How to Find Out All the Dependencies of a Gem

How do I find out which gem has a specific dependency?

In the bash shell you can do:

gem dependency name_of_the_gem --reverse-dependencies

For instance:

$ gem dependency activesupport --reverse-dependencies                        
Gem activesupport-2.3.14
Used by
actionpack-2.3.14 (activesupport (= 2.3.14))
activerecord-2.3.14 (activesupport (= 2.3.14))
activeresource-2.3.14 (activesupport (= 2.3.14))

How do I check that all the dependencies of my installed Ruby gems are satisfied?

I know you said you weren't interested in answers about Bundler, but…

Bundler will handle gem dependency resolution for you and is compatible with Rails 2.3. I've used Bundler with a number of Rails 2 apps and not had any problems with it.

There are instructions for installing Bundler on Rails 2.3 here: http://gembundler.com/rails23.html

Command for displaying a gem's dependencies?

The following information was pulled from the rubygems command reference linked below.

http://guides.rubygems.org/command-reference/#gem-dependency

The first command you're asking for is "gem dependency". Below is the command description.

gem dependency GEMNAME [options]

Options:
-v, --version VERSION            Specify version of gem to uninstall
-r, --[no-]reverse-dependencies  Include reverse dependencies in the output
-p, --pipe                       Pipe Format (name --version ver)

Common Options:
    --source URL                 Use URL as the remote source for gems
-h, --help                       Get help on this command
    --config-file FILE           Use this config file instead of default
    --backtrace                  Show stack backtrace on errors
    --debug                      Turn on Ruby debugging

Arguments:
GEMNAME   name of gems to show

Summary:
Show the dependencies of an installed gem

Defaults:
--version '> 0' --no-reverse

The second command you'll need is "gem install". Dependencies get installed automatically. Read the quote below from the command reference for more detail.

"gem install" will install the named
gem. It will attempt a local
installation (i.e. a .gem file in the
current directory), and if that fails,
it will attempt to download and
install the most recent version of the
gem you want.

If a gem is being installed remotely,
and it depends on other gems that are
not installed, then gem will download
and install those, after you have
confirmed the operation.

How to see the dependency tree just from Gemfile?

gem dependency (with no args) should show you all gems from current system with their dependencies.

bundle exec gem dependency will show you for the current Gemfile

Edit:

You can also do gem dependency -R (or just dep instead of dependency) if you want to find out which gems use specific (or all) gems.

For deeper dependencies I'd parse output (regex maybe?) of first gem dependencies, pick gem's names and call gem dep on each of them, but that's just a loose idea.

Ruby - How to find out all dependencies of a given Gem in a .rb file?

You can yank the code out of lib/rubygems/commands/dependency_command.rb. Here's a simple method I made.

require 'rubygems/commands/dependency_command'

def get_dependencies name, local = true
  cmd = Gem::Commands::DependencyCommand.new

  dependency = cmd.gem_dependency name, nil, nil

  specs =
  if local
    dependency.matching_specs.uniq.sort
  else
    cmd.fetch_remote_specs(dependency).uniq.sort
  end

  dependencies = []
  specs.each do |spec|
    dependencies.concat spec.dependencies.sort_by { |dep| dep.name }.map { |dep| [dep.name, dep.requirement] }
  end
  dependencies
end

puts get_dependencies('diamond_shell', false)

Get all dependencies of a specific version of a Gem on RubyGems

I found the solution. The best (and only way) is to use the dependency file of a gem, who list all dependencies of each version of the gem.
To get this file, it's necessary to call the dependency address of the API : http://guides.rubygems.org/rubygems-org-api/#misc-methods (the last one).

After that, this few line do all the stuff to get dependencies of a specific version :

  url          = URI("https://rubygems.org/api/v1/dependencies?gems=#{gem_name}")
  dependencies = Net::HTTP.get(url)
  data         = Marshal.load(dependencies).each do |dependency|
    break dependency if dependency[:number] == gem_version
  end

How to list gem dependencies for some non-intalled gem?

I know that this is a bit of a necro, but I think that you can use gem install --explain rails.

from http://guides.rubygems.org/command-reference/#gem-install

--explain - Rather than install the gems, indicate which would be installed

You may have to run that command from another system that CAN access the rubygems.org server, get the list of gems, pull them down, and install them on your system instead of hitting rubygems directly (till you get your network problem figured out)

How to trace and check dependencies in bundled Ruby gems

To see a visual representation of the dependency tree run bundle viz:

apt-get install graphviz && gem install ruby-graphviz && bundle viz

It will generate a PNG file of the tree.

How to check system package dependency of a gem

You can't, per se. The dependency is on the libcurl headers, so you could try to find those headers in the available search paths, and then make a recommendation of which package to install based on the platform being installed on, but the libcurl headers could be installed by any number of packages, or even from source, so there's no one way to say "curb requires libcurl-devel" in a general case.