Line Filters in Ruby

A line filter is a common type of program that reads input on stdin, processes it, and then prints some derived result to stdout. grep and sed are common line filters.

Here’s an example line filter in Ruby that writes a capitalized version of all input text. You can use this pattern to write your own Ruby line filters.

# We don't need to explicitly import libraries in Ruby
# as they are automatically available

# Wrap STDIN with a buffered reader
ARGF.each_line do |line|
  # Convert the line to uppercase
  ucl = line.upcase

  # Write out the uppercased line
  puts ucl
end

# Check for errors during reading
# In Ruby, exceptions are raised automatically, so we don't need to check explicitly

To try out our line filter, first make a file with a few lowercase lines.

$ echo 'hello'   > /tmp/lines
$ echo 'filter' >> /tmp/lines

Then use the line filter to get uppercase lines.

$ cat /tmp/lines | ruby line_filters.rb
HELLO
FILTER

In this Ruby version:

1. We use ARGF which is a special stream that acts as a concatenation of all the files mentioned on the command line, or uses STDIN if no files are specified.

2. The each_line method on ARGF allows us to iterate over each line of input.

3. We use line.upcase to convert each line to uppercase.

4. puts is used to print each processed line.

5. Ruby automatically handles EOF and raises exceptions for other errors, so we don’t need explicit error checking like in the original example.

This Ruby script provides the same functionality as the original, reading lines from standard input, converting them to uppercase, and writing them to standard output.