LWM-Linux/06 - Linux File Operations/Directories and Symlinks (Shortcuts).md

Directories and Symlinks

Directories:

1. Definition:

A directory (also called a folder) is a container in a file system that can hold files and other directories. It organizes files and provides a hierarchical structure.

2. Types of directories:

• Root directory: The top-level directory in the file system hierarchy, typically represented by "/" on Unix-like systems.
• Home directory: The default directory for a user, usually containing personal files and configurations. "~"
• Parent directory: The directory one level above the current directory, represented by "..".
• Current directory: The directory you're currently in, represented by ".".
• Subdirectory: Any directory contained within another directory. "/usr/bin" - bin is a subdirectory of /usr.

3. Directory operations:

• Create: mkdir (make directory)
• Delete: rmdir (remove directory - must be empty) or rm -rf ("f"orce remove "r"ecursively, even if not empty)
• Change: cd (change directory)
• List contents: ls (list)
• View path: pwd ("p"rint "w"orking "d"irectory)

4. Directory permissions:

On Unix-like systems, directories have read (r), write (w), and execute (x) permissions. The execute permission allows users to enter the directory.

• See the section on Permissions for examples.

5. Hidden directories:

In Unix-like systems, directories starting with a dot (.) are hidden by default.

Symlinks (Symbolic Links) or shortcuts (Windows):

1. Definition:

A symlink is a special type of file that points to another file or directory. It's similar to a shortcut in Windows or an alias in macOS.

2. Types of symlinks:

• Soft links (symbolic links): Point to a file or directory by name. They can span file systems and can link to non-existent targets.
• Hard links: Point directly to the inode of a file. They can't span file systems or link to directories.

3. Creating symlinks:

Use the ln command with the -s option: ln -s target_path link_path # Create a symbolic link at link_path to the target_path

4. Advantages of symlinks:

• Save space by avoiding duplicate files
• Create shortcuts to frequently accessed files or directories
• Maintain multiple versions of files or configurations
• Facilitate easier software updates and management

5. Symlink behavior:

• When you access a symlink, the system automatically redirects to the target.
• Deleting a symlink doesn't affect the target file or directory.
• If the target is deleted, the symlink becomes a "dangling" or "broken" link.

6. Identifying symlinks:

• Use ls -l to see detailed file information. Symlinks are indicated by an "l" at the start of the permissions string.
• The file command can also identify symlinks.

7. Following symlinks:

• Some commands (like cp) don't follow symlinks by default. Use the -L or --follow options to change this behavior.

8. Symlinks in different operating systems:

• Unix-like systems (Linux, macOS): Native support for symlinks.
• Windows: Limited support in older versions, full support since Windows Vista.

9. Security considerations:

• Symlinks can potentially be used in symlink attacks, where an attacker creates a link to a sensitive file.
• Many systems implement symlink protections to mitigate these risks.

10. Use cases:

- Creating shortcuts in the file system
- Managing shared libraries
- Facilitating easier software updates