Install JRuby and PiCrate on debian (Rakefile)

gem_path.sh

if [ -z $GEM_HOME ]; then
  mkdir -p ~/.gem/ruby/2.5.0/bin
  echo "export GEM_HOME=\"\${HOME}/.gem/ruby/2.5.0\"" >> ~/.profile
  echo "export GEM_PATH=\"\${HOME}/.gem/ruby/2.5.0\"" >> ~/.profile
  echo "export PATH=\"\${PATH}:\${GEM_PATH}/bin\"" >> ~/.profile
  echo "gem: --no-document" >> ~/.gemrc
  source ~/.profile
fi

Rakefile

# frozen_string_literal: true

require 'rake/clean'

WARNING = <<-WARN.freeze
   WARNING: Something has gone wrong with download
   check that the JRUBY_VERSION and download path
   are still valid

WARN

JRUBY_VERSION = '9.2.8.0'.freeze
URL = [
  'https://repo1.maven.org',
  'maven2',
  'org',
  'jruby',
  'jruby-dist',
  JRUBY_VERSION,
  "jruby-dist-#{JRUBY_VERSION}-bin.tar.gz"
].join('/').freeze

SHA256 = [
  URL,
  'sha256'
].join('.').freeze

CLOBBER << "jruby-dist-#{JRUBY_VERSION}-bin.tar.gz"
CLOBBER << "jruby-dist-#{JRUBY_VERSION}-bin.tar.gz.sha256"

task default: %i[download_jruby install_jruby install_gem]

desc 'Downloading JRuby'
task :download_jruby do
  begin
    sh "wget #{URL}"
    sh "wget #{SHA256}"
  rescue
    warn(WARNING)
  end
  value = File.read("jruby-dist-#{JRUBY_VERSION}-bin.tar.gz.sha256")
  check_sha256("jruby-dist-#{JRUBY_VERSION}-bin.tar.gz", value)
end

desc 'Install JRuby'
task :install_jruby do
  tar_ball = File.join(Dir.pwd, "jruby-dist-#{JRUBY_VERSION}-bin.tar.gz")
  Dir.chdir('/opt') do
    sh "sudo tar xzvf #{tar_ball}"
  end
  sh "sudo update-alternatives --install /usr/bin/jruby jruby /opt/jruby-#{JRUBY_VERSION}/bin/jruby 51"
  sh "sudo update-alternatives --install /usr/bin/jirb jirb /opt/jruby-#{JRUBY_VERSION}/bin/jirb 51"
  sh "sudo update-alternatives --install /usr/bin/jgem jgem /opt/jruby-#{JRUBY_VERSION}/bin/jgem 51"
end

def check_sha256(filename, expected_hash)
  require 'digest'
  sha256 = Digest::SHA256.new
  File.open(filename, 'r') do |f|
    while (buf = f.read(4096))
      sha256.update(buf)
    end
  end
  return if sha256.hexdigest == expected_hash

  raise "bad sha256 checksum for #{filename} (expected #{expected_hash} got #{sha256.hexdigest})"
end

desc 'install gem'
task :install_gem do
  sh 'jgem install picrate'
end

This rakefile is for first time install of JRuby on a debian system (real target is raspbian for raspberryPI) for PiCrate users.
Put this file in its own folder say installer then rake

suggesting

  echo "export PATH=\"\${PATH}:\${GEM_PATH}/bin\"" >> ~/.profile

for L5 of gem_path.sh: fixes typo and is more polite with paths.

@scruss thanks! Also seems as though there has been mix-up on released gem which I need to fix.