring Slide Show"

   SummerDir="~/Pictures/Summer Slide Show"

     FallDir="~/Pictures/Fall Slide Show"

   WinterDir="~/Pictures/Winter Slide Show"



CheckTripWire () {

    # Our last season is in "~/Season Slide Show/CurrentSeason"

    LastSeasonFilename="$SlideShowDir"/CurrentSeason

    LastSeason=$(cat "$LastSeasonFilename")



    [[ "$LastSeason" == "$Season" ]] && return 0 # Season still the same



    # We now know our season has changed.



    rm -f "$SlideShowDir"/{*,.*}           # Erase all files in target

    # Reference: https://askubuntu.com/questions/60228/how-to-remove-all-files-from-a-directory



    echo "$Season" > "$LastSeasonFilename" # Record new season in target



    # Copy new slide show based on season

    if (( "$Season" == SPRING)) ; then

        cp -R "$SpringDir"/. "$SlideShowDir"/

        # Reference: https://stackoverflow.com/questions/3643848/copy-files-from-one-directory-into-an-existing-directory

    elif (( "$Season" == SUMMER)) ; then

        cp -R "$SummerDir"/. "$SlideShowDir"/

    elif (( "$Season" == FALL)) ; then

        cp -R "$FallDir"/. "$SlideShowDir"/

    else

        cp -R "$WinterDir"/. "$SlideShowDir"/

    fi



} # End of CheckTripWire () function.



# Start of Mainline



DOY=$(date '+%j')                     # DOY = Current Day of Year

# Reference: https://stackoverflow.com/questions/10112453/how-to-get-day-of-the-year-in-shell



if ((DOY>=079 && DOY<172)) ; then

    Season="SPRING"                   # Spring has sprung!

    # Reference: https://stackoverflow.com/questions/12614011/using-case-for-a-range-of-numbers-in-bash

elif ((DOY>=172 && DOY<265)) ; then

    Season="SUMMER"                   # Hit the beach!

elif ((DOY>=265 && DOY<355)) ; then

    Season="FALL"                     # Rake those leaves!

else

    Season="WINTER"                   # Shovel the snow!

fi



# Current season establish, now see if we tripped the wire

CheckTripWire



exit 0 # Command not necessary but good habit to signify no Abend.

Save the file in gedit. Now mark it as executable using:

chmod a+x ~/bin/season.sh

Next we need to add it to startup applications. Reference: How do I start applications automatically on login?

Note: You probably already have your slide show setup in startup applications. You will want to use season.sh BEFORE your regular slide show as it deletes and copies files which would crash the slide show program if it started first.

Testing

You will want to test season.sh script when you create it and not wait a year to see if it works properly or not. Reference: https://serverfault.com/questions/138325/faking-the-date-for-a-specific-shell-session

Enhancements

After initially developing a script it is common to enhance it Days, Weeks, Months or even Years later. This section discusses some enhancements you might want to make to session.sh down the road.

Compress files to save disk space

Consider keeping the off-season images compressed in TAR (Tape Archive) format to save on disk space. Then replace the cp (Copy) command with the tar command to un-compress the files. Reference: https://www.rootusers.com/23-tar-command-examples-for-linux/:

For example, we would change:

cp -R "$SpringDir"/. "$SlideShowDir"/

To:

tar -xf "$SpringDir"archive.tar -C "$SlideShowDir"/

... and so on for the other seasons.

Setup variables for season start

Using variables for season start days makes it easier to modify the script and makes the code easier to read (aka code readability).

Consider setting up Variables for start of season:

SpringStart=079

SummerStart=179

FallStart=265

WinterStart=355

Define the variables at the top of the script to make them easier to spot and change. You might want to do this for leap years. You might want to change to "Meteorological" season starts instead of "Astronomical" start dates.

Then change these lines:

if ((DOY>=079 && DOY<172)) ; then

elif ((DOY>=172 && DOY<265)) ; then

elif ((DOY>=265 && DOY<355)) ; then

To this:

if ((DOY>="$SpringStart" && DOY<"$SummerStart")) ; then

elif ((DOY>="$SummerStart" && DOY<"$FallStart")) ; then

elif ((DOY>="$FallStart" && DOY<"$WinterStart")) ; then

NOTE: I was hoping to finish this answer in an hour but it's been two hours and I have to do some Saturday Shopping. To Be Continued...

Perhaps this is an easier way:

1. 
   

   Create a symlink from ~/images/mybackgrounds to ~/images/spring:

   
   
   

   ln -s ~/images/spring ~/images/mybackgrounds
   
   

   
   



2. Use one of these methods to display a background slideshow using images from ~/images/mybackgrounds.



3. 
   

