Animation assembler

#!/bin/bash

# Monitors clipboard for PNG images and writes them as files. When stopped
# turns the saved files into a GIF animation.

function is_installed()
{
    local app="$1"

    if command -v "$app" > /dev/null; then
        return 0
    else
        return 1
    fi
}

function announce()
{
    local message="$1"

    if is_installed "espeak"; then
        espeak -v "whisper" -s 300 "$message"
    else
        echo "$message"
    fi
}

function leave()
{
    local message="$1"
    local exit_status="$2"

    announce "$message"
    exit "$exit_status"
}

for app in "xclip" "gio" "convert" "clipnotify" "play" "zenity"; do
    if ! is_installed "$app"; then
        leave "$app is not installed! Exiting!" 1
    fi
done

cd ~/Images || leave "Couldn't change the directory! Exiting!" 1

gio trash -f frame_*.png

echo -n | xclip -i -selection clipboard

script_PID="$$"

{
    zenity                               \
        --question                       \
        --title="Animation assembler"    \
        --text="Generate the animation?" \
        --no-wrap                        \
        --cancel-label='Abort'

    if [ "$?" -eq 0 ]; then
        convert -quiet -delay 10 -loop 0 frame_*.png output.gif
        announce "Done!"
    else
        announce "Aborted!"
    fi
    kill "$script_PID"
} &

counter=0

while clipnotify -s clipboard; do
    until xclip -o -selection clipboard -t TARGETS | grep "image/png"; do
        sleep 0.3
    done

    frame_number=$(printf "%03d" "$counter")
    current_frame_path="frame_${frame_number}.png"
    (( counter++ ))
    xclip -o -selection clipboard -t image/png > "$current_frame_path" && \
        play -n synth 0.1 sine 220 vol -20dB 2> /dev/null
    echo -n | xclip -i -selection clipboard
done