How to get the full path of a shell script to itself as data

readme.md

WIP: Searching for a stable way for a "Bash script to get the full path to itself" agnostic of the way the script is called / sourced from somewhere and the system hosting bash.

See https://stackoverflow.com/q/242538/3320256 and https://stackoverflow.com/q/4774054/3320256

build.sh

#!/bin/bash
set -o nounset  # Exit if undefined variable is used.
set -o errexit  # Exit after first command failure.
set -o pipefail  # Exit if any part of the pipe fails.
# set -o xtrace

echo "--- ${BASH_SOURCE[0]:-NOTSET}"
echo "See https://stackoverflow.com/q/242538/3320256 and https://stackoverflow.com/q/4774054/3320256"
echo -e "                    "
echo "--- direct variant:"
./nightly/inner.sh

echo -e "                    "
echo "--- source variant:"
. ./nightly/inner.sh

__dir="$(dirname "$(readlink -f "${BASH_SOURCE[0]:-NOTSET}")")"
BASEDIR="$("${__dir}/nightly/sets.sh")"
echo -e "                    "
echo "BASEDIR:            $BASEDIR"

inner.sh

#!/bin/bash
# nightly/inner.sh
set -o nounset  # Exit if undefined variable is used.
set -o errexit  # Exit after first command failure.
set -o pipefail  # Exit if any part of the pipe fails.
# set -o xtrace

set +o nounset
echo "called from         "$(pwd)/$line
set -o nounset
echo "\$0                  "$(readlink -f "$0")
echo "BASH_SOURCE         "$(readlink -f "$BASH_SOURCE")
echo "BASH_SOURCE index 0 "$(readlink -f "${BASH_SOURCE[0]:-NOTSET}")
echo "BASH_SOURCE index 1 "$(readlink -f "${BASH_SOURCE[1]:-NOTSET}")
echo "~ index 0 folder    "$(dirname "$(readlink -f "${BASH_SOURCE[0]:-NOTSET}")")

sets.sh

#!/bin/bash
# nightly/sets.sh
set -o nounset  # Exit if undefined variable is used.
set -o errexit  # Exit after first command failure.
set -o pipefail  # Exit if any part of the pipe fails.
# set -o xtrace

cd "$(dirname "$(readlink -f "$0")")/../.." || exit; pwd