33d3eeadb2
\getenv fetches the value of an environment variable into a psql variable. This is the inverse of the \setenv command that was added over ten years ago. We'd not seen a compelling use-case for \getenv at the time, but upcoming regression test refactoring provides a sufficient reason to add it now. Discussion: https://postgr.es/m/1655733.1639871614@sss.pgh.pa.us