If you are in the Ruby business (which probably means „in the Ruby on Rails business“ nowadays) sooner or later you’ll have to play around with different Ruby versions on the same machine as you might run into crashing ruby processes or performance issues . At least you’ll notice that running the standard Debian/Ubuntu Ruby versions might get you into serious trouble as it is several times slower than a manually compiled version (for reference see this launchpad bug and this blog entry.).

So a common situation with is: you have Ruby and a lot of Ruby gems installed and need to switch to a different Ruby version while making sure that you have all gems installed in the new version that you had in the old version. As gems differ from version to version you should also be interested in installing exactly the same gem versions again and not justing doing a install of all recent versions.

As far as I know there is no official way of syncing gems between two ruby installations. So the common way is something like asking ruby for a list of currently installed gems like

$ gem list
 
*** LOCAL GEMS ***
 
actionmailer (2.3.2, 2.2.2, 2.1.1)
actionpack (2.3.2, 2.2.2, 2.1.1)
activerecord (2.3.2, 2.2.2, 2.1.1)
activeresource (2.3.2, 2.2.2, 2.1.1)
activesupport (2.3.2, 2.2.2, 2.1.1)
[...]
ZenTest (4.0.0)

and then running a

$ gem install actionmailer -v 2.3.2
$ gem install actionmailer -v 2.2.2
$ gem install actionmailer -v 2.1.1
[...]
$ gem install Zentest -v 4.0.0

for every gem and every gem version you probably need. As a couple of gems are native extensions they’ll get compiled and you need to wait some seconds or minutes.

As I had to do this task more than once I wrote a small wrapper script that automates the process completely by fetching the list of gems and installing them again on another ruby version:

#!/bin/sh
GEM_FROM=/path/to/old/gem
GEM_TO=/path/to/new/gem
${GEM_FROM} list | sed -n '4,$ p' | \
 while read gem versions; do
  for version in $(echo ${versions} | sed "s/[(),]//g"); do
  echo ${gem} ${version}
  ${GEM_TO} install --no-rdoc --no-ri ${gem} -v ${version}
 done
done

The script uses some regular expression sed magic, friendly tweaked by Mnemonikk (thank you). Please note, that I prefer not to install rdoc and ri, as it saves time and disk space. Feel free to change this to your needs.

The only caveeat in this script are gems that cannot be installed as they come from unknown external repositories or were manually downloaded/installed. Therefore try to make sure to check this after a run of the gem sync script – it won’t stop when a gem cannot be installed which is intended behaviour.

So far about this. Hope, it helps you out when dealing with different Ruby versions. Do you have similar best practices for keeping Ruby gems in sync?