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Add info on env variables for Formula Cookbook #5737

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Feb 16, 2019
Merged

Add info on env variables for Formula Cookbook #5737

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d5f6e4c
Add info on env variables for Formula Cookbook
davidjb Feb 15, 2019
a3a3e3b
Formula Cookbook: env var docs tweaks.
MikeMcQuaid Feb 16, 2019
d415600
Formula Cookbook: fix typo.
MikeMcQuaid Feb 16, 2019
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10 changes: 10 additions & 0 deletions docs/Formula-Cookbook.md
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Expand Up @@ -696,6 +696,16 @@ Homebrew provides two formula DSL methods for launchd plist files:
* [`plist_name`](https://www.rubydoc.info/github/Homebrew/brew/master/Formula#plist_name-instance_method) will return e.g. `homebrew.mxcl.<formula>`
* [`plist_path`](https://www.rubydoc.info/github/Homebrew/brew/master/Formula#plist_path-instance_method) will return e.g. `/usr/local/Cellar/foo/0.1/homebrew.mxcl.foo.plist`

### Using environment variables

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 (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 (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.

## Updating formulae

Eventually a new version of the software will be released. In this case you should update the [`url`](https://www.rubydoc.info/github/Homebrew/brew/master/Formula#url-class_method) and [`sha256`](https://www.rubydoc.info/github/Homebrew/brew/master/Formula#sha256%3D-class_method). If a [`revision`](https://www.rubydoc.info/github/Homebrew/brew/master/Formula#revision%3D-class_method) line exists outside any `bottle do` block *and* the new release is stable rather than devel, it should be removed.
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