flock

LOCK_EX   | Exclusive lock. Only one process may hold an
          | exclusive lock for a given file at a time.
----------+------------------------------------------------
LOCK_NB   | Don't block when locking. May be combined
          | with other lock options using logical or.
----------+------------------------------------------------
LOCK_SH   | Shared lock. Multiple processes may each hold a
          | shared lock for a given file at the same time.
----------+------------------------------------------------
LOCK_UN   | Unlock.

# update a counter using write lock
# don't use "w" because it truncates the file before lock.
File.open("counter", File::RDWR|File::CREAT, 0644) {|f|
  f.flock(File::LOCK_EX)
  value = f.read.to_i + 1
  f.rewind
  f.write("#{value}\n")
  f.flush
  f.truncate(f.pos)
}

# read the counter using read lock
File.open("counter", "r") {|f|
  f.flock(File::LOCK_SH)
  p f.read
}