Ruby script to increment a build number

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This is used to increment a build number in a tracking file for use in a build / CI script:

#!/usr/bin/ruby

def increment(filename)
    # Read it
    file_str = ''
    if File.exists?(filename)
        file_str = File.open(filename, 'r') {|file| file.read }
    end

    old_version = file_str.to_i
    new_version = old_version+1

    # Write
    File.open(filename, 'w') {|file| file.write(new_version) }

    puts "Incrementing version number in #{filename} to #{new_version}" 
end

if ARGV.length < 1
    puts 'Usage: VersionIncrement '
else
    increment(ARGV.first)
end