Homebrew公式变量定义与使用问题：install方法无法访问变量

问题原因 #

你定义的os和arch是类作用域下的局部变量，而install是Formula的实例方法——两者作用域完全隔离：

• 类级代码（比如on_macos、url设置）在公式加载阶段执行，此时局部变量os/arch处于类作用域内，所以url能正常解析；
• install方法在安装阶段以实例对象的方式调用，无法访问类作用域的局部变量，因此报错undefined local variable or method。

修复方案 #

方案1：改用类实例变量（@前缀） #

将局部变量改为带@的类实例变量，让实例方法可以访问：

class Foobar < Formula
  desc "..."
  homepage "https://github.com/..."
  version "0.17"
  license "Apache-2.0"

  @os = "XXX"
  @arch = "XXX"

  on_macos do
    @os = "osx"
    if Hardware::CPU.arm?
      @arch = "arm64"
      sha256 "..."
    elsif Hardware::CPU.intel?
      @arch = "x86_64"
      sha256 "..."
    end
  end

  on_linux do
    @os = "linux"
    if Hardware::CPU.intel?
      @arch = "x86_64"
      sha256 "..."
    elsif Hardware::CPU.arm? && Hardware::CPU.is_64_bit?
      @arch = "arm64"
      sha256 "..."
    end
  end

  url "https://github.com/.../releases/download/mycmd-v#{version}/mycmd-#{@os}-#{@arch}"

  def install
    bin.install "mycmd-#{@os}-#{@arch}" => "mycmd"
  end

  test do
    system "#{bin}/mycmd", "--version"
  end
end

方案2：在install方法内直接推导文件名（更推荐） #

避免变量作用域问题，直接在install里根据当前环境重新判断，逻辑和类级代码一致：

class Foobar < Formula
  desc "..."
  homepage "https://github.com/..."
  version "0.17"
  license "Apache-2.0"

  on_macos do
    if Hardware::CPU.arm?
      sha256 "..."
    elsif Hardware::CPU.intel?
      sha256 "..."
    end
  end

  on_linux do
    if Hardware::CPU.intel?
      sha256 "..."
    elsif Hardware::CPU.arm? && Hardware::CPU.is_64_bit?
      sha256 "..."
    end
  end

  url do
    os = OS.mac? ? "osx" : "linux"
    arch = if Hardware::CPU.arm? && Hardware::CPU.is_64_bit?
             "arm64"
           else
             "x86_64"
           end
    "https://github.com/.../releases/download/mycmd-v#{version}/mycmd-#{os}-#{arch}"
  end

  def install
    os = OS.mac? ? "osx" : "linux"
    arch = if Hardware::CPU.arm? && Hardware::CPU.is_64_bit?
             "arm64"
           else
             "x86_64"
           end
    bin.install "mycmd-#{os}-#{arch}" => "mycmd"
  end

  test do
    system "#{bin}/mycmd", "--version"
  end
end

这种写法更符合Homebrew的设计规范，避免作用域依赖，同时保证url和install的逻辑一致。

内容的提问来源于stack exchange，提问作者emmby