Add command line helps

exe/yard-to-rbs-inline

@@ -1,8 +1,6 @@
 #!/usr/bin/env ruby
 
-if Dir.exist?(File.join(__dir__, "..", ".git"))
-  $LOAD_PATH.unshift(File.expand_path('../lib', __dir__))
-end
+$LOAD_PATH.unshift(File.expand_path("../lib", __dir__)) if Dir.exist?(File.join(__dir__, "..", ".git"))
 
-require 'yard_to_rbs_inline'
-YardToRbsInline::Cli.start(ARGV)
+require "yard_to_rbs_inline"
+exit(YardToRbsInline::Cli.start(ARGV))

lib/yard_to_rbs_inline/cli.rb

@@ -1,19 +1,31 @@
 # rbs_inline: enabled
 
-require 'optparse'
+require "optparse"
 
 module YardToRbsInline
   class Cli
     def self.start(argv)
       opt = OptionParser.new
       dry_run = false
 
-      opt.on('-n', '--dry-run', 'Dry run') { dry_run = true }
+      opt.banner = <<~STRING
+        Usage: yard-to-rbs-inline [options] <files ...>
+
+      STRING
+
+      opt.on("-n", "--dry-run", "Dry run") { dry_run = true }
       opt.parse!(argv)
 
+      if argv.empty?
+        warn opt.help
+        return 1
+      end
+
       argv.each do |file|
         Converter.convert(file, dry_run: dry_run)
       end
+
+      0
     end
   end
 end