## /etc/passwd
The **`/etc/passwd`** file in Linux contains essential information related to user accounts. Let's dive into the details:
1. **File Purpose**:
- The **`/etc/passwd`** file is a plain text-based database that stores information for **all user accounts** on the system.
- It includes details such as **user ID (UID)**, **group ID (GID)**, **home directory**, and **default shell**.
- Ownership: The file is owned by the **root user** and has **644 permissions**.
- Access: Only the **root user** or users with **sudo privileges** can modify the file, while it remains readable by all system users.
2. **File Format**:
- Each line in the **`/etc/passwd`** file represents a **user account**.
- Fields are separated by **colons (:)**.
- The typical format of an entry is as follows:
```
username:password:UID:GID:GECOS:home_directory:login_shell
```
3. **Fields Explained**:
- **Username**: The string used when logging into the system. Must be unique and limited to 32 characters.
- **Password**: In older Linux systems, the encrypted password was stored here. Modern systems set it to **'x'**, indicating that the actual password is stored in the **`/etc/shadow`** file.
- **UID (User Identifier)**: A number assigned to each user, used by the OS to refer to the user.
- **GID (Group Identifier)**: Refers to the user's primary group. When a user creates a file, its group is set to this group.
- **GECOS**: Contains comma-separated values:
- User's full name or application name.
- Room number.
- Work phone number.
- Home phone number.
- Other contact information.
- **Home Directory**: Absolute path to the user's home directory, where files and configurations reside (usually under **`/home`**).
- **Login Shell**: Absolute path to the user's login shell (e.g., **`/bin/bash`**).
Manually editing the **`/etc/passwd`** file should be avoided unless you understand its structure. Always use appropriate commands (e.g., **`usermod`** or **`useradd`**) for managing user accounts.
## /etc/shadow
In modern Linux systems, user passwords are stored in an encrypted format within the **`/etc/shadow`** file. This file contains essential information related to user accounts, including their password hashes and optional password aging details. Let's break down what you need to know:
1. **`/etc/passwd`** File:
- The `/etc/passwd` file stores user account information, such as usernames, home directories, and default shells.
- However, it **does not** contain the actual passwords; instead, it holds an **'x'** placeholder for the password field.
2. **`/etc/shadow`** File:
- The encrypted passwords and other relevant data are stored in the `/etc/shadow` file.
- Each entry in this file corresponds to a user listed in the `/etc/passwd` file.
- The fields in a typical `/etc/shadow` entry include:
- **Username**: The login name.
- **Password**: The encrypted password hash (usually in the format `$id$salt$hashed`).
- Common algorithms include MD5, Blowfish, SHA-256, and SHA-512.
- **Last password change**: The number of days since January 1, 1970, when the password was last changed.
- **Minimum**: The minimum days required between password changes.
- **Maximum**: The maximum validity period for the password.
- **Warn**: The number of days before password expiration when the user is warned to change it.
- **Inactive**: The number of days after password expiration before the account is disabled.
- **Expire**: The absolute date when the login may no longer be used.
3. **Access to `/etc/shadow`**:
- Only **root users** or commands with the **suid bit** can access the `/etc/shadow` file.
- To view the contents of `/etc/shadow`, follow these steps:
1. Log in as the **root user**.
2. Execute the following command:
```
# cat /etc/shadow
```
3. You'll be prompted to provide the root user password.
Remember that handling passwords directly from the `/etc/shadow` file requires caution, as it involves sensitive information. Always follow best practices and security guidelines when managing user accounts and passwords in Linux..
## /etc/groups
Certainly! The **`/etc/group`** file in Linux plays a crucial role in defining user groups. Let's delve into the details:
1. **Purpose of `/etc/group`**:
- The **`/etc/group`** file stores information about **user groups** on the system.
- Each line in this file represents a **group entry**.
- It helps organize users into logical groups, facilitating efficient management and access control.
