ARGF: The Diamond Operator on Steroids

2026-03-14

ARGF is Ruby's equivalent of Perl's diamond operator <>. It automatically reads from files listed in ARGV, or from STDIN if no files are given. But here's the thing: it's actually MORE powerful than Perl's version. If that doesn't get your attention, you might be reading the wrong blog.

Part 1: The Perl Translation

# Perl: read from files or stdin
while (<>) {
    print if /pattern/;
}

# Ruby: identical behavior
ARGF.each_line do |line|
  puts line if line =~ /pattern/
end

Same behavior. Different syntax. Both read from filenames passed on the command line, or from STDIN if no files are given.

Part 2: Basic Usage

# These are all equivalent:
$&lt;.each_line { |l| puts l }
ARGF.each_line { |l| puts l }

# Run with:
# ruby script.rb file1.txt file2.txt    (reads both files)
# cat file.txt | ruby script.rb          (reads stdin)
# ruby script.rb                         (waits for stdin)

Part 3: ARGF Methods

Reading lines one at a time:

ARGF.each_line do |line|
  # process each line from all input files/stdin
end

# Or read all lines at once
all_lines = ARGF.readlines

Slurping entire input into one string:

# Read everything into one string
content = ARGF.read

# Equivalent one-liner:
ruby -0777 -ne 'puts $_.length' file.txt

Getting the current filename:

ARGF.each_line do |line|
  puts "#{ARGF.filename}:#{ARGF.lineno}: #{line}"
end

Skipping to next file (this is where ARGF flexes on Perl):

ARGF.each_line do |line|
  if line =~ /SKIP_THIS_FILE/
    ARGF.skip      # Move to next file in ARGV
    next
  end
  puts line
end

Part 4: ARGF vs Perl's Diamond Operator

Feature	Perl `<>`	Ruby ARGF
Read from files/stdin	Yes	Yes
Current filename	`$ARGV`	`ARGF.filename`
Line number	`$.`	`ARGF.lineno` or `$.`
Skip to next file	`close ARGV`	`ARGF.skip`
Read all at once	`local $/; <>`	`ARGF.read`
Get all lines	`my @lines = <>`	`ARGF.readlines`
Binary read	Via `read`	`ARGF.binmode; ARGF.read`

Ruby wins on readability here. ARGF.skip is a lot clearer than close ARGV.

Part 5: Practical One-Liners

# Grep across multiple files with filename
ruby -ne 'puts "#{$FILENAME}:#{$.}: #{$_}" if /ERROR/' *.log

# Count total lines across all input files
ruby -ne 'END { puts $. }' file1.txt file2.txt

# Cat with line numbers (like cat -n)
ruby -ne 'printf "%6d  %s", $., $_' file.txt

Part 6: The -n and -p Connection

When you use -n or -p switches, Ruby wraps your code in an ARGF loop automatically:

# ruby -ne 'CODE' is equivalent to:
while gets
  CODE
end

# ruby -pe 'CODE' is equivalent to:
while gets
  CODE
  print $_
end

The gets method reads from ARGF by default, which is why -n and -p automatically handle files and stdin. ARGF is always there, quietly doing the work.