Add a \getenv command to psql.

\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

doc/src/sgml/ref/psql-ref.sgml

@@ -2237,6 +2237,28 @@ Tue Oct 26 21:40:57 CEST 1999
       </varlistentry>
 
 
+      <varlistentry>
+        <term><literal>\getenv <replaceable class="parameter">psql_var</replaceable> <replaceable class="parameter">env_var</replaceable></literal></term>
+
+        <listitem>
+        <para>
+         Gets the value of the environment
+         variable <replaceable class="parameter">env_var</replaceable>
+         and assigns it to the <application>psql</application>
+         variable <replaceable class="parameter">psql_var</replaceable>.
+         If <replaceable class="parameter">env_var</replaceable> is
+         not defined in the <application>psql</application> process's
+         environment, <replaceable class="parameter">psql_var</replaceable>
+         is not changed.  Example:
+<programlisting>
+=&gt; <userinput>\getenv home HOME</userinput>
+=&gt; <userinput>\echo :home</userinput>
+/home/postgres
+</programlisting></para>
+        </listitem>
+      </varlistentry>
+
+
       <varlistentry>
         <term><literal>\gexec</literal></term>
 

src/bin/psql/command.c

@@ -98,6 +98,8 @@ static backslashResult process_command_g_options(char *first_option,
 												 bool active_branch,
 												 const char *cmd);
 static backslashResult exec_command_gdesc(PsqlScanState scan_state, bool active_branch);
+static backslashResult exec_command_getenv(PsqlScanState scan_state, bool active_branch,
+										   const char *cmd);
 static backslashResult exec_command_gexec(PsqlScanState scan_state, bool active_branch);
 static backslashResult exec_command_gset(PsqlScanState scan_state, bool active_branch);
 static backslashResult exec_command_help(PsqlScanState scan_state, bool active_branch);
@@ -348,6 +350,8 @@ exec_command(const char *cmd,
 		status = exec_command_g(scan_state, active_branch, cmd);
 	else if (strcmp(cmd, "gdesc") == 0)
 		status = exec_command_gdesc(scan_state, active_branch);
+	else if (strcmp(cmd, "getenv") == 0)
+		status = exec_command_getenv(scan_state, active_branch, cmd);
 	else if (strcmp(cmd, "gexec") == 0)
 		status = exec_command_gexec(scan_state, active_branch);
 	else if (strcmp(cmd, "gset") == 0)
@@ -1481,6 +1485,43 @@ exec_command_gdesc(PsqlScanState scan_state, bool active_branch)
 	return status;
 }
 
+/*
+ * \getenv -- set variable from environment variable
+ */
+static backslashResult
+exec_command_getenv(PsqlScanState scan_state, bool active_branch,
+					const char *cmd)
+{
+	bool		success = true;
+
+	if (active_branch)
+	{
+		char	   *myvar = psql_scan_slash_option(scan_state,
+												   OT_NORMAL, NULL, false);
+		char	   *envvar = psql_scan_slash_option(scan_state,
+													OT_NORMAL, NULL, false);
+
+		if (!myvar || !envvar)
+		{
+			pg_log_error("\\%s: missing required argument", cmd);
+			success = false;
+		}
+		else
+		{
+			char	   *envval = getenv(envvar);
+
+			if (envval && !SetVariable(pset.vars, myvar, envval))
+				success = false;
+		}
+		free(myvar);
+		free(envvar);
+	}
+	else
+		ignore_slash_options(scan_state);
+
+	return success ? PSQL_CMD_SKIP_LINE : PSQL_CMD_ERROR;
+}
+
 /*
  * \gexec -- send query and execute each field of result
  */

src/test/regress/expected/psql.out

@@ -282,6 +282,18 @@ select '2000-01-01'::date as party_over
 (1 row)
 
 \unset FETCH_COUNT
+-- \setenv, \getenv
+-- ensure MYVAR isn't set
+\setenv MYVAR
+-- in which case, reading it doesn't change the target
+\getenv res MYVAR
+\echo :res
+:res
+-- now set it
+\setenv MYVAR 'environment value'
+\getenv res MYVAR
+\echo :res
+environment value
 -- show all pset options
 \pset
 border                   1

src/test/regress/sql/psql.sql

@@ -141,6 +141,18 @@ select 'drop table gexec_test', 'select ''2000-01-01''::date as party_over'
 
 \unset FETCH_COUNT
 
+-- \setenv, \getenv
+
+-- ensure MYVAR isn't set
+\setenv MYVAR
+-- in which case, reading it doesn't change the target
+\getenv res MYVAR
+\echo :res
+-- now set it
+\setenv MYVAR 'environment value'
+\getenv res MYVAR
+\echo :res
+
 -- show all pset options
 \pset