How can I use pip in oh-my-zsh? I was trying to install nltk through pip, but it told me zsh: command not found: pip. When I check plugins under .oh-my-zsh/custom/plugins, there is a folder named pip. I don't know what the problem is. 
Edit:
$ echo $PATH
/home/xxx/bin:/usr/local/bin:/home/xxx/bin:/home/xxx/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
$ type pip
pip is an alias for noglob pip
 
     
     
     
     
     
     
     
     
     
     
     
    