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Formula Cookbook: env var docs tweaks.

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# Formula Cookbook
A *formula* is a package definition written in Ruby. It can be created with `brew create <URL>` where `<URL>` is a zip or tarball, installed with `brew install <formula>`, and debugged with `brew install --debug --verbose <formula>`. Formulae use the [Formula API](https://www.rubydoc.info/github/Homebrew/brew/master/Formula) which provides various Homebrew-specific helpers.
A *formula* is a package definition written in Ruby. It can be created with `brew create <URL>` where `<URL>` is a zip or tarball, installed with `brew install <formula>`, and debugged with `brew install --drefebug --verbose <formula>`. Formulae use the [Formula API](https://www.rubydoc.info/github/Homebrew/brew/master/Formula) which provides various Homebrew-specific helpers.
## Homebrew terminology
@ -700,9 +700,9 @@ Homebrew provides two formula DSL methods for launchd plist files:
Homebrew has multiple levels of environment variable filtering which affects variables available to formulae.
Firstly, the overall environment in which Homebrew runs is filtered to avoid environment contamination breaking from-source builds ([ref](https://github.com/Homebrew/brew/issues/932)). In particular, this process filters all but the given whitelisted variables, but allows environment variables prefixed with `HOMEBREW_`. The specific implementation can be seen in the [`brew`](https://github.com/Homebrew/brew/blob/master/bin/brew) script.
Firstly, the overall environment in which Homebrew runs is filtered to avoid environment contamination breaking from-source builds (https://github.com/Homebrew/brew/issues/932). In particular, this process filters all but the given whitelisted variables, but allows environment variables prefixed with `HOMEBREW_`. The specific implementation can be seen in [`bin/brew`](https://github.com/Homebrew/brew/blob/master/bin/brew).
The second level of filtering removes sensitive environment variables (such as credentials like keys, passwords or tokens) to avoid malicious subprocesses obtaining them ([ref](https://github.com/Homebrew/brew/pull/2524)). This has the effect of preventing any such variables from reaching a formula's Ruby code as they are filtered before it is called. The specific implementation can be seen in the [`clear_sensitive_environment` method](https://github.com/Homebrew/brew/blob/master/Library/Homebrew/extend/ENV.rb).
The second level of filtering removes sensitive environment variables (such as credentials like keys, passwords or tokens) to avoid malicious subprocesses obtaining them (https://github.com/Homebrew/brew/pull/2524). This has the effect of preventing any such variables from reaching a formula's Ruby code as they are filtered before it is called. The specific implementation can be seen in the [`ENV.clear_sensitive_environment!` method](https://github.com/Homebrew/brew/blob/master/Library/Homebrew/extend/ENV.rb).
In summary, environment variables used by a formula need to conform to these filtering rules in order to be available.