Fetch Ruby interpreter from GitBash for Windows or Cygwin similar CLI.

ruby-bash-win.sh

#!/usr/bin/env bash
################################################################################
# This script can install RUBY binaries on Windows in Bash.
#
# Link to this file:
#   https://gist.github.com/andkirby/caf9e825e83f38ad98721956d6e0107c/raw/ruby-bash-win.sh
#
# Download latest version by command in GitBash Windows console:
#   $ curl -Ls bit.ly/ruby-bash-win | bash
#
# Example to download 2.5.x version:
#   $ curl -Ls bit.ly/ruby-bash-win | bash -s -- 2.5
#
# Direct link usage:
#   $ curl -Ls https://gist.github.com/andkirby/caf9e825e83f38ad98721956d6e0107c/raw/ruby-bash-win.sh | bash
#
# Downloaded files hierarchy:
#   ~/bin/
#       /ruby             - Last installed RUBY version.
#       /ruby25           - RUBY 2.5.x
#       /ruby23           - RUBY 2.3.x
#   ~/.ruby/
#       /ruby-install     - Script to execute this online file
################################################################################

gist_version=2018-08-07

set -o errexit
set -o pipefail
set -o nounset
#set -o xtrace

ERR_FATAL=1
ERR_LOGIC=2
ERR_PARAMS=3
ERR_FILE_SYSTEM=4
ERR_CONNECTION=6

zip7_bin=${BASH_INSTALLER_7ZIP_BIN:-7za}

# colors
t_red='\e[0;31m'
t_green='\e[0;32m'
t_yellow='\e[0;33m'
t_cyan='\e[0;36m'
t_reset='\e[0m'

echo_head() {
  echo -e "${t_yellow}${@}${t_reset}"
}
echo_note() {
  echo -e "${t_cyan}${@}${t_reset}"
}
echo_notice() {
  echo -e "${t_yellow}notice:${t_reset} ${@}"
}
echo_warning() {
  echo -e "${t_red}warning:${t_reset} ${@}"
}

show_help () {
    cat << EOF
Download RUBY binaries
${gist_version}

Run this command in GitBash Windows console
  $ curl -Ls https://gist.github.com/andkirby/caf9e825e83f38ad98721956d6e0107c/raw/ruby-bash-win.sh | bash
Example to download 2.5.x version.
  $ curl -Ls https://gist.github.com/andkirby/caf9e825e83f38ad98721956d6e0107c/raw/ruby-bash-win.sh | bash -s -- 2.5
Short version (bit.ly):
  $ curl -Ls bit.ly/ruby-bash-win | bash

OPTIONS
  -3, --32
      Download 32-bit version.
  -f, --force
      Force downloading. It will try to overwrite exist files.
EOF
}

check_error () {
  local status=${1}
  shift
  if [ '0' != "${status}" ]; then
    echo "error: $@" > /dev/stderr
    exit ${status}
  fi
}

init_options() {

  x32='x64'
  minor_version=''

  if [ -n "${1:-}" ] && [ "${1:0:1}" != '-' ]; then
    minor_version="${1:-}"
    test -n "${1:-}" && shift

    # check if not empty and it's not an option
    if [[ ! "${minor_version}" =~ ^[0-9]+\.[0-9]+$ ]]; then
      check_error ${ERR_PARAMS} "Invalid version format (${minor_version}). Please use this one: #.##" > /dev/stderr
      exit
    fi
  fi

  # Process options
  # validate and redefine options
  declare -A options
  options=(
    [3]=32
    [f]=force
    [D]=no-download
    [h]=help
  )
  OPTS=`getopt -o $(echo ${!options[*]} | tr -d ' ') -l $(echo ${options[*]} | tr ' ' ',') -- "$@"`
  eval set -- "${OPTS}"

  nts='-nts'
  force=0
  xdebug_install=0
  while true; do
    case "${1}" in
      -f|--force)
        force=1
        shift
        ;;
      -3|--32)
        x32='x86'
        shift 1
        ;;
      -h|--help)
        show_help
        exit 0
        ;;
      -\?)
        show_help
        exit ${ERR_PARAMS}
        ;;
      --)
        shift
        break
        ;;
      *)
        check_error ${ERR_PARAMS} "${0}: unparseable option ${1}."
        ;;
    esac
  done
}

init_user_home_bin() {
  if [ ! -d "$(cd; pwd)/bin" ]; then
    mkdir -p "$(cd; pwd)/bin"
  fi
}

