brew.sh: allow --prefix to be run as root without warning

[akirk]

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In the files .bashrc or .zshrc the command brew --prefix xyz is often used to set the correct $PATH for executables. For example: export PATH="$(brew --prefix php70)/bin:$PATH"

When the *rc files are executed as root (e.g. via sudo -s) then --prefix should be considered a legitimate use of the brew command while under uid 0.

Old behavior:

$ brew --prefix gcc
Error: Running Homebrew as root is extremely dangerous. As Homebrew does not
drop privileges on installation you are giving all build scripts full access
to your system. As a result of the macOS sandbox not handling the root user
correctly HOMEBREW_NO_SANDBOX has been set so the sandbox will not be used. If
we have not merged a pull request to add privilege dropping by November 1st
2016 running Homebrew as root will be disabled. No Homebrew maintainers plan
to work on this functionality.
/usr/local/Cellar/gcc/6.2.0
$ brew --prefix gcc 2>/dev/null
/usr/local/Cellar/gcc/6.2.0

New behavior:

$ brew --prefix gcc
/usr/local/Cellar/gcc/6.2.0

[MikeMcQuaid]

Seems reasonable. Can you also add --repository in there too? Thanks!

[akirk]

@MikeMcQuaid that's already handled in https://github.com/Homebrew/brew/blob/master/Library/Homebrew/brew.sh#L63, also --prefix without arguments is covered by that case block.

This PR is about the case when --prefix has an argument.

[MikeMcQuaid]

Thanks for the quick response!