Install and configure Steam for Flatpak on elementary OS 6

setup-steam4flatpak.sh

#!/usr/bin/env bash
# Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted.

# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
readonly ERROR_ASSERT_FAIL=1

function assertContinueOnSuccess() # $1=line number of the command that is being asserted to have completed successfully.
{
    local -r OUTCOME="$?"
    local LOCATION="<unknown>"

    if [ -n "$1" ]
    then
        LOCATION="$1"
    fi

    if [ "$OUTCOME" -ne 0 ]
    then
        echo "The operation at line ${LOCATION} failed with result ${OUTCOME}"
        exit "$ERROR_ASSERT_FAIL"
    else
        return 0
    fi
}

flatpak install --system --runtime org.freedesktop.Platform.VulkanLayer.MangoHud && assertContinueOnSuccess $LINENO
flatpak install --user --app com.valvesoftware.Steam && assertContinueOnSuccess $LINENO

readonly `flatpak info --show-metadata com.valvesoftware.Steam | grep FLATPAK_STEAM_XDG_DIRS_PREFIX`

mkdir "${FLATPAK_STEAM_XDG_DIRS_PREFIX}/.config/MangoHud"
cp "`flatpak info --show-location org.freedesktop.Platform.VulkanLayer.MangoHud`/files/share/doc/mangohud/MangoHud.conf.example" "${FLATPAK_STEAM_XDG_DIRS_PREFIX}/.config/MangoHud"
# TODO: Download & install customized MangoHUD config from gist.github.com
#flatpak override --user --env=MANGOHUD=1 com.valvesoftware.Steam

mkdir "${FLATPAK_STEAM_XDG_DIRS_PREFIX}/media"
# TODO: mount devices (e.g., external disks) containing Steam libraries