Trailing Conditionals: Put the Important Stuff First

2026-03-18

Just like Perl, Ruby lets you put conditions at the end of a statement. The action comes first, the condition comes second. It reads like English and saves you from writing three lines when one will do. If you already write print if /ERROR/ in Perl, you're going to feel right at home.

Part 1: Postfix if and unless

# Postfix if
puts line if line =~ %r~error~i
# Perl: print $line if $line =~ /error/i;

# Postfix unless
puts line unless line.empty?
# Perl: print $line unless $line eq '';

# With method calls
next unless line.include?("WARNING")     # skip iteration
exit 1 if errors.any?                    # bail out
retry if attempts < 3                    # retry block

Part 2: unless vs if !

unless is Ruby's (and Perl's) negative conditional. Reads more naturally than if !:

# These are equivalent:
puts "missing" unless File.exist?(path)
puts "missing" if !File.exist?(path)

# unless reads better for simple negation
# Avoid unless with complex conditions though:
# BAD: do_thing unless !ready? && !paused?  (confusing)
# GOOD: do_thing if ready? || paused?

Rule of thumb: if you need unless with && or ||, you've gone too far. Switch to if and flip the logic.

Part 3: Ternary Operator

Same as Perl and C:

status = count > 0 ? "found" : "none"
label = valid? ? "OK" : "FAIL"

# In interpolation:
puts "Status: #{count > 0 ? 'found' : 'none'}"

# Perl: my $status = $count > 0 ? "found" : "none";

Part 4: Postfix while and until

# Rarely used but available
line = gets until line =~ %r~\S~    # skip blank lines
attempts += 1 while retrying?

Part 5: Practical One-Liners

# Print non-empty, non-comment lines
ruby -ne 'puts $_ unless $_ =~ %r~^\s*(#|$)~'

# Number only matching lines
ruby -ne 'printf "%d: %s", $., $_ if /ERROR/'

# Skip header line
ruby -ne 'puts $_ unless $. == 1' data.csv

Trailing conditionals are the single best feature for one-liners. The action is what you see first, the condition is the fine print. Perfect for scanning code and knowing immediately what it does.