System File in Linux.md

🔐 Linux User & Group Configuration Files Explained 🐧

Linux manages users and groups using several important configuration files. These files store essential information like user accounts, passwords, groups, and security settings. Understanding these files is crucial for system administration and security.

This guide covers:
1️⃣ /etc/passwd - Stores user account information.
2️⃣ /etc/shadow - Stores encrypted passwords & security policies.
3️⃣ /etc/group - Stores group information.
4️⃣ /etc/gshadow - Stores secure group details.

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📜 1. /etc/passwd – User Account Information

📌 Purpose: This file contains basic details of all user accounts in the system, including username, UID, GID, home directory, and default shell.

📌 Access: Public (All users can read it).

📌 File Format:

Each line represents a user account with the following format:

username:x:UID:GID:user_info:home_directory:shell

📄 Example Entry:

john:x:1001:1001:John Doe:/home/john:/bin/bash

🔍 Breakdown:

• john → Username
• x → Placeholder for password (stored in /etc/shadow)
• 1001 → User ID (UID)
• 1001 → Group ID (GID)
• John Doe → User description
• /home/john → Home directory
• /bin/bash → Default shell

🛠️ Commands to View & Modify:

• View all users:

  cat /etc/passwd

• Add a new user:

  sudo useradd -m -s /bin/bash john

• Modify user details:

  sudo usermod -c "New Info" john

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🔒 2. /etc/shadow – Secure Password Storage

📌 Purpose: Stores encrypted passwords and password aging policies for users.

📌 Access: Restricted (Only root can read it).

📌 File Format:

Each line represents a user password entry with the following format:

username:encrypted_password:last_changed:min_age:max_age:warn:inactive:expire:reserved

📄 Example Entry:

john:$6$somehash:17542:0:99999:7::::

🔍 Breakdown:

• john → Username
• $6$somehash → Encrypted password
• 17542 → Days since the last password change
• 0 → Minimum days before changing password
• 99999 → Maximum password validity days
• 7 → Warning days before password expires

🛠️ Commands to Manage Passwords:

• View shadow file (Root Only):

  sudo cat /etc/shadow

• Change a user’s password:

  sudo passwd john

• Force a password reset after 30 days:

  sudo chage -M 30 john

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👥 3. /etc/group – Group Information

📌 Purpose: Stores group names, GIDs, and group members.

📌 Access: Public (All users can read it).

📌 File Format:

Each line represents a group entry in the following format:

groupname:x:GID:member1,member2,member3

📄 Example Entry:

developers:x:1002:alice,bob,john

🔍 Breakdown:

• developers → Group name
• x → Placeholder (group password is in /etc/gshadow)
• 1002 → Group ID (GID)
• alice, bob, john → Members of the group

🛠️ Commands to Manage Groups:

• View all groups:

  cat /etc/group

• Create a new group:

  sudo groupadd developers

• Add a user to a group:

  sudo usermod -aG developers john

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🔐 4. /etc/gshadow – Secure Group Management

📌 Purpose: Stores group passwords and administrator settings.

📌 Access: Restricted (Only root can read it).

📌 File Format:

Each line represents a secure group entry in this format:

groupname:encrypted_password:group_admins:group_members

📄 Example Entry:

developers:$6$somehash:alice,bob:alice,bob,john

🔍 Breakdown:

• developers → Group name
• $6$somehash → Encrypted password
• alice,bob → Group administrators
• alice,bob,john → Group members

🛠️ Commands to Modify Groups Securely:

• View gshadow file (Root Only):

  sudo cat /etc/gshadow

• Set a group password:

  sudo gpasswd developers

• Assign a group administrator:

  sudo gpasswd -A alice developers

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🔍 5. Summary Table

📂 File	📌 Purpose	🔒 Access
/etc/passwd	Stores user account information	Public
/etc/shadow	Stores encrypted passwords	Root Only
/etc/group	Stores group details	Public
/etc/gshadow	Stores secure group settings	Root Only

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🎯 6. Key Takeaways

✔ Linux stores user & group details in /etc/passwd and /etc/group 📁
✔ Passwords are securely encrypted in /etc/shadow and /etc/gshadow 🔒
✔ Root user has exclusive access to /etc/shadow and /etc/gshadow 🛑
✔ Users & groups can be managed using commands like useradd, passwd, and groupadd ⚙️

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