久しぶりにbrew updateをかけようとしたらエラーが出てアップデートできなかったので、brew doctorで出たWarningと解決手順をメモ。

環境

MacBook Pro (Retina, 13-inch、Early 2015)
OS X El Capitan バージョン10.11.6（15G1004）
XQuartz 2.7.8
Xcode Version 8.0 (8A218a)

５つのWarningが出たよ

brew doctorを叩いて出たWarningは以下の5つ。

Warning: Your XQuartz (2.7.8) is outdated
Please install XQuartz 2.7.9:
  https://xquartz.macosforge.org

Warning: You have unlinked kegs in your Cellar
Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
those kegs to fail to run properly once built. Run `brew link` on these:
    mono
 
Warning: You have Xcode 8 installed without the CLT;
this causes certain builds to fail on OS X El Capitan (10.11).
Please install the CLT via:
  sudo xcode-select --install

Warning: Broken symlinks were found. Remove them with `brew prune`:
    /usr/local/share/man/man8/prlexec.8
 
Warning: Some keg-only formula are linked into the Cellar.
Linking a keg-only formula, such as gettext, into the cellar with
`brew link <formula>` will cause other formulae to detect them during
the `./configure` step. This may cause problems when compiling those
other formulae.
 
Binaries provided by keg-only formulae may override system binaries
with other strange results.
 
You may wish to `brew unlink` these brews:
    openssl

順番に解消していく。

解決方法

1.XQuartzのアップデート

Warning: Your XQuartz (2.7.8) is outdated
Please install XQuartz 2.7.9:
https://xquartz.macosforge.org

XQuartzが古い。公式サイトから最新版をインストールする。

2.monoのシンボリックリンクを貼り直す

Warning: You have unlinked kegs in your Cellar
Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
those kegs to fail to run properly once built. Run `brew link` on these:
    mono

monoのシンボリックリンクが切れてる。
brew link monoでシンボリックリンクを貼る。

Error: Could not symlink bin/al
Target /usr/local/bin/al
already exists. You may want to remove it:
  rm '/usr/local/bin/al'

こんなErrorが出たらoverwriteオプションつけて強制的に上書きする。
$ brew link --overwrite mono

3.CLTのアップデート

Warning: You have Xcode 8 installed without the CLT;
this causes certain builds to fail on OS X El Capitan (10.11).
Please install the CLT via:
  sudo xcode-select --install

XcodeのCLT（Command Line Tools）が古いのでアップデートする。

4.デッドシンボリックリンクを削除

Warning: Broken symlinks were found. Remove them with `brew prune`:
    /usr/local/share/man/man8/prlexec.8

これは単純に書かれてるコマンド叩いたら権限がないとのエラーが出た。

$ brew prune /usr/local/share/man/man8/prlexec.8
Error: Permission denied - /usr/local/share/man/man8/prlexec.8

親フォルダの所有者を実行ユーザーにして権限を付与。
$ sudo chown -R unsoluble_sugar /usr/local/share/man/man8
再度brew pruneを叩いて解決。

$ brew prune /usr/local/share/man/man8/prlexec.8
Pruned 1 symbolic links and 5 directories from /usr/local

5.opensslのシンボリックリンクを貼り直す

Warning: Some keg-only formula are linked into the Cellar.
Linking a keg-only formula, such as gettext, into the cellar with
`brew link <formula>` will cause other formulae to detect them during
the `./configure` step. This may cause problems when compiling those
other formulae.
 
Binaries provided by keg-only formulae may override system binaries
with other strange results.

You may wish to `brew unlink` these brews:
    openssl

opensslのシンボリックリンクを削除。
$ brew unlink openssl
貼り直す。
$ brew install openssl
これだけ。

Homebrewをアップデート

brew doctorで問題が解消されていることを確認。

$ brew doctor
Your system is ready to brew.

brew updateでHomebrewをアップデート。

$ brew update
Updated 1 tap (homebrew/core).
==> Updated Formulae
ace                            gnome-builder                  gucharmap
clutter-gtk                    gnome-icon-theme               mongodb
evince                         gsettings-desktop-schemas      neofetch
gedit                          gtk+3                          nmap
git-archive-all                gtkmm3

おしまい。