main: Add basic help output

Add basic usage text to the binary and print it when invoked with -h
or with no arguments at all.

Signed-off-by: Karl Palsson <karlp@remake.is>
Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

1 files changed, 47 insertions, 1 deletions

diff --git a/main.c b/main.c
index e4902d9..960b003 100644
--- a/main.c
+++ b/main.c
@@ -37,6 +37,42 @@ struct match_item {
 	struct list_head list;
 };
 
+static void
+print_usage(char *app)
+{
+	printf(
+	"== Usage ==\n\n"
+	"  # %s [-i <file> | -s \"json...\"] {-t <pattern> | -e <pattern>}\n"
+	"  -q		Quiet, no errors are printed\n"
+	"  -h, --help	Print this help\n"
+	"  -i path	Specify a JSON file to parse\n"
+	"  -s \"json\"	Specify a JSON string to parse\n"
+	"  -l limit	Specify max number of results to show\n"
+	"  -F separator	Specify a field separator when using export\n"
+	"  -t <pattern>	Print the type of values matched by pattern\n"
+	"  -e <pattern>	Print the values matched by pattern\n"
+	"  -e VAR=<pat>	Serialize matched value for shell \"eval\"\n\n"
+	"== Patterns ==\n\n"
+	"  Patterns are JsonPath: http://goessner.net/articles/JsonPath/\n"
+	"  This tool implements $, @, [], * and the union operator ','\n"
+	"  plus the usual expressions and literals.\n"
+	"  It does not support the recursive child search operator '..' or\n"
+	"  the '?()' and '()' filter expressions as those would require a\n"
+	"  complete JavaScript engine to support them.\n\n"
+	"== Examples ==\n\n"
+	"  Display the first IPv4 address on lan:\n"
+	"  # ifstatus lan | %s -e '@[\"ipv4-address\"][0].address'\n\n"
+	"  Extract the release string from the board information:\n"
+	"  # ubus call system board | %s -e '@.release.description'\n\n"
+	"  Find all interfaces which are up:\n"
+	"  # ubus call network.interface dump | \\\n"
+	"  	%s -e '@.interface[@.up=true].interface'\n\n"
+	"  Export br-lan traffic counters for shell eval:\n"
+	"  # devstatus br-lan | %s -e 'RX=@.statistics.rx_bytes' \\\n"
+	"	-e 'TX=@.statistics.tx_bytes'\n",
+		app, app, app, app, app);
+}
+
 static struct json_object *
 parse_json(FILE *fd, const char *source, const char **error)
 {
@@ -382,10 +418,20 @@ int main(int argc, char **argv)
 	struct json_object *jsobj = NULL;
 	const char *jserr = NULL, *source = NULL, *separator = " ";
 
-	while ((opt = getopt(argc, argv, "i:s:e:t:F:l:q")) != -1)
+	if (argc == 1)
+	{
+		print_usage(argv[0]);
+		goto out;
+	}
+
+	while ((opt = getopt(argc, argv, "hi:s:e:t:F:l:q")) != -1)
 	{
 		switch (opt)
 		{
+		case 'h':
+			print_usage(argv[0]);
+			goto out;
+
 		case 'i':
 			input = fopen(optarg, "r");