linux doc: usermanage

Linux/LPIC1/18-usermanage.md

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+# 🧑‍💻 Linux User & Group Management Cheat Sheet
+
+---
+
+## 🔍 Viewing User Info from `/etc/shadow`
+
+```bash
+cat /etc/shadow
+```
+
+Example entry:
+
+```bash
+radin:$y$j9T$gxn.Bgl/nVnzPEeWn0Hrz.$CEeQGtg1TlZ/jwu9Zsz2kkIq3dBtbhJ/bzhVT7cJ1.9:20270:0:99999:7:::
+```
+
+| Field             | Description                                  | Example                                         |
+| ----------------- | -------------------------------------------- | ----------------------------------------------- |
+| 👤 Username       | User’s login name                            | `radin`                                         |
+| 🔒 Password Hash  | Encrypted password                           | `$y$j9T$gxn.Bgl/nVnzPEeWn0Hrz.$CEeQGtg1TlZ/...` |
+| 🗓️ Last Changed  | Days since Jan 1, 1970 when password changed | `20270` (which is 20270 + 1970 = year)          |
+| ⏳ Min Age         | Minimum days between password changes        | `0`                                             |
+| ⏰ Max Age         | Maximum days before password must be changed | `99999`                                         |
+| ⚠️ Warning Period | Days before expiration user is warned        | `7`                                             |
+
+---
+
+## 🛡️ Viewing Groups from `/etc/group`
+
+```bash
+cat /etc/group
+```
+
+Example:
+
+```bash
+radin:x:1000:
+sudo:x:27:radin
+```
+
+| Field         | Description                          | Example         |
+| ------------- | ------------------------------------ | --------------- |
+| 👥 Group Name | Name of the group                    | `radin`, `sudo` |
+| 🔑 Password   | Password (usually `x` means none)    | `x`             |
+| 🆔 GID        | Group ID                             | `1000`, `27`    |
+| 👤 Users      | Users in the group (comma-separated) | `radin`         |
+
+---
+
+## 👥 Check Groups for a User
+
+```bash
+groups <username>
+```
+
+---
+
+## 🆔 View User and Group IDs
+
+```bash
+id <username>
+```
+
+Example output:
+
+```bash
+uid=1000(radin) gid=1000(radin) groups=1000(radin),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),100(users),114(lpadmin)
+```
+
+---
+
+## ➕ Adding Users
+
+| Command                                | Description                               |
+| -------------------------------------- | ----------------------------------------- |
+| `useradd <username>`                   | Add user without home directory           |
+| `useradd -m <username>`                | Add user **with** home directory          |
+| `useradd -s /bin/bash -m <username>`   | Add user with bash shell & home directory |
+| `useradd -G group1,root -m <username>` | Add user with home and extra groups       |
+
+---
+
+## 🔑 Changing Password
+
+```bash
+passwd <username>
+```
+
+---
+
+## ❌ Deleting Users
+
+| Command                 | Description                          |
+| ----------------------- | ------------------------------------ |
+| `userdel <username>`    | Delete user                          |
+| `userdel -r <username>` | Delete user and their home directory |
+| `userdel -f <username>` | Force delete user                    |
+
+---
+
+## 🔧 Modifying Users (`usermod`)
+
+| Command                                  | Description                               |
+| ---------------------------------------- | ----------------------------------------- |
+| `usermod [options] username`             | Modify user with various options          |
+| `usermod -l <newusername> <oldusername>` | Rename user                               |
+| `usermod -d <dir> -m <username>`         | Change user home directory and move files |
+| `usermod -G <groups> <username>`         | Set new groups (replaces old groups)      |
+| `usermod -aG <group> <username>`         | Add user to a supplementary group         |
+| `usermod -s <shell> <username>`          | Change user login shell                   |
+