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Previously, text between backquotes in a psql metacommand's arguments was always passed to the shell literally. That considerably hobbles the usefulness of the feature for scripting, so we'd foreseen for a long time that we'd someday want to allow substitution of psql variables into the shell command. IMO the addition of \if metacommands has brought us to that point, since \if can greatly benefit from some sort of client-side expression evaluation capability, and psql itself is not going to grow any such thing in time for v10. Hence, this patch. It allows :VARIABLE to be replaced by the exact contents of the named variable, while :'VARIABLE' is replaced by the variable's contents suitably quoted to become a single shell-command argument. (The quoting rules for that are different from those for SQL literals, so this is a bit of an abuse of the :'VARIABLE' notation, but I doubt anyone will be confused.) As with other situations in psql, no substitution occurs if the word following a colon is not a known variable name. That limits the risk of compatibility problems for existing psql scripts; but the risk isn't zero, so this needs to be called out in the v10 release notes. Discussion: https://postgr.es/m/9561.1490895211@sss.pgh.pa.us
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: http://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.