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"

status=0
grep 'anything' absent.txt 2>diagnostic.txt || status=$?
printf 'file error: exit=%s\n' "$status"