On macOS, using grep:
> daml damlc inspect-dar some.dar \
| grep --only-matching '^daml-stdlib-\d*\.\d*\.\d*'
daml-stdlib-2.8.5
On macOS, using sed:
> daml damlc inspect-dar some.dar \
| sed -n 's/^daml-stdlib-\([0-9]*\.[0-9]*\.[0-9]*.*\)-.*$/\1/p'
2.8.5