   Set up crontab entries to change the symlink on particular days. Create a file called ~/mycrontab with these contents:

   
   
   

   # min  hr     day     mon  dow
   
   0      9      21      3    *     ln -sf ~/images/spring ~/images/mybackgrounds
   
   0      9      21      6    *     ln -sf ~/images/summer ~/images/mybackgrounds
   
   0      9      21      9    *     ln -sf ~/images/fall ~/images/mybackgrounds
   
   0      9      21      12   *     ln -sf ~/images/winter ~/images/mybackgrounds
   
   

   
   
   

   Run

   
   
   

   crontab ~/mycrontab
   
   

   
   
   

   to register the crontab entries. On March 21 at 9AM, crond will run the command

   
   
   

   ln -sf ~/images/spring ~/images/mybackgrounds
   
   

   
   

thus linking ~/images/mybackgrounds to ~/images/spring. On Jun 21 at 9AM,
crond will change the symlink so that ~/images/mybackgrounds points to
~/images/summer. The slideshow program is configured to select a file from
~/images/mybackgrounds. The path to ~/images/mybackgrounds stays the same,
but now all the contents are different because the symlink points to a different
location. The crontab entries for Sep 21 and Dec 21 pull the same trick.

Step 1: Create a slideshow.py script

Save this in a file called ~/bin/slideshow.py:

#!/usr/bin/env python

import os

import datetime as DT

import itertools as IT

import bisect

import random

import subprocess



# customize cutoffs and image_dirs however you like, but note that there must be

# the same number of items in each, and the items in cutoffs must be in sorted order.

cutoffs = [(3, 21), (6, 21), (9, 21), (12, 21)]

image_dirs = ['~/images/winter', '~/images/spring', '~/images/summer', '~/images/fall']

image_dirs = list(map(os.path.expanduser, image_dirs))



today = DT.date.today()

year = today.year



# convert the cutoffs to actual dates

cutoff_dates = [DT.date(year, m, d) for m, d in cutoffs]

# find the index into cutoff_dates where today would fit and still keep the list sorted

idx = bisect.bisect(cutoff_dates, today)

# use idx to get the corresponding image directory 

image_dir = next(IT.islice(IT.cycle(image_dirs), idx, idx+1))



# list all the files in image_dir (even in subdirectories, and following symlinks)

files = [os.path.join(root, filename)

         for root, dirs, files in os.walk(image_dirs[idx], followlinks=True)

         for filename in files]

# pick a file at random

imagefile = os.path.abspath(random.choice(files))



# find the current process's effective user id (EUID)

euid = str(os.geteuid())

# find the pid of the current EUID's gnome-session

pid = subprocess.check_output(['pgrep', '--euid', euid, 'gnome-session']).strip().decode()

# load all the environment variables of gnome-session

env = open('/proc/{}/environ'.format(pid), 'rb').read().strip(b'x00')

env = dict([item.split(b'=', 1) for item in env.split(b'x00')])

# get the value of DBUS_SESSION_BUS_ADDRESS environment variable

key = b'DBUS_SESSION_BUS_ADDRESS'

env = {key: env[key]}

# call gsettings to change the background to display the selected file

# with the DBUS_SESSION_BUS_ADDRESS environment variable set appropriately

subprocess.call(['gsettings', 'set', 'org.gnome.desktop.background', 'picture-uri',

                 'file://{}'.format(imagefile)], env=env)

Step 2: Make it executable:

chmod 755 ~/bin/slideshow.py

To test that things are working as expected, you can open a terminal and run
slideshow.py repeatedly. You should see the background changing. Note that
slideshow.py looks for images in one of 4 directories, ~/images/spring,
~/images/summer, ~/images/fall, or ~/images/winter depending on the season.

Step 3: Configure crontab

You can use cron to periodically
run a command to change the background, say, once every day or once every minute
minutes.

Make a file called, say, ~/mycrontab, and
put something like this inside:

# min  hr     day     mon  dow

# 0      9      *       *    *    ~/bin/slideshow.py   # run once at 9AM

*      *      *       *    *    ~/bin/slideshow.py   # run once every minute

Then run

crontab ~/mycrontab

to register the change to your crontab.

You should now see the background changing once every minute. (You might even enjoy keeping it this way.)

crontab will ignore lines which begin with #. So if you want the background
to change once per day, uncomment the second line and comment-out the third so
that ~/mycrontab now looks like this:

# min  hr     day     mon  dow

0      9      *       *    *    ~/bin/slideshow.py   # run once at 9AM

# *      *      *       *    *    ~/bin/slideshow.py   # run once every minute

Note however that cron will only run this command if you are logged into the machine
at 9AM that day.