Copying gems from previous version of Ruby in rbenv

You can copy the gems/ and bin/ folders, but this will lead to problems. The files in bin/ have hardcoded paths in them.

I'd recommend reinstalling them, which would be as easy as this:

$ rbenv local 1.9.3-p286
$ gem list | cut -d" " -f1 > my-gems
$ rbenv local 1.9.3-p327
$ gem install $(cat my-gems)

I've been looking at this specifically from the perspective of upgrading and reinstalling without downloading. It's not trivial, and I recommend you do some cleanup of your gems to minimize the amount of processing/installation that needs to be done (e.g., I had five versions of ZenTest installed; I did 'gem cleanup ZenTest' before doing this). Be careful with 'gem cleanup', though, as it removes all but the LAST version: if you need to support an older version of Rails, manually clean up the versions you don't need.

I called this script 'migrate-gems.sh':

#! /bin/bash

if [ ${#} -ne 2 ]; then
  echo >&2 Usage: $(basename ${0}) old-version new-version
  exit 1
fi

home_path=$(cd ~; pwd -P)
old_version=${1}
new_version=${2}

rbenv shell ${old_version}

declare -a old_gem_paths old_gems
old_gem_paths=($(gem env gempath | sed -e 's/:/ /'))

rbenv shell ${new_version}

for ogp in "${old_gem_paths[@]}"; do
  case "${ogp}" in
    ${home_path}/.gem/ruby*|*/.gem/ruby*)
      # Skip ~/.gem/ruby.
      continue
      ;;
  esac

  for old_gem in $(ls -1 ${ogp}/cache/*.gem); do
    gem install --local --ignore-dependencies ${ogp}/cache/${old_gem}
  done
done

There are three pieces that make this work:

1. gem env gempath contains the paths (:-separated) where gems are installed. Because gems are shared in ~/.gem/ruby, I skip this one.
2. gem install accepts --local, which forces no network connections.
3. gem install accepts --ignore-dependencies, which disables dependency checking.

I had a fairly large list of gems to move over today and I didn't want to download from rubygems.org (plus, I needed older versions), so I whipped this up fairly quickly.