cmd: add 'help' subcommand to zpool and zfs

'program help subcommand' is a reasonably common pattern for
multifunction command-line programs. This commit adds support for that
style to the zpool and zfs commands.

When run as 'zpool help [<topic>]' or 'zfs help [<topic>]', executes the
'man' program on the PATH with the most likely manpage name for the
requested topic: "zpool-<topic>" or "zfs-<topic>" for subcommands, or
"zpool<topic>" or "zfs<topic>" for the "concepts" and "props" topics.
If no topic is supplied, uses the top "zpool" or "zfs" pages.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kay Pedersen <mail@mkwg.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #15288

cmd/zfs/zfs_main.c

@@ -132,6 +132,8 @@ static int zfs_do_zone(int argc, char **argv);
 static int zfs_do_unzone(int argc, char **argv);
 #endif
 
+static int zfs_do_help(int argc, char **argv);
+
 /*
  * Enable a reasonable set of defaults for libumem debugging on DEBUG builds.
  */
@@ -606,6 +608,9 @@ usage(boolean_t requested)
 		(void) fprintf(fp,
 		    gettext("\nFor the delegated permission list, run: %s\n"),
 		    "zfs allow|unallow");
+		(void) fprintf(fp,
+		    gettext("\nFor further help on a command or topic, "
+		    "run: %s\n"), "zfs help [<topic>]");
 	}
 
 	/*
@@ -8730,6 +8735,25 @@ zfs_do_version(int argc, char **argv)
 	return (zfs_version_print() != 0);
 }
 
+/* Display documentation */
+static int
+zfs_do_help(int argc, char **argv)
+{
+	char page[MAXNAMELEN];
+	if (argc < 3 || strcmp(argv[2], "zfs") == 0)
+		strcpy(page, "zfs");
+	else if (strcmp(argv[2], "concepts") == 0 ||
+	    strcmp(argv[2], "props") == 0)
+		snprintf(page, sizeof (page), "zfs%s", argv[2]);
+	else
+		snprintf(page, sizeof (page), "zfs-%s", argv[2]);
+
+	execlp("man", "man", page, NULL);
+
+	fprintf(stderr, "couldn't run man program: %s", strerror(errno));
+	return (-1);
+}
+
 int
 main(int argc, char **argv)
 {
@@ -8785,6 +8809,12 @@ main(int argc, char **argv)
 	if ((strcmp(cmdname, "-V") == 0) || (strcmp(cmdname, "--version") == 0))
 		return (zfs_do_version(argc, argv));
 
+	/*
+	 * Special case 'help'
+	 */
+	if (strcmp(cmdname, "help") == 0)
+		return (zfs_do_help(argc, argv));
+
 	if ((g_zfs = libzfs_init()) == NULL) {
 		(void) fprintf(stderr, "%s\n", libzfs_error_init(errno));
 		return (1);

cmd/zpool/zpool_main.c

@@ -126,6 +126,8 @@ static int zpool_do_version(int, char **);
 
 static int zpool_do_wait(int, char **);
 
+static int zpool_do_help(int argc, char **argv);
+
 static zpool_compat_status_t zpool_do_load_compat(
     const char *, boolean_t *);
 
@@ -538,6 +540,10 @@ usage(boolean_t requested)
 				(void) fprintf(fp, "%s",
 				    get_usage(command_table[i].usage));
 		}
+
+		(void) fprintf(fp,
+		    gettext("\nFor further help on a command or topic, "
+		    "run: %s\n"), "zpool help [<topic>]");
 	} else {
 		(void) fprintf(fp, gettext("usage:\n"));
 		(void) fprintf(fp, "%s", get_usage(current_command->usage));
@@ -11051,6 +11057,25 @@ zpool_do_version(int argc, char **argv)
 	return (zfs_version_print() != 0);
 }
 
+/* Display documentation */
+static int
+zpool_do_help(int argc, char **argv)
+{
+	char page[MAXNAMELEN];
+	if (argc < 3 || strcmp(argv[2], "zpool") == 0)
+		strcpy(page, "zpool");
+	else if (strcmp(argv[2], "concepts") == 0 ||
+	    strcmp(argv[2], "props") == 0)
+		snprintf(page, sizeof (page), "zpool%s", argv[2]);
+	else
+		snprintf(page, sizeof (page), "zpool-%s", argv[2]);
+
+	execlp("man", "man", page, NULL);
+
+	fprintf(stderr, "couldn't run man program: %s", strerror(errno));
+	return (-1);
+}
+
 /*
  * Do zpool_load_compat() and print error message on failure
  */
@@ -11118,6 +11143,12 @@ main(int argc, char **argv)
 	if ((strcmp(cmdname, "-V") == 0) || (strcmp(cmdname, "--version") == 0))
 		return (zpool_do_version(argc, argv));
 
+	/*
+	 * Special case 'help'
+	 */
+	if (strcmp(cmdname, "help") == 0)
+		return (zpool_do_help(argc, argv));
+
 	if ((g_zfs = libzfs_init()) == NULL) {
 		(void) fprintf(stderr, "%s\n", libzfs_error_init(errno));
 		return (1);

tests/zfs-tests/tests/functional/cli_root/zfs_program/zfs_program_json.ksh

@@ -117,7 +117,9 @@ usage:
 
 For the property list, run: zfs set|get
 
-For the delegated permission list, run: zfs allow|unallow")
+For the delegated permission list, run: zfs allow|unallow
+
+For further help on a command or topic, run: zfs help [<topic>]")
 cnt=0
 for cmd in ${neg_cmds[@]}; do
 	log_mustnot zfs program $cmd $TESTPOOL $TESTZCP $TESTDS 2>&1