diff --git a/lib/gel/catalog.rb b/lib/gel/catalog.rb
index 14e063d..00c1e68 100644
--- a/lib/gel/catalog.rb
+++ b/lib/gel/catalog.rb
@@ -15,7 +15,7 @@ class Gel::Catalog
   autoload :DependencyIndex, File.expand_path("catalog/dependency_index", __dir__)
   autoload :LegacyIndex, File.expand_path("catalog/legacy_index", __dir__)
 
-  def initialize(uri, httpool: Gel::Httpool.new, work_pool:, cache: ENV["GEL_CACHE"] || "~/.cache/gel", initial_gems: [])
+  def initialize(uri, httpool: Gel::Httpool.new, work_pool:, cache: Gel::Config.cache_dir, initial_gems: [])
     @uri = normalize_uri(uri)
     @httpool = httpool
     @work_pool = work_pool
diff --git a/lib/gel/command/env.rb b/lib/gel/command/env.rb
index 51ca885..fd58851 100644
--- a/lib/gel/command/env.rb
+++ b/lib/gel/command/env.rb
@@ -1,7 +1,211 @@
 # frozen_string_literal: true
 
 class Gel::Command::Env < Gel::Command
+  GEL_ROOT = File.expand_path("../../..", __dir__)
+  SELF_BIN = File.expand_path("exe/gel", GEL_ROOT)
+  HOME_PREFIX = File.join(File.expand_path("~"), "")
+
   def run(command_line)
-    raise "TODO"
+    base_store = Gel::Environment.store
+    base_store = base_store.inner if base_store.is_a?(Gel::LockedStore)
+
+    config_file = Gel::Environment.config.path
+
+    gemfile = Gel::Environment.find_gemfile(error: false)
+
+    puts
+    puts "#### Gel Environment ####"
+    puts "```"
+    path_entry "Context", Dir.pwd
+    if gemfile
+      path_entry "  Gemfile", gemfile
+      path_entry "  Lockfile", Gel::Environment.lockfile_name(gemfile)
+    end
+    puts
+    gel_version = Gel::VERSION
+    if File.exist?(File.expand_path(".git", GEL_ROOT))
+      gel_version = "git #{git_revision(GEL_ROOT)} (#{gel_version})"
+    end
+    text_entry "Gel", gel_version
+    path_entry "  bin", SELF_BIN
+    path_entry "  Store", base_store.root
+    path_entry "  Cache", Gel::Config.cache_dir
+    path_entry "  Config", config_file
+    puts
+    text_entry "Ruby", RUBY_DESCRIPTION
+    path_entry "  bin", Gem.ruby
+    list_entry "  RUBYLIB", ENV['RUBYLIB']&.split(File::PATH_SEPARATOR) || [], subtype: :path
+    text_entry "  RUBYOPT", ENV['RUBYOPT']
+    puts
+    text_entry "System", ""
+    path_entry "  ~", File.expand_path("~")
+    # This doesn't use :path, because a raw '~' in PATH is noteworthy
+    list_entry "  PATH", ENV['PATH']&.split(File::PATH_SEPARATOR) || []
+    path_entry "  SHELL", ENV['SHELL'] || "-"
+    puts "```"
+
+    if command_line.include?("-v") || command_line.include?("--verbose")
+      puts
+      puts "#### Commands ####"
+      puts "```"
+      {
+        "gel" => SELF_BIN,
+        "ruby" => Gem.ruby,
+        "gem" => File.expand_path("../gem", Gem.ruby),
+        "bundle" => File.expand_path("../bundle", Gem.ruby),
+      }.each do |name, expected|
+        if actual = which(name)
+          text_entry name, abbreviated_path(actual), flag: actual == expected ? "" : "!"
+        else
+          text_entry name, "(not present)", flag: "?"
+        end
+      end
+
+      puts
+      stub_set = Gel::Environment.store.stub_set
+      if stub_dir = stub_set&.dir
+        if ENV["PATH"].split(File::PATH_SEPARATOR).map { |s| File.expand_path(s) }.include?(File.expand_path(stub_dir))
+          stubs = stub_set.all_stubs
+          if stubs.empty?
+            puts "# No stubs present"
+          else
+            good_stubs, broken_stubs =
+              stubs.map { |stub| [stub, which(stub)] }.
+              partition { |_, actual| stub_set.own_stub?(actual) }
+
+            puts "# #{good_stubs.size} active stubs in #{abbreviated_path(stub_dir)}"
+            unless broken_stubs.empty?
+              puts "# #{broken_stubs.size} stubs broken"
