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

grep '^ERROR' < data/a.log