# Fetch 7z from https://www.7-zip.org/
init_7zip() {
  # Unfortunately, that's the only link with .zip file
  # The rest must installed by using GUI
  # See: https://www.7-zip.org/download.html
  local zip7_url='https://www.7-zip.org/a/7za920.zip'

  if ! ${zip7_bin} --help 2> /dev/null > /dev/null; then
    curl -Ls ${zip7_url} --output /tmp/7za920.zip
    unzip -p /tmp/7za920.zip 7za.exe > ~/bin/7za.exe
    zip7_bin='7za'
  fi
}

find_archive() {
  reg_version=${minor_version:-'[0-9]+.[0-9]+'}'.[0-9]+'
  reg_version=$(echo ${reg_version} | sed 's|\.|\\.|g')

  # find archive
  curl -sL ${releases_url} \
      | grep -E 'browser_download_url.*-('${reg_version}').*'${x32}'.7z\"' | head -1 | grep -oE 'http[^"]+'
}

has_installed?() {
  test -f ${install_path}/bin/ruby.exe
}

######################
# Init
init_options ${@:-}

releases_url=https://api.github.com/repos/oneclick/rubyinstaller2/releases

root_dir=$(cd; pwd)'/.ruby'

if [ ! -d ${root_dir} ]; then
  printf 'Making RUBY home dir ~/.ruby/...'
  mkdir -p ${root_dir}
  echo 'OK'
fi

######################
# Find target RUBY version

# download last binary
if [ -n "${minor_version:-}" ]; then
  echo_head 'Looking for the latest RUBY v'${minor_version}'.x releases ('${releases_url}')...'
else
  echo_head 'Looking for the latest RUBY release ('${releases_url}')...'
fi

archive_url=$(find_archive)
archive_file=$(echo ${archive_url} | sed -r 's|^.*/([^/]+)$|\1|')

[ -z "${archive_file}" ] && check_error ${ERR_CONNECTION} 'Cannot fetch last RUBY version.'

download_version=$(echo ${archive_file} | grep -Eo '[0-9]+\.[0-9]+\.[0-9]+(-[0-9]+)?')
install_path=${root_dir}/$(echo ${archive_file} | sed -r 's|\.7z$||')

# -----------------
init_user_home_bin
bin_path=$(cd; pwd)'/bin'

echo_head "Found RUBY version: ${download_version}"

######################
# Fetch RUBY archive
if [ ${force} != 1 ] && [ -f ${install_path}/bin/ruby.exe ]; then
  echo_notice 'This version already downloaded.'
else
  if [ ${force} != 1 ] && [ -f ${root_dir}/${archive_file} ]; then
    echo_notice 'Archive already downloaded by path '${root_dir}/${archive_file}
  else
    echo 'Fetching '${archive_file}'...'
    curl -Ls ${archive_url} --output ${root_dir}/${archive_file}
    echo 'OK'
  fi

  echo "Unpacking: ${root_dir}/${archive_file}..."
  init_7zip
  ${zip7_bin} x -y -o${root_dir} ${root_dir}/${archive_file} > /dev/null || check_error ${ERR_FILE_SYSTEM} 'Could not unpack archive: '${root_dir}/${archive_file}
fi
# validate unpacked ruby.exe
if ! has_installed?; then
  check_error ${ERR_FATAL} "No binary file by path: ${install_path}/bin/ruby.exe"
fi
# Remove archive
rm -f ${root_dir}/${archive_file}

######################
# Generate binary input files
short_version="$(echo ${download_version} \
  | grep -Eo '^([0-9]+)\.([0-9]+)' | tr -d '.')"${x32/x64/}

download_path_print=$(echo ${install_path} | sed -r 's|'$(cd; pwd)'|~|g')

cat << EOF > ${bin_path}/ruby
#!/usr/bin/env bash
${install_path}/bin/ruby.exe \${@}
EOF

cat << EOF > ${bin_path}/ruby${short_version}
#!/usr/bin/env bash
${install_path}/bin/ruby.exe \${@}
EOF

echo 'Binary files:'
echo '  '${bin_path/$(cd; pwd)/'~'}'/ruby => '${download_path_print}'/bin/ruby.exe'
echo '  '${bin_path/$(cd; pwd)/'~'}'/ruby'${short_version}' => '${download_path_print}'/bin/ruby.exe'

# generate install input file
if [ ! -f $(cd; pwd)/.ruby/ruby-install ]; then
  cat << 'EOF' > $HOME/.ruby/ruby-install
#!/usr/bin/env bash
curl -Ls https://gist.github.com/andkirby/caf9e825e83f38ad98721956d6e0107c/raw/ruby-bash-win.sh | bash -s -- ${@}
EOF
fi

echo '  ~/.ruby/ruby-install'
echo
echo 'SWITCH'
echo 'To switch main '${bin_path/$(cd; pwd)/'~'}'/ruby binary file to any version. You may use "~/.ruby/ruby-install".'
echo 'It will use downloaded version but it will try to fetch latest #.##.x version.'
echo

If there is an issue with a required SSL file please take a look this solution:
https://stackoverflow.com/a/35452500/213759