+              broken_stubs.sort_by { |x, y| [y || "", x] }.each do |name, actual|
+                if actual
+                  text_entry name, abbreviated_path(actual) + " (#{stub_type(actual, stub_set)})", flag: "!"
+                else
+                  text_entry name, "(not found)", flag: "?"
+                end
+              end
+            end
+          end
+        else
+          puts "# Stub directory #{abbreviated_path(stub_dir)} not in PATH"
+          stubs = stub_set.all_stubs
+          puts "# #{stubs.size} stubs present"
+        end
+      else
+        puts "# No stub directory"
+      end
+      puts "```"
+
+      puts
+      puts "#### Runtime ####"
+      puts "```"
+      $LOADED_FEATURES.each do |name|
+        puts abbreviated_path(name)
+      end
+      puts "```"
+
+      puts
+      puts "#### Load Path ####"
+      puts "```"
+      $LOAD_PATH.each do |name|
+        puts abbreviated_path(name)
+      end
+      puts "```"
+
+      puts
+      puts "#### Files ####"
+
+      file_body config_file
+
+      if gemfile
+        file_body gemfile
+        file_body Gel::Environment.lockfile_name(gemfile)
+      end
+    end
+
+    puts
+  end
+
+  private
+
+  def abbreviated_path(path)
+    path = File.expand_path(path)
+    if path.start_with?(HOME_PREFIX)
+      File.join("~", path[HOME_PREFIX.length..-1])
+    else
+      path
+    end
+  end
+
+  def text_entry(label, value, flag: nil)
+    puts("%-16s %1s %s" % [label, flag, value])
+  end
+
+  def path_entry(label, path)
+    text_entry(label, abbreviated_path(path), flag: File.exist?(path) ? "" : "?")
+  end
+
+  def list_entry(label, list, subtype: :text)
+    if list.empty?
+      text_entry(label, "(none)")
+    else
+      first, *rest = list
+      send("#{subtype}_entry", label, first)
+      rest.each do |item|
+        send("#{subtype}_entry", "", item)
+      end
+    end
+  end
+
+  def file_body(path)
+    puts
+    puts "##### `#{abbreviated_path(path)}` #####"
+    if File.exist?(path)
+      puts "```"
+      puts File.read(path)
+      puts "```"
+    else
+      puts "(not present)"
+    end
+  end
+
+  def which(name)
+    ENV['PATH'].split(File::PATH_SEPARATOR).each do |path|
+      if File.exist?(file = File.join(path, name)) && File.executable?(file)
+        return file
+      end
+    end
+    nil
+  end
+
+  def stub_type(path, stub_set)
+    if stub_set.parse_stub(path)
+      return "gel stub"
+    end
+
+    content = File.read(path, 512)
+    if content.include?("This file was generated by Bundler")
+      return "bundler stub"
+    end
+
+    if content.include?("This file was generated by RubyGems") ||
+        content.include?("Gem.activate_bin_path(") ||
+        content.include?("Gem.bin_path(")
+      return "gem stub"
+    end
+
+    if content.match?(/\A#!.*ruby/)
+      return "ruby stub"
+    end
+
+    if content.start_with?("#!")
+      "script"
+    else
+      "binary"
+    end
+  end
+
+  def git_revision(path)
+    require "shellwords"
+    `git -C #{Shellwords.escape path} rev-parse --short HEAD`.strip
   end
 end
diff --git a/lib/gel/config.rb b/lib/gel/config.rb
index 15d3741..0ad5b91 100644
--- a/lib/gel/config.rb
+++ b/lib/gel/config.rb
@@ -30,6 +30,12 @@
 #     GEL_AUTH="https://user@pass:private-gem-server.local"
 
 class Gel::Config
+  def self.cache_dir
+    File.expand_path(ENV["GEL_CACHE"] || "~/.cache/gel")
+  end
+
+  attr_reader :path
+
   def initialize(root_path = "~/.config/gel")
     @root = File.expand_path(root_path)
     @path = File.join(@root, "config")
diff --git a/lib/gel/git_depot.rb b/lib/gel/git_depot.rb
index 0929aee..636e926 100644
--- a/lib/gel/git_depot.rb
+++ b/lib/gel/git_depot.rb
@@ -5,7 +5,7 @@
 class Gel::GitDepot
   attr_reader :mirror_root
 
