Many of TextMate’s commands use `ruby` and there also is a Ruby → Run command to run your current ruby script (⌘R).
All these commands work out-of-the-box on a standard Tiger/Leopard/Snow Leopard install, but there are ways to break some or all of them:
1. Overwriting or removing `/usr/bin/ruby`.
2. Setting `RUBYOPT` e.g. to `rubygems` but not having that installed for all your versions of `ruby`
3. Setting `TM_RUBY` to a bogus value.
4. Setting `PATH` to a bogus value.
5. Installing a version of `ruby` which is not compatible with TextMate’s ruby extensions (64 bit builds of `ruby` on Snow Leopard)
Never ever do #1. This is bad! Really bad! Please just don’t do it!
Assuming you did not delete `/usr/bin/ruby` the proper way to get TextMate to use your custom installed ruby is either [updating `PATH`] so that the directory with your new ruby is listed before `/usr/bin` or alternatively to set `TM_RUBY` to the version of ruby you wish to use (e.g. `/usr/local/bin/ruby`).
Be aware though that the latter will not affect all TextMate commands, it will however affect the version of ruby used to validate your script (⌃⇧V) and to run it (⌘R). There should be no real reason wanting the other commands to use your new ruby.