rails paperclip and passenger `is not recognized by the 'identify' command`

This is related to ImageMagick. The command_path option needs to point to the location where identify is installed. From the command line, you can determine this with which identify.

$ which identify
/some/path/to/identify

Afterwards, set command_path to that path (in config/environments/development.rb):

Paperclip.options[:command_path] = "/some/path/to"

This can also be caused by using fairly old versions of Paperclip (for example, version 2.4.x) with newer, incompatible versions of the Cocaine gem.

I was able to resolve this by doing the following:

brew uninstall imagemagick
brew install imagemagick