♦️ BEGIN, END, and the END of the World

2026-04-07

BEGIN and END blocks are the unsung heroes of Ruby one-liners. They give you setup and teardown for free, turning a simple -ne loop into a proper little program. Plus, Ruby lets you embed data right inside your script with __END__. Because sometimes your config file and your script should just be the same file.

Part 1: BEGIN and END Blocks

Like Perl's BEGIN and END blocks, these run before and after your main code.

# Perl:
BEGIN { $count = 0 }
# main code runs here
END { print "Total: $count\n" }

# Ruby:
BEGIN { $count = 0 }
# main code runs here
END { puts "Total: #{$count}" }

Part 2: One-Liner Power

This is where BEGIN/END really shine. Adding setup and teardown to one-liners:

# Count error lines
ruby -ne 'BEGIN{$n=0}; $n+=1 if /error/i; END{puts $n}' logfile.txt
# Perl: perl -ne 'BEGIN{$n=0} $n++ if /error/i; END{print "$n\n"}' logfile.txt

# Track unique IPs and report count
ruby -ne 'BEGIN{$ips={}}; $ips[$1]=1 if /(\d+\.\d+\.\d+\.\d+)/; END{puts $ips.keys.length}' access.log

# Sum a column
ruby -ane 'BEGIN{$sum=0}; $sum += $F[2].to_f; END{puts $sum}' data.txt

BEGIN runs before ANY code, even require statements that appear before it in the file. END runs after all code completes, even if an exception was raised. They're bulletproof bookends.

Part 3: DATA and END

Ruby has a mechanism for embedding data inside your script, just like Perl's __DATA__ section.

# Perl:
while (<DATA>) {
    chomp;
    print "Got: $_\n";
}
__DATA__
line one
line two
line three

# Ruby:
DATA.each_line do |line|
  line.chomp!
  puts "Got: #{line}"
end
__END__
line one
line two
line three

Part 4: Practical Embedded Config

#!/usr/bin/env ruby

# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Parse embedded config
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
config = {}
DATA.each_line do |line|
  line.chomp!
  next if line.empty? || line =~ %r~^#~
  key, value = line.split(%r~\s*=\s*~, 2)
  config[key] = value
end

puts config.inspect

__END__
# Server configuration
host = localhost
port = 5432
user = admin
database = production

One file. Script and config together. No external dependencies. Perfect for quick utilities where you don't want to manage a separate config file.

Part 5: Key Differences from Perl

Perl	Ruby	Notes
`__DATA__`	`__END__`	Different keyword
`<DATA>`	`DATA.each_line`	Ruby uses IO object methods
Works in any file	Only works in main script	Not available in required files

One gotcha: DATA is an IO object positioned right after the __END__ line. You can only read it once unless you seek back to the beginning with DATA.rewind.