13-CLI-File-System-Permissions

## Managing File System Permissions from the Command Line

### Objectives
After completing this section, you should be able to change the permissions and ownership of files using command-line tools.

### Changing File and Directory Permissions

The `chmod` command is used to change file and directory permissions. Permissions can be specified using either the symbolic method or the numeric method.

#### Changing Permissions with the Symbolic Method

```bash
chmod WhoWhatWhich file|directory
```
- **Who**: `u` (user), `g` (group), `o` (other), `a` (all)
- **What**: `+` (add), `-` (remove), `=` (set exactly)
- **Which**: `r` (read), `w` (write), `x` (execute)

The symbolic method uses letters to represent user categories and permissions. The operation can add, remove, or set permissions exactly.

##### Examples:
- **Remove read and write permission for group and other on `file1`:**
  ```bash
  chmod go-rw file1
  ```
- **Add execute permission for everyone on `file2`:**
  ```bash
  chmod a+x file2
  ```

##### Recursive Permission Change:
- The `-R` option applies changes recursively:
  ```bash
  chmod -R g+rwX demodir
  ```
  This command sets read and write permissions for the group on `demodir` and its contents, adding execute permission only to directories and files that already have execute set for user, group, or other.

#### Changing Permissions with the Numeric Method

```bash
chmod ### file|directory
```
Each digit in the numeric method represents permissions for the user, group, and others, calculated as follows:
- **Read**: 4
- **Write**: 2
- **Execute**: 1

##### Example:
- **Set read and write permissions for user, read permission for group and other on `samplefile`:**
  ```bash
  chmod 644 samplefile
  ```

- **Set read, write, and execute permissions for user, read and execute permissions for group, and no permission for others on `sampledir`:**
  ```bash
  chmod 750 sampledir
  ```

### Changing File and Directory User or Group Ownership

File ownership can be changed using the `chown` command.

#### Changing User Ownership

```bash
chown user file|directory
```
- **Example:**
  ```bash
  chown student test_file
  ```

#### Changing Group Ownership

```bash
chown :group file|directory
```
- **Example:**
  ```bash
  chown :admins test_dir
  ```

#### Changing Both User and Group Ownership

```bash
chown user:group file|directory
```
- **Example:**
  ```bash
  chown visitor:guests test_dir
  ```

#### Recursive Ownership Change

- The `-R` option changes ownership recursively:
  ```bash
  chown -R student test_dir
  ```

### Important Notes

- Always use a colon (`:`) to separate user and group in `chown` commands. Do not use a period (`.`), as it can cause confusion due to periods being valid characters in usernames.
  - **Correct:**
    ```bash
    chown owner:group filename
    ```
  - **Incorrect:**
    ```bash
    chown owner.group filename
    ```

### References
- `ls(1)`, `chmod(1)`, `chown(1)`, and `chgrp(1)` man pages

This guide covers the essential commands and methods for managing file system permissions and ownership from the command line in Linux.