i3status

i3status is a small program (about 1500 SLOC) for generating a status bar for i3bar, dzen2, xmobar or similar programs. It is designed to be very efficient by issuing a very small number of system calls, as one generally wants to update such a status line every second. This ensures that even under high load, your status bar is updated correctly. Also, it saves a bit of energy by not hogging your CPU as much as spawning the corresponding amount of shell commands would.