A digital picture frame with weather forecast

Updating Photos

The simplest way to update the Raspberry Pi might be to wait for some special trigger file to be generated. In this case, you would just list the files in a directory on Dropbox, and when the trigger file exists, you would transfer the files.

Although I might use this method to coordinate a file transfer at work, this solution doesn't exactly scream user friendly. A different method would be to copy over all the files periodically to the Raspberry Pi. In this case, the synchronization is just a matter of removing all existing photos, gathering a list of all images on Dropbox, iterating through that list, and copying over each file. In the future, I might revise the synchronization mechanism, but for now, I will do this once a week.

The first step is to gather up a list of files to be copied over and then to iterate through that list. A rather cute way of doing this is to use both head and tail to get a specific entry from the list:

NAME=`head -${IDX} $WORKFILE | tail -1`

Downloading the file from Dropbox is then just a matter of running dbxcli with the name of the current file to retrieve.

Selecting and Displaying a Picture

Displaying a picture is as easy as running the feh program with the name of the file to be displayed over the desktop. The entire goal of the picture frame is to allow for dynamic updating of photos, but even keeping that in consideration, the photos will remain fairly static most of the time; therefore, I maintain the current list of files to display, which is generated each time the files are copied from Dropbox.

Displaying all photographs is just a matter of looping through the list of existing files. Each photo should be displayed for at least a few minutes. I decided that the script shouldn't run continuously; instead, from the list of pictures, it should choose the current image, display it, and exit. The script, then, is run by a crontab file every few minutes. For this to work, the script will need to maintain its state between runs.

The pickpicture method in Listing 2 retrieves the current item in the list to be displayed, advances the index, and then displays the current picture.

Listing 2

Display Bash Code

01 pickpicture()
02 {
03    IDXFILE=$PICDIR/index.dat
04    cd $PICDIR
05
06    IDX=`cat $IDXFILE`
07    CNT=`cat $PICLIST | wc -l`
08    NAME=`head -$IDX $PICLIST | tail -1`
09
10    IDX=$(($IDX + 1))
11
12    if [ $IDX -gt $CNT ]
13    then
14       echo 1 > $IDXFILE
15    else
16       echo $IDX > $IDXFILE
17    fi
18
19    showpicture $NAME
20
21    cd $INSTDIR
22 }

Downloading Weather Data

Retrieving the weather for a given city is quite easy. The OpenWeather site does the authentication once at the beginning and gives you a time-unlimited key, so you only need to pass the key in with each call:

CMD="http://api.openweathermap.org/data/2.5/weather?id=${CITYID}&appid=${APPID}&units=metric"
curl -s $CMD > $TODAY

The script simply makes a call to the OpenWeather URL with the appropriate parameters, and you receive a JSON object in return from the curl utility. The returned weather structure is not overly complicated: Simply select each field of interest.

Extracting the values was straightforward except for sunrise and sunset, because these values are stored as a Unix UTC value. The good news is that this can easily be converted to a more user-friendly value with the date command:

# parse out data from data file
sunset=`cat $TODAY | jq -r '.sys' |jq -r '.sunset'`
sunset=`date --date=@$sunset +'%H:%M:%S'`

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