Docker launcher for proper dbus session support in between separate exec calls

docker-dbus-session-launcher.sh

#!/usr/bin/env bash
set -euf -o pipefail

declare uid
uid="$(id -u)"

declare tmp_dir="${TMPDIR:-/run/user/${uid}}"
if ! [[ -d "$tmp_dir" ]]; then
  # Fallback to unsafe tmp dir (docker case).
  tmp_dir="/tmp"
fi

declare dbus_session_address_file="$tmp_dir/current-user-${uid}-dbus-session-address.txt"

declare dbus_session_address
if [[ -f "$dbus_session_address_file" ]]; then
  dbus_session_address="$(cat "$dbus_session_address_file")"
else
  declare dbus_session_conf_files=( \
    "/usr/share/dbus-1/session.conf" \
    "/run/current-system/sw/share/dbus-1/session.conf" \
  )

  declare dbus_session_conf_file
  dbus_session_conf_file="$(find "${dbus_session_conf_files[@]}" 2>/dev/null || true | head -n 1)"


  if [[ -z "$dbus_session_conf_file" ]] || ! [[ -f "$dbus_session_conf_file" ]]; then
    1>&2 printf "ERROR: No 'session.conf' file found at expected locations:"
    1>&2 printf " '%q'" "${dbus_session_conf_files[@]}"
    1>&2 printf ".\n"
    exit 1
  fi

  declare dbus_daemon_args=( --fork \
    --config-file="$dbus_session_conf_file" \
    --print-address \
  )
  1>&2 printf "$ dbus_session_address=\"\$(dbus-daemon"
  1>&2 printf " %q" "${dbus_daemon_args[@]}"
  1>&2 printf ")\"\n"
  dbus_session_address="$(dbus-daemon "${dbus_daemon_args[@]}")"

  1>&2 printf "$ echo \"\$dbus_session_address\" > '%s'\n" "$dbus_session_address_file"
  echo "$dbus_session_address" > "$dbus_session_address_file"
fi

if [[ -z "$dbus_session_address" ]]; then
  1>&2 printf "ERROR: Unexpected empty 'dbus_session_address'.\n"
  exit 1
fi

printf "$ export DBUS_SESSION_BUS_ADDRESS=%s\n" "$dbus_session_address"
export "DBUS_SESSION_BUS_ADDRESS=$dbus_session_address"

if [ $# -gt 0 ]; then
    exec bash -c "$*"
else
    exec bash -i
fi

See ./docker-dbus-session-launcher.sh for a
nice little launcher that will allow one to exec services and commands depending
on a dbus session from within a docker container.

To be run used as your entry point implementation.

my-docker-entry-point.sh:

./docker-dbus-session-launcher.sh "$@"
# ..

This launche makes sure to:

1. launch a dbus session daemon it does not already exists, reuse the existing
   daemon otherwise.
2. export the DBUS_SESSION_BUS_ADDRESS env var.

So, in doing so this dbus session will be reused in between call to docker exec.

This little launcher should work on both nixos, ubuntu based docker containers
and most likely many others. If not, it is easy to adapt.