User/Group/Password Files

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 /etc/passwd 
 used to store user and system accounts 
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 /etc/shadow 
 Used to store user passwords (encrypted) 
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 /etc/group 
 Used to store group information, and which users are included in a group. 
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 /etc/skel 
 This directory is used to define the default items that will get added to a new users home directory 
 For example, this could contain a README file that contains rules for using the server. 
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 /etc/default/useradd 
 This directory is used to define the default options used when the useradd command is used. 
 For example, this could specify a location that user directories are created, default group to be added to, and more. 
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 /etc/sudoers 
 This file is used to define which users have sudo access/