set -euo pipefail
export LC_ALL=C
grep_work=$(mktemp -d "${TMPDIR:-/tmp}/grep-example.XXXXXX")
trap 'rm -rf -- "$grep_work"' EXIT
cd "$grep_work"
mkdir -p data
cat > data/a.log <<'GRP_INPUT_1'
INFO alpha
ERROR disk
GRP_INPUT_1
cat > data/b.log <<'GRP_INPUT_2'
WARN beta
ERROR network
GRP_INPUT_2

grep -nH '^ERROR' data/a.log data/b.log