Linux Authentication Files

There are the following files related to users/groups/passwords:

1. /etc/passwd    It is related to user info.
2. /etc/shadow    It is related to user password info
3. /etc/group       It is related to group info
4. /etc/gshadow  It is related to group password info
5. /etc/default/useradd  It is related to default attribute of a new user
6. /etc/login.defs  It is related to password default info.

#vim /etc/passwd

• Username: It is used when user logs in. It should be between 1 and 32 characters in length.
•  Password: An x character indicates that encrypted password is stored in /etc/shadow file.
•  UID: Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99
• are reserved for other predefined accounts.
•  GID: The primary group ID (stored in /etc/group file)
• Label: The comment field. It allow you to add extra information about the users such as user's full name,
• phone number etc.
• Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /
• Login shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell.

The permission of this file is 644 means -rw-r--r-- 
To check permission : ls -ll /etc/passwd