set -euo pipefail
export LC_ALL=C
sed_work=$(mktemp -d "${TMPDIR:-/tmp}/sed-example.XXXXXX")
trap 'rm -rf -- "$sed_work"' EXIT
cd "$sed_work"
mkdir -p data
cat > data/settings.txt <<'SED_INPUT_1'
# title
mode=old
note=old
# end
SED_INPUT_1

sed '/^mode=/s/=old$/=new/' data/settings.txt