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/events.txt <<'GRP_INPUT_1'
INFO start
WARN retry
ERROR disk
INFO ready
ERROR network
GRP_INPUT_1

grep -H '^ERROR' data/events.txt