gistfile1.txt

… Create a file:

octobrain:~ will$ echo foo > /tmp/foo

… launch IRB:

octobrain:~ will$ irb
ruby-1.9.2-p0 > f = File.open('/tmp/foo')
 => #<File:/tmp/foo> 
ruby-1.9.2-p0 > f.flock(File::LOCK_EX)
 => 0

… Meanwhile in another terminal:

octobrain:~ will$ cat /tmp/foo 
foo
octobrain:~ will$ echo bar > /tmp/foo
octobrain:~ will$ cat /tmp/foo 
bar

– Back in IRB:

ruby-1.9.2-p0 > f.close
 => nil 
ruby-1.9.2-p0 > 

… The file contents:

octobrain:~ will$ cat /tmp/foo 
bar