Get the current phase of the moon and use it to print an emoji moon phase image. OS X only, requires some shell knowledge. (Emoji moon phases will not show up here, but they will when you copy and paste or download the file.)

.bash_prompt

# Get the current phase of the moon for use as prompt
# Adapted from: http://wan.pengganas.net/entry/calculating-phase-of-moon-in-ruby
function moonphase() {
/usr/bin/env ruby <<-EORUBY
  # Convert a date to Julian.
  def julian(year, month, day)
    a = (14-month)/12
    y = year+4800-a
    m = (12*a)-3+month

    return day + (153*m+2)/5 + (365*y) + y/4 - y/100 + y/400 - 32045
  end

  Define the phases of the moon.
  def phase(year, month, day)
    p = (julian(year, month, day) - julian(2000, 1, 6)) % 29.530588853

    if    p < 1.84566 then  return "🌑"  # new
    elsif p < 5.53699 then  return "🌒"  # waxing crescent
    elsif p < 9.22831 then  return "🌓"  # first quarter
    elsif p < 12.91963 then return "🌔"  # waxing gibbous
    elsif p < 16.61096 then return "🌕"  # full
    elsif p < 20.30228 then return "🌖"  # waning gibbous
    elsif p < 23.99361 then return "🌗"  # last quarter
    elsif p < 27.68493 then return "🌘"  # waning crescent
    else                    return "🌑"  # new
    end
  end

  # Get today's phase.
  today = Time.new
  print "#{phase(today.year, today.month, today.day)}"
EORUBY
}

# Set the prompt.
PS1="\$(moonphase)  "

comments.md

Note the two spaces after the emoji moon; it leaves a decent space between prompt and your command.

This is the simplest possible prompt; just a moon phase. You will probably want to incorporate this into something fancier. My .bash_prompt file has its own line for the current path, Git smarts, and more.

Oh and to use - run ./moonPhase.sh yyyy mm dd

where moonPhase.sh is the name of your script!