ENV

ENV is a hash-like accessor for environment variables.

Interaction with the Operating System

The ENV object interacts with the operating system’s environment variables:

• When you get the value for a name in ENV, the value is retrieved from among the current environment variables.

• When you create or set a name-value pair in ENV, the name and value are immediately set in the environment variables.

• When you delete a name-value pair in ENV, it is immediately deleted from the environment variables.

Names and Values

Generally, a name or value is a String.

Valid Names and Values

Each name or value must be one of the following:

• A String.

• An object that responds to #to_str by returning a String, in which case that String will be used as the name or value.

Invalid Names and Values

A new name:

• May not be the empty string:

  Example

  Run in Playground

  ENV[''] = '0'
  # Raises Errno::EINVAL (Invalid argument - ruby_setenv())
  

• May not contain character "=":

  Example

  Run in Playground

  ENV['='] = '0'
  # Raises Errno::EINVAL (Invalid argument - ruby_setenv(=))
  

A new name or value:

• May not be a non-String that does not respond to #to_str:

  Example

  Run in Playground

  ENV['foo'] = Object.new
  # Raises TypeError (no implicit conversion of Object into String)
  ENV[Object.new] = '0'
  # Raises TypeError (no implicit conversion of Object into String)
  

• May not contain the NUL character "\0":

  Example

  Run in Playground

  ENV['foo'] = "\0"
  # Raises ArgumentError (bad environment variable value: contains null byte)
  ENV["\0"] == '0'
  # Raises ArgumentError (bad environment variable name: contains null byte)
  

• May not have an ASCII-incompatible encoding such as UTF-16LE or ISO-2022-JP:

  Example

  Run in Playground

  ENV['foo'] = '0'.force_encoding(Encoding::ISO_2022_JP)
  # Raises ArgumentError (bad environment variable name: ASCII incompatible encoding: ISO-2022-JP)
  ENV["foo".force_encoding(Encoding::ISO_2022_JP)] = '0'
  # Raises ArgumentError (bad environment variable name: ASCII incompatible encoding: ISO-2022-JP)
  

About Ordering

ENV enumerates its name/value pairs in the order found in the operating system’s environment variables. Therefore the ordering of ENV content is OS-dependent, and may be indeterminate.

This will be seen in:

• A Hash returned by an ENV method.

• An Enumerator returned by an ENV method.

• An Array returned by ENV.keys, ENV.values, or ENV.to_a.

• The String returned by ENV.inspect.

• The Array returned by ENV.shift.

• The name returned by ENV.key.

About the Examples

Some methods in ENV return ENV itself. Typically, there are many environment variables. It’s not useful to display a large ENV in the examples here, so most example snippets begin by resetting the contents of ENV:

• ENV.replace replaces ENV with a new collection of entries.

• ENV.clear empties ENV.