set -euo pipefail
export LC_ALL=C
sed_work=$(mktemp -d "${TMPDIR:-/tmp}/sed-example.XXXXXX")
trap 'rm -rf -- "$sed_work"' EXIT
cd "$sed_work"
mkdir -p data
cat > data/log.txt <<'SED_INPUT_1'
INFO start
WARN retry
ERROR disk
INFO ready
SED_INPUT_1

sed '/^INFO/!{s/^/[other] /;}' data/log.txt