i3status is a small program (about 3000 SLOC) for generating a status bar
for 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.

Homepage:
https://i3wm.org/i3status/
