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/context.txt <<'GRP_INPUT_1'
one
two
HIT first
four
five
six
HIT last
eight
nine
GRP_INPUT_1

grep -B 1 'HIT first' data/context.txt