diff --git a/grader_id b/grader_id
--- a/grader_id
+++ b/grader_id
@@ -1,5 +1,7 @@
 #!/usr/bin/ruby
 
+require 'fileutils'
+
 def talk(str)
   if TALKATIVE
     puts str
@@ -85,5 +87,5 @@
 RAILS_ENV = 'development'
 require RAILS_APP_DIR + '/config/environment'
 
-current_dir = `pwd`
+current_dir = FileUtils.pwd
 grade(ARGV[0].to_i)
diff --git a/lib/engine.rb b/lib/engine.rb
--- a/lib/engine.rb
+++ b/lib/engine.rb
@@ -31,7 +31,7 @@
     # takes a submission, asks room_maker to produce grading directories,
     # calls grader scripts, and asks reporter to save the result
     def grade(submission)
-      current_dir = `pwd`.chomp
+      current_dir = FileUtils.pwd
 
       user = submission.user
       problem = submission.problem
diff --git a/std-script/judge b/std-script/judge
--- a/std-script/judge
+++ b/std-script/judge
@@ -66,7 +66,7 @@
 
 log "Making test result and sandbox directories..."
 
-current_dir = `pwd`
+current_dir = FileUtils.pwd
 current_dir.strip!
 
 if ARGV.length >= 3
@@ -99,7 +99,7 @@
   exit(127)
 end
 execute("#{problem_home}/script/compile #{language} #{source_file}", "Compilation error!")
-compile_message = `cat compiler_message`
+compile_message = open("compiler_message").read
 compile_message.strip!
 call_and_log("Cannot move the compiler message to #{test_result_dir}.") {
   FileUtils.mv("compiler_message", test_result_dir)
diff --git a/std-script/run b/std-script/run
--- a/std-script/run
+++ b/std-script/run
@@ -111,7 +111,7 @@
   result_file.write run_stat.strip 
   result_file.write "\n"
   result_file.close 
-  `rm run_result`
+  FileUtils.rm "run_result"
   # `rm output.txt`   --- keep the output
 
   comment_file.write comment