[DOCS] How to prompt for a value if a variable is not set

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.2/static/app-psql.html
Description:

I want to make a small script to show the primary key for a table and put
the name into a variable.

Called from another script, the tablename is set in a variable called
tablename.  But if the variable is not set, I want the script to prompt for
it.

Is there a way to do this?
thx,
-r

--
Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-docs