你的Mac中可以使用git ，但无法使用gitk，一般就两个原因

1. mac原生git版本太低了。
2. git-gui未安装

解决办法：重新使用brew安装git

安装git

1. 升级brew

   brew update
   

2. 安装git

   ~ brew install git
   Updating Homebrew...
   ==> Installing dependencies for git: pcre2
   ==> Installing git dependency: pcre2
   ==> Downloading https://homebrew.bintray.com/bottles/pcre2-10.35.catalina.bottle
   ==> Downloading from https://akamai.bintray.com/6a/6a1e59a5db23d684f92d2bf695601
   ######################################################################## 100.0%
   ==> Pouring pcre2-10.35.catalina.bottle.tar.gz
   🍺  /usr/local/Cellar/pcre2/10.35: 230 files, 6.0MB
   ==> Installing git
   ==> Downloading https://homebrew.bintray.com/bottles/git-2.26.2.catalina.bottle.
   ==> Downloading from https://akamai.bintray.com/74/74d378f6530877383f38b6d1a923e
   ######################################################################## 100.0%
   ==> Pouring git-2.26.2.catalina.bottle.tar.gz
   ==> Caveats
   The Tcl/Tk GUIs (e.g. gitk, git-gui) are now in the `git-gui` formula.
   
   Bash completion has been installed to:
     /usr/local/etc/bash_completion.d
   
   zsh completions and functions have been installed to:
     /usr/local/share/zsh/site-functions
   
   Emacs Lisp files have been installed to:
     /usr/local/share/emacs/site-lisp/git
   ==> Summary
   🍺  /usr/local/Cellar/git/2.26.2: 1,470 files, 44.8MB
   ==> Caveats
   ==> git
   The Tcl/Tk GUIs (e.g. gitk, git-gui) are now in the `git-gui` formula.
   
   Bash completion has been installed to:
     /usr/local/etc/bash_completion.d
   
   zsh completions and functions have been installed to:
     /usr/local/share/zsh/site-functions
   
   Emacs Lisp files have been installed to:
     /usr/local/share/emacs/site-lisp/git
   

3. 使用命令查看git版本

   ~ git git --version
   git version 2.24.2 (Apple Git-127)
   

   发现git版本没有变化，不是上面安装的2.26.2版本

4. 查看当git目录

   ~ which git
   /usr/bin/git
   

   发现git目录还是/usr/bin/git ,brew安装目录在/usr/local/bin/git中才对，因为这里的git才是关联了/usr/local/Cellar/git/2.26.2目录

   ~ ls -l  /usr/local/bin/git
   lrwxr-xr-x  1 username  admin  28  5 11 11:19 /usr/local/bin/git -> ../Cellar/git/2.26.2/bin/git
   
   ~ ls -l /usr/bin/git      
   -rwxr-xr-x  1 root  wheel  31488  3 17 23:42 /usr/bin/git
   

   造成这样情况是我们.zshrc文件没设置好，我们可以运行监测brew

   ~ brew doctor
   # 我们可以看到这么一段提示，我用的是zsh，原生的可能会提示echo 'export PATH="/usr/local/bin:$PATH"' >> ~/.bash_profile，下面步骤只要把zshrc变为bash_profile即可
   Warning: Homebrew's sbin was not found in your PATH but you have installed
   formulae that put executables in /usr/local/sbin.
   Consider setting the PATH for example like so:
     echo 'export PATH="/usr/local/sbin:$PATH"' >> ~/.zshrc
   

   根据提示，我们将echo ‘export PATH="/usr/local/sbin:$PATH"’添加到~/.zshrc文件中，并运行source ~/.zshrc生效配置

   #Mac原生git
   ~ which git
   /usr/bin/git
   #添加后
   echo 'export PATH="/usr/local/sbin:$PATH"' >> ~/.zshrc
   source ~/.zshrc
   #brew管理的git
   ~ which git      
   /usr/local/bin/git
   

   再次运行brew doctor发现上面报警消失，我们这时候再检测git版本，发现版本已经正常显示为brew安装的2.26.2版本

   ~ git --version  
   git version 2.26.2
   

   但此时在运行gitk，发现还是有问题，还需安装一个git-gui插件

   ~ gitk
   zsh: command not found: gitk
   

5. 安装git-gui

   通过安装git-gui使git带有gitk功能

   ~ brew install git-gui 
   Updating Homebrew...
   ==> Installing dependencies for git-gui: tcl-tk
   ==> Installing git-gui dependency: tcl-tk
   ==> Downloading https://homebrew.bintray.com/bottles/tcl-tk-8.6.10.catalina.bottle.1.tar.gz
   ==> Downloading from https://akamai.bintray.com/47/4740b30b97f0308ecc59c1308945c38ddca5d3da
   ######################################################################## 100.0%
   ==> Pouring tcl-tk-8.6.10.catalina.bottle.1.tar.gz
   ==> Caveats
   tcl-tk is keg-only, which means it was not symlinked into /usr/local,
   because macOS already provides this software and installing another version in
   parallel can cause all kinds of trouble.
   
   If you need to have tcl-tk first in your PATH run:
     echo 'export PATH="/usr/local/opt/tcl-tk/bin:$PATH"' >> ~/.zshrc
   
   For compilers to find tcl-tk you may need to set:
     export LDFLAGS="-L/usr/local/opt/tcl-tk/lib"
     export CPPFLAGS="-I/usr/local/opt/tcl-tk/include"
   
   For pkg-config to find tcl-tk you may need to set:
     export PKG_CONFIG_PATH="/usr/local/opt/tcl-tk/lib/pkgconfig"
   
   ==> Summary
   🍺  /usr/local/Cellar/tcl-tk/8.6.10: 3,036 files, 51MB
   ==> Installing git-gui
   ==> Downloading https://homebrew.bintray.com/bottles/git-gui-2.26.2.catalina.bottle.tar.gz
   ==> Downloading from https://akamai.bintray.com/27/272b5e5ab0d3f511f640c743053ed8f78bc291e0
   ######################################################################## 100.0%
   ==> Pouring git-gui-2.26.2.catalina.bottle.tar.gz
   🍺  /usr/local/Cellar/git-gui/2.26.2: 79 files, 2.2MB
   ==> Caveats
   ==> tcl-tk
   tcl-tk is keg-only, which means it was not symlinked into /usr/local,
   because macOS already provides this software and installing another version in
   parallel can cause all kinds of trouble.
   
   If you need to have tcl-tk first in your PATH run:
     echo 'export PATH="/usr/local/opt/tcl-tk/bin:$PATH"' >> ~/.zshrc
   
   For compilers to find tcl-tk you may need to set:
     export LDFLAGS="-L/usr/local/opt/tcl-tk/lib"
     export CPPFLAGS="-I/usr/local/opt/tcl-tk/include"
   
   For pkg-config to find tcl-tk you may need to set:
     export PKG_CONFIG_PATH="/usr/local/opt/tcl-tk/lib/pkgconfig"
   

6. 现在可以愉快的使用gitk了

   ~ gitk
   

常用命令

#
~ git --version     
git version 2.26.2
#
~ type  -a git
git is /usr/local/bin/git
git is /usr/bin/git
#
~ which git   
/usr/bin/git