5.4 Special Directories and Files (Weight: 1)

📘Linux Essentials (LPI 010-160)

1. Symbolic Links (Symlinks)

What is a Symbolic Link?

A symbolic link (also called a soft link or symlink) is a special type of file that points to another file or directory.

Instead of containing actual data, it contains a reference path to another file.

When a program or user accesses the symbolic link, the system redirects the access to the original file.

Key Characteristics

It is a separate file that points to another file.
It contains the path to the target file or directory.
If the target file is removed, the symbolic link becomes broken.
Symbolic links can point to:
- Files
- Directories
- Files located on other filesystems.

Viewing Symbolic Links

You can identify symbolic links using the ls -l command.

Example:

ls -l

Output example:

lrwxrwxrwx 1 user user 12 Mar 10 10:00 config -> /etc/config

Explanation:

Field	Meaning
l	File type is symbolic link
rwxrwxrwx	Link permissions
config	Name of link
->	Points to
/etc/config	Target file

The first character l indicates a symbolic link.

Creating Symbolic Links

The command used is:

ln -s target link_name

Example:

ln -s /etc/nginx/nginx.conf nginx_config

Result:

nginx_config becomes a symbolic link.
It points to /etc/nginx/nginx.conf.

Removing Symbolic Links

Symbolic links are removed like normal files:

rm link_name

Example:

rm nginx_config

This removes only the link, not the original file.

Broken Symbolic Links

A symbolic link becomes broken when the target file does not exist.

Example situation:

ln -s /etc/app/config.conf config

If /etc/app/config.conf is deleted, the link config remains but does not work.

ls -l may display it like:

config -> /etc/app/config.conf

But accessing it will result in an error.

Symbolic Links in IT Environments

Symbolic links are widely used in system administration.

Configuration File Management

Example:

Applications may expect configuration files in /etc.

But the real configuration may be stored elsewhere.

Example:

/etc/app.conf -> /opt/app/config/app.conf

This allows:

applications to use /etc/app.conf
administrators to store configuration elsewhere.

Software Version Management

Example structure:

/opt/java/java17
/opt/java/java21

A symbolic link can be used:

/opt/java/current -> /opt/java/java21

Programs can always reference:

/opt/java/current

When upgrading, the administrator simply updates the symbolic link.

Shared Resources

Example:

A shared library stored in one location:

/usr/lib/libapp.so

Multiple programs may access it via symbolic links in different directories.

2. Special Permissions

Normally Linux permissions include:

Read (r)
Write (w)
Execute (x)

But Linux also supports three special permissions that modify normal permission behavior.

These are:

Permission	Name
SUID	Set User ID
SGID	Set Group ID
Sticky Bit	Restricted deletion flag

These permissions are commonly used in multi-user systems.

3. SUID (Set User ID)

What is SUID?

The SUID permission allows a file to run with the permissions of the file owner, not the user executing it.

This is usually applied to executable programs.

Example Behavior

If a program is owned by root and has the SUID bit, then:

Any user running that program temporarily runs it with root privileges.

Example in Linux Systems

A well-known example is the passwd program.

/usr/bin/passwd

This program allows users to change their passwords.

The program needs to modify the file:

/etc/shadow

This file is normally only writable by root.

But users can still change passwords because passwd runs with SUID root permissions.

Viewing SUID Permissions

Use:

ls -l

Example:

-rwsr-xr-x 1 root root 54256 Mar 10 10:00 passwd

Notice:

rws

The s replaces the x in the owner’s permission.

Meaning:

SUID is enabled

Setting SUID

Using chmod:

chmod u+s filename

Example:

chmod u+s program

Removing SUID

chmod u-s filename

4. SGID (Set Group ID)

What is SGID?

The SGID permission allows a program to run with the group permissions of the file, instead of the user’s group.

SGID on Executable Files

If SGID is set on a program:

it runs with the group privileges of the file owner group.

SGID on Directories

SGID has another important behavior on directories.

When SGID is set on a directory:

new files created inside the directory inherit the directory’s group.

Normally files inherit the creator’s group, but SGID overrides this behavior.

Example in IT Environments

A shared project directory:

/srv/project

Multiple developers belong to the group:

devteam

Directory permissions:

drwxrwsr-x

With SGID:

all files created in this directory belong to devteam
this ensures consistent group ownership.

Viewing SGID

Example:

drwxrwsr-x 2 root devteam 4096 Mar 10 10:00 project

Notice:

rws

in the group permission section.

Setting SGID

chmod g+s directory

Example:

chmod g+s /srv/project

Removing SGID

chmod g-s directory

5. Sticky Bit

What is the Sticky Bit?

The sticky bit is used mainly on directories.

It restricts file deletion.

When the sticky bit is set:

Users can delete files only if they:

own the file
own the directory
are root.

Why Sticky Bit is Important

Without the sticky bit:

any user with write permission could delete other users’ files in the directory.

With sticky bit:

users cannot delete files owned by others.

Common Linux Example

The directory:

/tmp

Permissions:

drwxrwxrwt

The t indicates the sticky bit.

This directory is used for temporary files by all users.

Sticky bit prevents users from deleting other users’ files.

Viewing Sticky Bit

Example:

drwxrwxrwt 10 root root 4096 Mar 10 10:00 /tmp

The t appears in the others execute position.

Setting Sticky Bit

chmod +t directory

Example:

chmod +t /shared/tmp

Removing Sticky Bit

chmod -t directory

6. Special Permission Numeric Values

Linux permissions can also be represented numerically.

Special permissions use an extra digit.

Format:

Special | Owner | Group | Others

Example:

Meaning:

Digit	Meaning
4	SUID
2	SGID
1	Sticky

Examples:

Permission	Numeric
SUID	4000
SGID	2000
Sticky	1000

Examples:

chmod 4755 program

chmod 2775 directory

chmod 1777 /tmp

7. Summary Table

Feature	Purpose	Used On
Symbolic Link	Points to another file	Files & directories
SUID	Run program as file owner	Executables
SGID	Run program as file group / inherit group	Files & directories
Sticky Bit	Restrict file deletion	Directories

8. Key Exam Points (Linux Essentials)

Students should remember:

Symbolic links are created using ln -s
Symbolic links contain paths to target files
If target is deleted → broken link
SUID runs programs as the file owner
SGID allows group inheritance in directories
Sticky bit prevents users from deleting others’ files
/tmp commonly uses the sticky bit
Special permissions appear as s or t in ls -l output
Numeric permission values:
- SUID = 4
- SGID = 2
- Sticky = 1