Around line 425, there is:

directories=(bin etc include lib sbin share var opt
             share/zsh share/zsh/site-functions
             var/homebrew var/homebrew/linked
             Cellar Caskroom Homebrew Frameworks)
mkdirs=()
for dir in "${directories[@]}"; do
  if ! [[ -d "${HOMEBREW_PREFIX}/${dir}" ]]; then
    mkdirs+=("${HOMEBREW_PREFIX}/${dir}")
  fi
done

I think the Homebrew directory here is an error. HOMEBREW_REPOSITORY should be created with a different logic.

Running this version on Apple Silicon currently fails because we're not giving /opt/homebrew the right ownership. It gets root:wheel, which is not appropriate. Whatever else happens, we always need to be owners of HOMEBREW_REPOSITORY.

brewadmin@administrators-Mac-5 ~ % bash ./install.sh    
==> This script will install:
/opt/homebrew/bin/brew
/opt/homebrew/share/doc/homebrew
/opt/homebrew/share/man/man1/brew.1
/opt/homebrew/share/zsh/site-functions/_brew
/opt/homebrew/etc/bash_completion.d/brew
/opt/homebrew
==> The following new directories will be created:
/opt/homebrew/bin
/opt/homebrew/etc
/opt/homebrew/include
/opt/homebrew/lib
/opt/homebrew/sbin
/opt/homebrew/share
/opt/homebrew/var
/opt/homebrew/opt
/opt/homebrew/share/zsh
/opt/homebrew/share/zsh/site-functions
/opt/homebrew/var/homebrew
/opt/homebrew/var/homebrew/linked
/opt/homebrew/Cellar
/opt/homebrew/Caskroom
/opt/homebrew/Homebrew
/opt/homebrew/Frameworks

Press RETURN to continue or any other key to abort
==> /usr/bin/sudo /bin/mkdir -p /opt/homebrew
==> /usr/bin/sudo /usr/sbin/chown root:wheel /opt/homebrew
==> /usr/bin/sudo /bin/mkdir -p /opt/homebrew/bin /opt/homebrew/etc /opt/homebrew/include /opt/homebrew/lib /opt/homebrew/sbin /opt/homebrew/share /opt/homebrew/var /opt/homebrew/opt /opt/homebrew/share/zsh /opt/homebrew/share/zsh/site-functions /opt/homebrew/var/homebrew /opt/homebrew/var/homebrew/linked /opt/homebrew/Cellar /opt/homebrew/Caskroom /opt/homebrew/Homebrew /opt/homebrew/Frameworks
==> /usr/bin/sudo /bin/chmod g+rwx /opt/homebrew/bin /opt/homebrew/etc /opt/homebrew/include /opt/homebrew/lib /opt/homebrew/sbin /opt/homebrew/share /opt/homebrew/var /opt/homebrew/opt /opt/homebrew/share/zsh /opt/homebrew/share/zsh/site-functions /opt/homebrew/var/homebrew /opt/homebrew/var/homebrew/linked /opt/homebrew/Cellar /opt/homebrew/Caskroom /opt/homebrew/Homebrew /opt/homebrew/Frameworks
==> /usr/bin/sudo /usr/sbin/chown brewadmin /opt/homebrew/bin /opt/homebrew/etc /opt/homebrew/include /opt/homebrew/lib /opt/homebrew/sbin /opt/homebrew/share /opt/homebrew/var /opt/homebrew/opt /opt/homebrew/share/zsh /opt/homebrew/share/zsh/site-functions /opt/homebrew/var/homebrew /opt/homebrew/var/homebrew/linked /opt/homebrew/Cellar /opt/homebrew/Caskroom /opt/homebrew/Homebrew /opt/homebrew/Frameworks
==> /usr/bin/sudo /usr/bin/chgrp admin /opt/homebrew/bin /opt/homebrew/etc /opt/homebrew/include /opt/homebrew/lib /opt/homebrew/sbin /opt/homebrew/share /opt/homebrew/var /opt/homebrew/opt /opt/homebrew/share/zsh /opt/homebrew/share/zsh/site-functions /opt/homebrew/var/homebrew /opt/homebrew/var/homebrew/linked /opt/homebrew/Cellar /opt/homebrew/Caskroom /opt/homebrew/Homebrew /opt/homebrew/Frameworks
==> Downloading and installing Homebrew...
/opt/homebrew/.git: Permission denied
Failed during: git init -q

I think the Homebrew directory here is an error. HOMEBREW_REPOSITORY should be created with a different logic.

Yes, that could reference HOMEBREW_REPOSITORY if needed or just be removed perhaps.

I've tested on ARM and Intel macOS. I'd like to have some Linux testing, but otherwise I think this is complete and ready for review.

I'd like to have some Linux testing, but otherwise I think this is complete and ready for review.

I gave it a quick test on Linux in /home/linuxbrew/.linuxbrew with and without sudo available, and it worked for me.

I think this is great, as really a shockingly large amount of stuff "just works" now - it's really impressive how fast that has happened.

But I wonder if you might want to consider describing it as more partial support, or support with significant caveats? As some not-insignificant proportion of formulae will likely not be compatible for a long time, if ever. So setting user expectations in some form may help?

But I wonder if you might want to consider describing it as more partial support, or support with significant caveats? As some not-insignificant proportion of formulae will likely not be compatible for a long time, if ever. So setting user expectations in some form may help?

Yup, that's the plan before we'd merge this 👍🏻