Add SHELL_ERROR and SHELL_EXIT_CODE magic variables to psql.

These are set after a \! command or a backtick substitution.
SHELL_ERROR is just "true" for error (nonzero exit status) or "false"
for success, while SHELL_EXIT_CODE records the actual exit status
following standard shell/system(3) conventions.

Corey Huinker, reviewed by Maxim Orlov and myself

Discussion: https://postgr.es/m/CADkLM=cWao2x2f+UDw15W1JkVFr_bsxfstw=NGea7r9m4j-7rQ@mail.gmail.com

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

@@ -4267,6 +4267,34 @@ bar
         </listitem>
       </varlistentry>
 
+      <varlistentry id="app-psql-variables-shell-error">
+       <term><varname>SHELL_ERROR</varname></term>
+       <listitem>
+        <para>
+         <literal>true</literal> if the last shell command
+         failed, <literal>false</literal> if it succeeded.
+         This applies to shell commands invoked via the <literal>\!</literal>
+         meta-command or backquote (<literal>`</literal>) expansion.
+         See also <varname>SHELL_EXIT_CODE</varname>.
+        </para>
+       </listitem>
+      </varlistentry>
+
+      <varlistentry id="app-psql-variables-shell-exit-code">
+       <term><varname>SHELL_EXIT_CODE</varname></term>
+       <listitem>
+        <para>
+         The exit status returned by the last shell command.
+         0&ndash;127 represent program exit codes, 128&ndash;255
+         indicate termination by a signal, and -1 indicates failure
+         to launch a program or to collect its exit status.
+         This applies to shell commands invoked via the <literal>\!</literal>
+         meta-command or backquote (<literal>`</literal>) expansion.
+         See also <varname>SHELL_ERROR</varname>.
+        </para>
+       </listitem>
+      </varlistentry>
+
       <varlistentry id="app-psql-variables-show-all-results">
         <term><varname>SHOW_ALL_RESULTS</varname></term>
         <listitem>

src/bin/psql/command.c

@@ -5041,6 +5041,21 @@ do_shell(const char *command)
 	else
 		result = system(command);
 
+	if (result == 0)
+	{
+		SetVariable(pset.vars, "SHELL_EXIT_CODE", "0");
+		SetVariable(pset.vars, "SHELL_ERROR", "false");
+	}
+	else
+	{
+		int			exit_code = wait_result_to_exit_code(result);
+		char		buf[32];
+
+		snprintf(buf, sizeof(buf), "%d", exit_code);
+		SetVariable(pset.vars, "SHELL_EXIT_CODE", buf);
+		SetVariable(pset.vars, "SHELL_ERROR", "true");
+	}
+
 	if (result == 127 || result == -1)
 	{
 		pg_log_error("\\!: failed");

src/bin/psql/help.c

@@ -451,6 +451,10 @@ helpVariables(unsigned short int pager)
 	HELP0("  SERVER_VERSION_NAME\n"
 		  "  SERVER_VERSION_NUM\n"
 		  "    server's version (in short string or numeric format)\n");
+	HELP0("  SHELL_ERROR\n"
+		  "    true if the last shell command failed, false if it succeeded\n");
+	HELP0("  SHELL_EXIT_CODE\n"
+		  "    exit status of the last shell command\n");
 	HELP0("  SHOW_ALL_RESULTS\n"
 		  "    show all results of a combined query (\\;) instead of only the last\n");
 	HELP0("  SHOW_CONTEXT\n"

src/bin/psql/psqlscanslash.l

@@ -19,6 +19,8 @@
 #include "postgres_fe.h"
 
 #include "psqlscanslash.h"
+#include "settings.h"
+
 #include "common/logging.h"
 #include "fe_utils/conditional.h"
 
@@ -772,6 +774,7 @@ evaluate_backtick(PsqlScanState state)
 	PQExpBufferData cmd_output;
 	FILE	   *fd;
 	bool		error = false;
+	int			exit_code = 0;
 	char		buf[512];
 	size_t		result;
 
@@ -783,6 +786,7 @@ evaluate_backtick(PsqlScanState state)
 	{
 		pg_log_error("%s: %m", cmd);
 		error = true;
+		exit_code = -1;
 	}
 
 	if (!error)
@@ -800,10 +804,19 @@ evaluate_backtick(PsqlScanState state)
 		} while (!feof(fd));
 	}
 
-	if (fd && pclose(fd) == -1)
+	if (fd)
 	{
-		pg_log_error("%s: %m", cmd);
+		/*
-		error = true;
+		 * Although pclose's result always sets SHELL_EXIT_CODE, we
+		 * historically have abandoned the backtick substitution only if it
+		 * returns -1.
+		 */
+		exit_code = pclose(fd);
+		if (exit_code == -1)
+		{
+			pg_log_error("%s: %m", cmd);
+			error = true;
+		}
 	}
 
 	if (PQExpBufferDataBroken(cmd_output))
@@ -826,5 +839,10 @@ evaluate_backtick(PsqlScanState state)
 		appendBinaryPQExpBuffer(output_buf, cmd_output.data, cmd_output.len);
 	}
 
+	/* And finally, set the shell error variables */
+	snprintf(buf, sizeof(buf), "%d", wait_result_to_exit_code(exit_code));
+	SetVariable(pset.vars, "SHELL_EXIT_CODE", buf);
+	SetVariable(pset.vars, "SHELL_ERROR", (exit_code == 0) ? "false" : "true");
+
 	termPQExpBuffer(&cmd_output);
 }

src/common/wait_error.c

@@ -127,3 +127,22 @@ wait_result_is_any_signal(int exit_status, bool include_command_not_found)
 		return true;
 	return false;
 }
+
+/*
+ * Return the shell exit code (normally 0 to 255) that corresponds to the
+ * given wait status.  The argument is a wait status as returned by wait(2)
+ * or waitpid(2), which also applies to pclose(3) and system(3).  To support
+ * the latter two cases, we pass through "-1" unchanged.
+ */
+int
+wait_result_to_exit_code(int exit_status)
+{
+	if (exit_status == -1)
+		return -1;				/* failure of pclose() or system() */
+	if (WIFEXITED(exit_status))
+		return WEXITSTATUS(exit_status);
+	if (WIFSIGNALED(exit_status))
+		return 128 + WTERMSIG(exit_status);
+	/* On many systems, this is unreachable */
+	return -1;
+}

src/include/port.h

@@ -495,6 +495,7 @@ extern char *escape_single_quotes_ascii(const char *src);
 extern char *wait_result_to_str(int exitstatus);
 extern bool wait_result_is_signal(int exit_status, int signum);
 extern bool wait_result_is_any_signal(int exit_status, bool include_command_not_found);
+extern int	wait_result_to_exit_code(int exit_status);
 
 /*
  * Interfaces that we assume all Unix system have.  We retain individual macros