swatch-cats: A simple monitor to watch the webcam watching my cats

gistfile1.txt

#!/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 monitoring a webcam and want to check the results

# Requires [feh](https://feh.finalrewind.org/),
# inotifywait(https://github.com/inotify-tools/inotify-tools).

echo "$$" > $HOME/.swatch-pid

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=""
ZOOM="${SWATCH_ZOOM:---auto-zoom}"

# Disabling because shellcheck has no good answer for "ls -t"
# shellcheck disable=SC2012
LATEST=$(ls cats-*.webp | grep -v '^feh_' | tail -1)

if [ -n "$LATEST" ]; then
    feh --fullscreen --draw-filename $ZOOM --start-at "$LATEST" $(ls cats-*.webp | grep -v '^feh_') &
    PID="$!"
fi

lastfile="-nofileyet-";
inotifywait -m . -e close_write --include '\.webp' |
    while read -r _path _action file; do
        if [[ "$file" == cats-* ]] && [[ "$file" != "$lastfile" ]]; then
            # 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 2
            echo "Update: $file"
            feh --fullscreen --draw-filename $ZOOM --start-at "$file" $(ls [0-9]*.webp | grep -v '^feh_') &
            lastfile="$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"
        fi
    done