Rails credentials.yml.encと環境変数の併用 ref: https://qiita.com/vochicong/items/d384824f2952c46064ac

Env.rb

class Env
  def self.method_missing(name, *default)
    ENV[name.to_s] ||
      default.first ||
      Rails.application.credentials.send(name) ||
      super
  end

  def self.respond_to_missing?(*)
    true
  end
end

file0.txt

class Env
  def self.method_missing(name, *default)
    ENV[name.to_s] ||
      default.first ||
      Rails.application.credentials.send(name) ||
      super
  end

  def self.respond_to_missing?(*)
    true
  end
end

file1.txt

$ RAILS_MASTER_KEY=289e1431050b365b62bb5917acabcc53 rails credentials:show
secret_key_base: 2105bc31227a27f81b901582a8bb43b35bebea2b9c3572b024184a0b06dad26fc3bb312fbc5a7069783798d22f55cf4f411ae19169dd2a78026dccfbbdc889d7
APP_CONFIG: encryptedConfig

file2.txt

$ rails runner 'puts Env.APP_CONFIG("default")'
default

file3.txt

$ APP_CONFIG=envVar rails runner 'puts Env.APP_CONFIG("default")'
envVar

file4.txt

$ RAILS_MASTER_KEY=289e1431050b365b62bb5917acabcc53 APP_CONFIG=envVar rails runner 'puts Env.APP_CONFIG("default")'
envVar

file5.txt

$ RAILS_MASTER_KEY=289e1431050b365b62bb5917acabcc53 rails runner 'puts Env.APP_CONFIG("default")'
default

file6.txt

$ RAILS_MASTER_KEY=289e1431050b365b62bb5917acabcc53 rails runner 'puts Env.APP_CONFIG'
encryptedConfig