Published May 30, 2023
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ARGF is a module in Ruby used for reading files passed as command line arguments or in standard input (STDIN) in a terminal. A Ruby script using ARGF may be thought of as implementing a subset of the functions of the Unix command cat.

cat file1
# Output: File 1 contents
cat file2
# Output: File2 contents
echo "standard input" | cat
# Output: standard input

The cat command can accept a filename or it can pipe (|) another command as standard input.

ARGF - The F Is for Files

# argf.rb

The ARGF module has a method .read that can be used to read the entire contents of a file(s) passed in as a command line argument(s).

ruby argf.rb file1 file2
# Output:
# File 1 contents
# File 2 contents

If no file is passed in, ARGF can also detect the standard input stream being piped in.

echo "STDIN" | ruby argf.rb
# Output:

A combination of files and standard input is accepted too.

echo "STDIN" | ruby argf.rb file1 file2 -
# Output:
# File 1 contents
# File 2 contents

ARGV - The V Is for Variable

Under the hood, there is a Ruby array termed ARGV which contains the names of all arguments passed to the file at the command line or through STDIN.

# argf_inspect_argv.rb
puts ARGV.inspect

The ARGV array disregards whether the arguments are valid file names or not.

ruby argf_inspect_argv.rb file1 file2
# Output: ["file1", "file2"]
ruby argf_inspect_argv.rb one two
# Output: ["one", "two"]


As is often the case with terminal commands, flags like --verbose or --all are often used.

ruby argf_inspect_argv.rb --all one two
# Output: ["--all", "one", "two"]

ARGV stores the names of the files and other arguments passed in through the command line.

The .shift method of ARGV can be used to remove any arguments from the ARGV array that are not files.

# argv_shift.rb
puts ARGV.inspect
ruby argv_shift.rb --all one two
["one", "two"]

Note: Files that are processed by are shifted off the ARGV array.


The method .readlines creates an array of strings. Elements within the array represent the file contents passed to the Ruby script. It’s commonly used for separating the lines of a file into array items so they can be iterated over with other methods such as .each.

# argv_readlines.rb
lines = ARGF.readlines
puts lines
puts lines.inspect
ruby argv_readlines.rb file1 file2
# Output:
# Contents of file1
# Contents of file2
# ["Contents of file1\n", "Contents of file2\n"]

.readlines will not accept command line arguments that are not valid file names.

ruby argv_readlines.rb one two
=> argv_readlines.rb:1:in `readlines': No such file or directory @ rb_sysopen - one (Errno::ENOENT) from argv_readlines.rb:1:in `<main>'

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