If you're using CLI and you're a postgres
user, then you can do this:
psql -c "SELECT version();"
Possible output:
version
-------------------------------------------------------------------------------------------------------------------------
PostgreSQL 11.1 (Debian 11.1-3.pgdg80+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 4.9.2-10+deb8u2) 4.9.2, 64-bit
(1 row)