-  def initialize(store, mirror_root = (ENV["GEL_CACHE"] || "~/.cache/gel") + "/git")
+  def initialize(store, mirror_root = File.join(Gel::Config.cache_dir, "git"))
     @store = store
     @mirror_root = File.expand_path(mirror_root)
   end
diff --git a/lib/gel/stub_set.rb b/lib/gel/stub_set.rb
index 994c2fc..3345516 100644
--- a/lib/gel/stub_set.rb
+++ b/lib/gel/stub_set.rb
@@ -3,7 +3,7 @@
 require_relative "db"
 
 class Gel::StubSet
-  attr_reader :root
+  attr_reader :root, :dir
 
   def initialize(root)
     @root = File.realpath(File.expand_path(root))
@@ -42,6 +42,12 @@ def rebuild!
     end
   end
 
+  def all_stubs
+    @db.reading do
+      @db.each_key.to_a
+    end
+  end
+
   def create_stub(exe)
     Dir.mkdir(@dir) unless Dir.exist?(@dir)
 
diff --git a/test/command/env_test.rb b/test/command/env_test.rb
new file mode 100644
index 0000000..7f05f35
--- /dev/null
+++ b/test/command/env_test.rb
@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "gel/command"
+
+class EnvTest < Minitest::Test
+  def test_env_command
+    output = capture_stdout { Gel::Command.run(%w[env]) }
+
+    assert_match(/^Context/, output)
+    assert_match(/^Gel/, output)
+    assert_match(/^Ruby/, output)
+    assert_match(/^System/, output)
+    assert_match(/^  PATH/, output)
+
+    refute_match(/^ruby /, output)
+    refute_match(/^#{Regexp.escape(home_relative_path(File.expand_path("../../lib/gel/command.rb", __dir__)))}$/, output)
+
+    refute_match(/^source "/, output)
+    refute_match(/^DEPENDENCIES/, output)
+
+    headings = output.lines(chomp: true).select { |line| line.start_with?("##") }
+    assert_equal([
+      "#### Gel Environment ####",
+    ], headings)
+  end
+
+  def test_env_command_verbose
+    output = capture_stdout { Gel::Command.run(%w[env -v]) }
+
+    assert_match(/^Context/, output)
+    assert_match(/^Gel/, output)
+    assert_match(/^Ruby/, output)
+    assert_match(/^System/, output)
+    assert_match(/^  PATH/, output)
+
+    assert_match(/^ruby /, output)
+    assert_match(/^#{Regexp.escape(home_relative_path(File.expand_path("../../lib/gel/command.rb", __dir__)))}$/, output)
+
+    assert_match(/^source "/, output)
+    assert_match(/^DEPENDENCIES/, output)
+
+    headings = output.lines(chomp: true).select { |line| line.start_with?("##") }
+    assert_equal([
+      "#### Gel Environment ####",
+      "#### Commands ####",
+      "#### Runtime ####",
+      "#### Load Path ####",
+      "#### Files ####",
+      "##### `#{home_relative_path Gel::Config.new.path}` #####",
+      "##### `#{home_relative_path File.expand_path("../../Gemfile", __dir__)}` #####",
+      "##### `#{home_relative_path File.expand_path("../../Gemfile.lock", __dir__)}` #####",
+    ], headings)
+  end
+
+  private
+
+  def home_relative_path(path)
+    if path.start_with?(ENV["HOME"])
+      path.sub(ENV["HOME"], "~")
+    else
+      path
+    end
+  end
+end
diff --git a/test/command/help_test.rb b/test/command/help_test.rb
index 4c3cf1f..1affef9 100644
--- a/test/command/help_test.rb
+++ b/test/command/help_test.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "test_helper"
+require "gel/command"
 
 class HelpTest < Minitest::Test
   def test_help
diff --git a/test/command/version_test.rb b/test/command/version_test.rb
index d953b24..4c918e8 100644
--- a/test/command/version_test.rb
+++ b/test/command/version_test.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "test_helper"
+require "gel/command"
 
 class VersionTest < Minitest::Test
   def test_version
diff --git a/test/sha512_test.rb b/test/sha512_test.rb
index 5f5d8af..f0ea0b5 100644
--- a/test/sha512_test.rb
+++ b/test/sha512_test.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 require "test_helper"
-require_relative "../lib/gel/support/sha512"
+require "gel/support/sha512"
 
 require "digest/sha2"