how to re-draw the line you just printed in Ruby, like to make a progress bar

15 Dec 2015

Here’s something I learned recently. Let’s say you have a program that is going to take a long time, and you want to mark the progress over time. You can print out some information like this:

tasks = Array.new(1000)
tasks.each.with_index do |task, index|
  sleep rand(0..0.1) # (something slow)
  percentage = (index + 1) / tasks.count.to_f
  puts "#{(percentage * 100).round(1)}%"

Which looks kinda like this:

progress bar before picture

Which is, let’s say, serviceable, but not, let’s say, beautiful. It stinks that it printed out all those lines when it didn’t really need to. I would rather it had sort of animated while it went. But how is this done?

This is one of those questions that’s itched at the back of my mind for a while and which, when I finally googled it, was a bit disappointing. It’s just another unix escape character, like \n (which prints a new line). It’s \r, which I now think of as “the backspace to the beginning of the line” magic character.

Armed with this knowledge and some clunky math we can write something like this:

  tasks = Array.new(1000)
  tasks.each.with_index do |task, i|
    width = `tput cols`.to_i
    sleep rand(0..0.1) # (something slow)
    percentage = (i + 1) / tasks.count.to_f
    summary = "#{(percentage * 100).round(1)}% ".rjust("100.0% ".length)
    remaining_chars_for_progress_bar = width - summary.length - 2
    chunks = (percentage * remaining_chars_for_progress_bar).ceil
    spaces = remaining_chars_for_progress_bar - chunks
    bar = "\r#{summary}[#{ '#' * chunks }#{' ' * spaces}]"
    print bar
rescue Interrupt
  system "say 'I was almost done, jeez'" if RUBY_PLATFORM.include?("darwin")

progress bar after gif

Probably you shouldn’t use this – there’s a very nice gem called ruby-progressbar which will work across platforms and lets you customize some things. But it’s nice information to have, because now you can do things like this:


I’ll leave it as an exercise to the reader how to write this one.