Use RAM as tmp overlay

tmpoverlay.sh

#!/bin/bash

set -e

# Assuming tmpfs on $TMPDIR / /tmp

function cleanup {
    echo
    if findmnt -t overlay "$D" &>/dev/null; then
        sudo umount -l "$D"
        if [ -d "$UPPER" ]; then
            if [ x"$(ls -1A ${UPPER})" != x ]; then
                echo "Copying from upper dir to lower dir"
                cp -av "$UPPER"/. "$D"
            fi
            rm -rv "$UPPER"
        fi
        test -d "$WORK" && sudo rm -rv "$WORK"
    fi
}

if [ x"$1" == x ]; then
    echo "${0} <directory>"
    exit
fi

D=$(realpath -e "$1")
if [ ! -d "$D" ]; then
    echo "${D} not a directory."
    exit 1
fi

UPPER=$(mktemp -d --tmpdir "$(id -u).upper.XXX")
WORK=$(mktemp -d --tmpdir "$(id -u).work.XXX")

echo "UPPER DIR: $UPPER"
echo "WORK DIR: $WORK"

trap cleanup EXIT

sudo mount -t overlay overlay \
     -olowerdir="$D",upperdir="$UPPER",workdir="$WORK" \
     "$D"

echo "Ctrl-C when you're done"

while true; do
      sleep 3600
done