A very (very!) simple screensaver for watching a folder with slow progressive downloads

stwatch.sh

#!/usr/bin/env bash

# Absolutely dumb progressive "screensaver". Basically, it watches a folder and, whenever a new
# image appears, puts that one up fullscreen, then tears down the prior one underneath. Useful for a
# folder where you're either doing slow downloads or rendering illustrations and just want to watch
# the results.

# Requires [feh](https://feh.finalrewind.org/),
# [tac](https://man7.org/linux/man-pages/man1/tac.1.html)
# inotifywait(https://github.com/inotify-tools/inotify-tools).

set -o errexit
set -o nounset
shopt -s nullglob

function _die() {
    echo "ERROR $? IN ${BASH_SOURCE[0]} AT LINE ${BASH_LINENO[0]}" 1>&2
    exit 1
}

trap _die ERR

PID=""

# Disabling because shellcheck has no good answer for "ls -t"
# shellcheck disable=SC2012
LATEST=$(ls -t ./*.png | head -1)

if [ -n "$LATEST" ]; then
    feh --fullscreen --draw-filename --auto-zoom "$LATEST" &
    PID="$!"
fi

inotifywait -m . -e create -e moved_to |
    while read -r _path _action file; do
        # Needed because create can be triggered before the file is completely coherent. Probably
        # not the best way, but haven't seen in glitch yet.
        sleep 1
        feh --fullscreen --draw-filename --auto-zoom "$file" &
        NPID="$!"
        if [ -n "$PID" ]; then
            if ps --pid "$PID" > /dev/null; then
                # This delay prevents a flash as the prior process gets killed faster than X can set
                # up the view for the new image.
                sleep 1
                kill $PID
            fi
        fi
        PID="$NPID"
    done