add description about required permissions and sudo

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

README.md

@@ -168,6 +168,16 @@ By default (`0.5`), the command detects log files that have not been collected u
 $ fluent-tailcheck --ratio 0.7 /var/log/td-agent/pos/secure
 ```
 
+### Permission
+
+`fluent-tailcheck` requires read permission to the specified pos files and the target log files.
+
+Please use `sudo` (on Linux-like) or a terminal with administrative privileges (on Windows).
+
+```console
+$ sudo fluent-tailcheck /var/log/td-agent/pos/secure
+```
+
 ### Result example
 
 #### No anomalies found

lib/fluent/tail_checker/collection_ratio_checker.rb

@@ -73,11 +73,18 @@ def get_file_size(pos_entry)
       end
 
       def get_file_size_from_path(pos_entry)
-        FileTest.size?(pos_entry.path)
+        size = FileTest.size?(pos_entry.path)
+        unless size
+          $stderr.puts "Skip #{pos_entry.path} because it is not found. Make sure you have the proper privilege, e.g. 'sudo'."
+        end
+        size
       end
 
       def get_file_size_from_inode(pos_entry)
-        return nil unless FileTest.exist?(pos_entry.path)
+        unless FileTest.exist?(pos_entry.path)
+          $stderr.puts "Skip #{pos_entry.path} because it is not found. Make sure you have the proper privilege, e.g. 'sudo'."
+          return nil
+        end
 
         stat = Fluent::FileWrapper.stat(pos_entry.path)
         # If follow_inodes is enabled, the inode of the current logfile should match the inode in the pos_file.

lib/fluent/tail_checker/tail_check.rb

@@ -45,6 +45,9 @@ def parse_command_line(argv)
           Example: fluent-tailcheck /path/to/pos/*
           Example: fluent-tailcheck --follow-inodes /path/to/pos_with_follow_inodes
 
+          Requires read permission to the specified pos files and the target log files.
+          Example: sudo fluent-tailcheck /path/to/pos1
+
           If you have any issues with this command, please report it to https://github.com/clear-code/fluent-tail_checker/issues.
 
           Options:
@@ -73,7 +76,7 @@ def validate_paths(paths)
         Enumerator.new do |y|
           paths.each do |path|
             unless FileTest.exist?(path)
-              $stderr.puts "File does not exist. Skipped. Path: #{path}"
+              $stderr.puts "Skip #{path} because it is not found. Make sure you have the proper privilege, e.g. 'sudo'."
               next
             end
 
@@ -117,7 +120,7 @@ def check
       def try_to_open_pos_file(path)
         PosFile.load(path)
       rescue => e
-        $stderr.puts "Can not open the file. Skipped. Path: #{path}, Error: #{e}"
+        $stderr.puts "Can not open the file. Skipped. Make sure you have the proper privilege, e.g. 'sudo'. Path: #{path}, Error: #{e}"
       end
     end
   end