Linux File Permissions Explained | Complete Guide with Octal Cheatsheet and Commands

Learn everything about Linux file permissions, including symbolic and octal notations, user-group-other levels, SUID, SGID, Sticky Bit, and chmod usage. This guide includes a visual cheatsheet, best practices, and real command-line examples.

  Linux   Jun 19, 2025   30  Add to Reading List
Linux File Permissions Explained | Complete Guide with Octal Cheatsheet and Commands

Table of Contents

Understanding Linux file permissions is essential for system administrators, ethical hackers, developers, and cybersecurity professionals. Proper file permissions protect sensitive data, prevent unauthorized access, and define who can read, write, or execute files or directories. In this blog, we’ll break down everything from permission notations to special bits like SUID, SGID, and the Sticky Bit, using real-world examples and terminal commands.

What Are Linux File Permissions?

Linux file permissions determine the level of access users have to a particular file or directory. These permissions are assigned to three categories of users:

  • Owner: The user who owns the file

  • Group: Users who belong to the file's group

  • Others: Everyone else

Each of these categories can be assigned three types of permissions:

Permission Symbol Meaning
Read r View file contents or list directory contents
Write w Modify file contents or create/delete files in a directory
Execute x Run a file as a program or enter a directory

 File Permission Structure

A typical Linux permission string looks like: 

-rwxr-xr--

Here’s how to read it:

  • First character: - indicates a regular file; d is for directories.

  • Next 3: rwx — Owner permissions (read, write, execute)

  • Next 3: r-x — Group permissions (read, execute)

  • Last 3: r-- — Other permissions (read only)

 Octal (Numeric) Representation of Permissions

Permissions can also be represented in binary or octal:

Binary Octal Symbol Meaning
000 0 --- No permissions
001 1 --x Execute only
010 2 -w- Write only
011 3 -wx Write + Execute
100 4 r-- Read only
101 5 r-x Read + Execute
110 6 rw- Read + Write
111 7 rwx Read + Write + Execute

So a permission string like rwxr-xr-- is 754 in octal.

 Special File Permissions: SUID, SGID, Sticky Bit

Linux offers three special bits that can alter how files are accessed and executed:

 SUID (Set User ID)

Allows users to run an executable with the permissions of the file owner.

  • Symbol: s in the owner execute field

  • Example: -rwsr-xr-x

  • Command: 

    chmod u+s filename

 SGID (Set Group ID)

Files: Similar to SUID, but applies to group permissions.
Directories: Newly created files inherit the group of the directory.

  • Symbol: s in the group execute field

  • Example: -rwxr-sr-x

  • Command: 

    chmod g+s directory

Sticky Bit

Applied to directories to restrict file deletion within them to file owners.

  • Symbol: t in the other execute field

  • Example: drwxrwxrwt

  • Command: 

    chmod +t directory

 Changing Permissions Using chmod

Linux provides the chmod command to change permissions in both symbolic and octal format.

✅ Symbolic Example:

chmod u+x script.sh   # Add execute to user
chmod g-w script.sh   # Remove write from group
chmod o=r file.txt    # Set read-only for others

✅ Octal Example:

chmod 755 filename     # rwxr-xr-x
chmod 700 filename     # rwx------
chmod 644 file.txt     # rw-r--r--

 Viewing Permissions with ls -l

ls -l

Sample Output: 

-rwxr-xr-- 1 user group  2048 Jun 19 12:30 example.sh

Breakdown:

  • -rwxr-xr--: Permissions

  • 1: Number of hard links

  • user: File owner

  • group: File’s group

  • 2048: File size in bytes

  • Jun 19 12:30: Last modified

  • example.sh: Filename

 Best Practices for Managing Permissions

  • Always follow the principle of least privilege.

  • Avoid using 777 unless absolutely necessary.

  • Use SUID/SGID/Sticky Bit carefully to avoid privilege escalation.

  • Use groups to manage multiple users with similar permissions.

✅ Common Permission Use Cases

Use Case Command
Make script executable chmod +x script.sh
Make file read-only chmod 444 filename.txt
Allow full access chmod 777 testfile
Secure config file chmod 600 config.ini
Set SUID chmod u+s /path/to/file
Set SGID chmod g+s /path/to/dir
Set Sticky Bit chmod +t /tmp/shared_dir

 Conclusion

Linux file permissions are one of the foundational elements of system security. With a deep understanding of permission notations, octal values, and special bits like SUID, SGID, and the Sticky Bit, you'll be equipped to secure any Linux environment effectively. Use the cheatsheet provided to reinforce your knowledge and apply best practices for file access control.

FAQs 

What are the 3 basic types of Linux file permissions?

Read (r), Write (w), and Execute (x).

What does chmod 755 mean in Linux?

It sets permissions to rwxr-xr-x, allowing full access for the owner, read-execute for group and others.

What is the difference between symbolic and octal permissions?

Symbolic uses letters like rwx, while octal uses numbers like 755.

How do I change file permissions in Linux?

Use the chmod command.

What is SUID in Linux?

Set User ID allows users to execute a file with the file owner's privileges.

How do you set SUID on a file?

Use: chmod u+s filename

What is the SGID bit used for?

SGID allows files in a directory to inherit the directory’s group.

How do you set the Sticky Bit on a folder?

Use: chmod +t folder_name

What does chmod 777 do?

Gives all permissions (read, write, execute) to everyone—it's risky.

How do I view file permissions in Linux?

Use: ls -l

What is rwxr-xr-- in file permissions?

It means owner can read/write/execute, group can read/execute, others can only read.

Can I set different permissions for a directory?

Yes, directories often need execute (x) permission to allow traversal.

What is the owner in Linux file permissions?

The user who created or owns the file.

How do I give read-only access to others?

Use: chmod o=r filename

What does chmod 600 do?

Grants read/write to owner, denies all access to group and others.

Why is chmod 777 considered insecure?

It allows anyone to modify or execute the file—potential security risk.

How do I recursively change permissions in a directory?

Use: chmod -R 755 /directory

What does chmod +x do?

Adds execute permission to the specified user or group.

What does chmod a+r mean?

Adds read permission for all (user, group, others).

How do I remove write permission for others?

Use: chmod o-w filename

What is the purpose of the Sticky Bit?

Restricts file deletion in shared directories to only the file's owner.

What is drwxrwxrwt?

Directory with read, write, execute for all and a Sticky Bit set.

How do I assign execute permission to a script?

Use: chmod +x script.sh

What is the default permission on Linux files?

Typically 644 for files and 755 for directories.

What does chmod 444 mean?

Read-only for all.

Can I change ownership and permissions together?

Yes, using chown for ownership and chmod for permissions.

How do I make a file executable for everyone?

Use: chmod a+x filename

What is the difference between chmod 775 and 755?

775 gives write permission to the group, 755 does not.

How do you give full control to a user?

Use: chmod 700 filename

How do I set permission using symbolic notation?

Example: chmod u+x,g-w filename

Can permissions affect program execution?

Yes. Lack of execute permission will prevent a script or binary from running.

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