2. **Format of an Entry**:
- Each line in the **`/etc/group`** file consists of several fields separated by colons (":").
- Here's the format:
```
group_name:password:GID:user_list
```
- Fields explained:
- **`group_name`**: The name of the group. When you run `ls -l`, this name appears in the group field.
- **`Password`**: Generally not used (left empty). It can store an encrypted password for privileged groups.
- **`GID` (Group ID)**: A unique numeric identifier assigned to the group. You can find this number in the **`/etc/passwd`** file.
- **`user_list`**: A comma-separated list of usernames belonging to the group.
3. **Why User Groups Matter**:
- **Sharing Resources**: Groups allow users to share files, directories, or devices with specific permissions.
- **User Management**: Managing users becomes easier when they belong to relevant groups.
- **Monitoring**: Group membership aids in monitoring user activities.
- **Access Control**: Group membership grants special access to files and directories permitted for that group.
4. **Example**:
- Consider the following entry:
```
developers:x:1001:tom,jane
```
- **`developers`**: Group name.
- **`x`**: Empty password field.
- **`1001`**: Group ID.
- **`tom, jane`**: Users who are part of this group.
- In the example, user **`tom`** can access files associated with both the **`Web developers`** and **`Sales`** groups.
5. **Viewing Group Information**:
- To view the current group settings, use one of these commands:
- `less /etc/group`
- `more /etc/group`
- `cat /etc/group`
- To find out which groups a user belongs to:
```
groups {username}
```
For instance:
```
groups vivek
```
Sample output:
```
vivek : vivek adm dialout cdrom plugdev lpadmin netdev admin sambashare libvirtd
```
Remember, managing user groups is essential for efficient system administration and access control. Group membership simplifies resource sharing and enhances security.
## Adding or deleting group(s) associated with a user
To remove a user from a group in **Linux**, you have a few options. Let's explore them:
1. **Using `gpasswd`**:
- The `gpasswd` command allows you to remove a user from a group. Execute the following:
```
gpasswd --delete user group
```
```
gpasswd --add user group
```
- Note that the new group configuration will take effect at the next login. If the user is currently logged in, the change won't be immediately visible.
## useradd, userdel, passwd
Certainly! Let's explore practical examples of using `useradd`, `userdel`, and the `passwd` command in Linux. I'll provide brief explanations along with the commands and flags.
1. **Adding a New User with `useradd`**:
- To create a new user named "testuser," use the following command:
```
sudo useradd testuser
```
- This will add "testuser" to the system, creating a new user account with default settings.
2. **Specifying a Home Directory for the New User**:
- You can set a custom home directory for the user using the `-d` flag:
```
sudo useradd -d /home/testuser_custom testuser
```
- Replace `/home/testuser_custom` with the desired path.
3. **Creating a User with a Specific User ID (UID)**:
- To assign a specific User ID (UID) to "bob," use:
```
sudo useradd -u 1001 bob
```
- Replace `1001` with the desired UID.
4. **Creating a User Without a Home Directory**:
- If you want to create a user without a home directory, use the `-M` flag:
```
sudo useradd -M guest
```
5. **Setting an Expiry Date for the User Account**:
- To create a user with an expiry date (e.g., after 30 days), use:
```
sudo useradd -e 2024-05-01 charlie
```
6. **Adding a Comment to the User Account**:
- You can include a comment (description) for the user:
```
sudo useradd -c "Sales Manager" dave
```
7. **Creating a System User**:
- System users are typically used for services and daemons. To create one:
```
sudo useradd -r -s /sbin/nologin nginx
```
8. **Assigning Multiple Groups to a User**:
- To add "emma" to both "developers" and "admins" groups:
```
sudo useradd -G developers,admins emma
```
9. **Removing a User with `userdel`**:
- To delete the user "testuser":
```
sudo userdel testuser
```
10. **Changing User Passwords with `passwd`**:
- To set or change the password for "testuser":
```
sudo passwd testuser
```
- Follow the prompts to set the new password.
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