错误代码：
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bash-3.2# brew update
Initialized empty Git repository in /usr/local/.git/
remote: Counting objects: 1163, done.
remote: Compressing objects: 100% (1067/1067), done.
remote: Total 1163 (delta 116), reused 391 (delta 55), pack-reused 0
Receiving objects: 100% (1163/1163), 1.23 MiB | 40.00 KiB/s, done.
Resolving deltas: 100% (116/116), done.
From https://github.com/Homebrew/brew
* [new branch] master -> origin/master
HEAD is now at 42be54e Merge pull request #3667 from commitay/cask-doctor-sip
To restore the stashed changes to /usr/local run:
'cd /usr/local && git stash pop'
Error: Running Homebrew as root is extremely dangerous and no longer supported.
As Homebrew does not drop privileges on installation you would be giving all
build scripts full access to your system.
解决方法就是给这个用户写权限，而不是sudo。
iMacdeMac:~ imac$ sudo chown -R imac /usr/local/