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/pairs.txt <<'SED_INPUT_1'
a1
a2
b1
b2
SED_INPUT_1

sed -n 'n;p' data/pairs.txt