What object does ENV return in Ruby?

Question

There's an ENV variable we all know. It returns an Object. But the return value of ENV is visually similar to a Hash. But it's really not.

For example:

> ENV
=> {"SHELL"=>"/bin/bash", "SESSION_MANAGER"=>"local/archlinux:@/tmp/.ICE-unix/613,unix/archlinux:/tmp/.ICE-unix/613", "COLORTERM"=>"truecolor", "XDG_CONFIG_DIRS"=>"/etc/xdg", "XDG_MENU_PREFIX"=>"xfce-", "SSH_AUTH_SOCK"=>"/tmp/ssh-Al0pdO1R5970/agent.622", "DESKTOP_SESSION"=>"Xfce Session", "SSH_AGENT_PID"=>"623", "GTK_MODULES"=>"canberra-gtk-module:canberra-gtk-module", "XDG_SEAT"=>"seat0", "PWD"=>"/home/sourav", "LOGNAME"=>"sourav", "XDG_SESSION_TYPE"=>"x11", "XAUTHORITY"=>"/home/sourav/.Xauthority", "HOME"=>"/home/sourav", "LANG"=>"en_GB.UTF-8", "XDG_CURRENT_DESKTOP"=>"XFCE", "VTE_VERSION"=>"5603", "INVOCATION_ID"=>"6d4dc7a91cc141e691436cb850e18f8d", "GLADE_CATALOG_PATH"=>":", "XDG_SESSION_CLASS"=>"user", "TERM"=>"xterm-256color", "USER"=>"sourav", "DISPLAY"=>":0.0", "SHLVL"=>"2", "XDG_VTNR"=>"1", "XDG_SESSION_ID"=>"1", "TILIX_ID"=>"f2480263-263e-408f-be36-8105e71256a6", "MOZ_PLUGIN_PATH"=>"/usr/lib/mozilla/plugins", "GLADE_MODULE_PATH"=>":", "XDG_RUNTIME_DIR"=>"/run/user/1000", "GLADE_PIXMAP_PATH"=>":", "JOURNAL_STREAM"=>"9:25041", "XDG_DATA_DIRS"=>"/usr/local/share:/usr/share", "PATH"=>"/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/home/sourav/.rvm/bin:/home/sourav/.rvm/bin:/home/sourav/.gem/ruby/2.6.0/bin", "DBUS_SESSION_BUS_ADDRESS"=>"unix:path=/run/user/1000/bus", "MAIL"=>"/var/spool/mail/sourav", "_"=>"/home/sourav/.irb", "LINES"=>"24", "COLUMNS"=>"80"}

Which looks similar to:

> ENV.to_h
=> {"SHELL"=>"/bin/bash", "SESSION_MANAGER"=>"local/archlinux:@/tmp/.ICE-unix/613,unix/archlinux:/tmp/.ICE-unix/613", "COLORTERM"=>"truecolor", "XDG_CONFIG_DIRS"=>"/etc/xdg", "XDG_MENU_PREFIX"=>"xfce-", "SSH_AUTH_SOCK"=>"/tmp/ssh-Al0pdO1R5970/agent.622", "DESKTOP_SESSION"=>"Xfce Session", "SSH_AGENT_PID"=>"623", "GTK_MODULES"=>"canberra-gtk-module:canberra-gtk-module", "XDG_SEAT"=>"seat0", "PWD"=>"/home/sourav", "LOGNAME"=>"sourav", "XDG_SESSION_TYPE"=>"x11", "XAUTHORITY"=>"/home/sourav/.Xauthority", "HOME"=>"/home/sourav", "LANG"=>"en_GB.UTF-8", "XDG_CURRENT_DESKTOP"=>"XFCE", "VTE_VERSION"=>"5603", "INVOCATION_ID"=>"6d4dc7a91cc141e691436cb850e18f8d", "GLADE_CATALOG_PATH"=>":", "XDG_SESSION_CLASS"=>"user", "TERM"=>"xterm-256color", "USER"=>"sourav", "DISPLAY"=>":0.0", "SHLVL"=>"2", "XDG_VTNR"=>"1", "XDG_SESSION_ID"=>"1", "TILIX_ID"=>"f2480263-263e-408f-be36-8105e71256a6", "MOZ_PLUGIN_PATH"=>"/usr/lib/mozilla/plugins", "GLADE_MODULE_PATH"=>":", "XDG_RUNTIME_DIR"=>"/run/user/1000", "GLADE_PIXMAP_PATH"=>":", "JOURNAL_STREAM"=>"9:25041", "XDG_DATA_DIRS"=>"/usr/local/share:/usr/share", "PATH"=>"/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/home/sourav/.rvm/bin:/home/sourav/.rvm/bin:/home/sourav/.gem/ruby/2.6.0/bin", "DBUS_SESSION_BUS_ADDRESS"=>"unix:path=/run/user/1000/bus", "MAIL"=>"/var/spool/mail/sourav", "_"=>"/home/sourav/.irb", "LINES"=>"24", "COLUMNS"=>"80"}

But:

> ENV.to_h.eql?(ENV)
=> false

So what kind of object does ENV return?

Answer 1

It's a custom Object with Hash-like functionality mostly implemented in C.

https://github.com/ruby/ruby/blob/trunk/hash.c#L4944

Answer 2

There is nothing in the Ruby Language Specification that requires ENV to be implemented in any particular way. As long as it responds to the right messages in the right way, it can be anything it wants.

For example, in Rubinius, ENV is implemented in a class called Rubinius::EnvironmentVariables that implements part of the Hash protocol and also mixes in Enumerable: https://github.com/rubinius/rubinius/blob/master/core/env.rb .