A Complete Guide to chmod in Linux That Covers Every Possible Command

Configuring file permissions right in a Linux environment is very important for data security and managing the operating system. An abbreviation for change mode, the chmod command is an essential tool for setting and changing permissions of files and deciding who can see and do what with each file. We will cover every aspect of chmod in Linux so you understand the syntax, usage, and application in varying situations.

1. Introduction to chmod and File Permissions in Linux

Overview of chmod

In Linux, though, having a Linux utility is one thing, but being able to utilize it is another. And one of the more important ones is chmod. The main benefit of setting permissions is that they allow you to specify who can read, write, or execute a given file — a technique that helps keep your system neat and clean on a single-user basis and in a multi-user system.

Importance of File Permissions

In Linux, permissions play a vital role by:

• Safeguarding sensitive information from unauthorized access.
• Minimizing errors by preventing unintended modification.
• Controlling executable files to protect against security risks.

It’s important to know and control permissions carefully since granting write permissions to all the users will leave a public folder open to threats or misuse.

Basic File Permissions Explained

Permissions in Linux are of three types:

1. Read (r): Grants the ability to view file contents.
2. Write (w): Allows editing, modifying, or deleting the file.
3. Execute (x): Permits that the file is executed as a script or program.

These permissions are assigned across three categories of users:

• User (u): The file’s owner.
• Group (g): The file’s assigned group.
• Others (o): All other users on the system.

The permissions on your files and directories on a Linux system can be managed using chmod and will give you control over its access by each user.

2. Linux File Permissions Explained

Permissions in Linux can be managed using two modes: numeric (octal) and symbolic. Each mode has its advantages and serves specific needs.

Numeric (Octal) Mode

Numeric mode represents permissions as a combination of numbers from 0 to 7. Binary values correspond to specific permissions for each number:

• 4: Read permission (r)
• 2: Write permission (w)
• 1: Execute permission (x)

By adding these values, you can represent combinations of permissions:

• 7 (4+2+1): Read, write, and execute (complete access)
• 6 (4+2): Read and write
• 5 (4+1): Read and execute
• 0: No permissions

Three digits represent permissions:

1. The first digit sets permissions for the owner.
2. The second digit applies to the group.
3. The third digit applies to others.

Examples

• 755: The owner has read, written, and executed; the group and others have read and executed.
• 644: The owner has read and written; the group and others have read only.

chmod 755 example.sh

chmod 644 notes.txt

Symbolic Mode

Symbolic mode allows permissions to be specified with letters and symbols for flexibility:

• u for user (owner)
• g for group
• o for others
• a for all

Symbols like +, –, and = let you add, remove, or set permissions, respectively:

chmod u+x script.sh   # Adds execution permission for user

chmod g-w file.txt    # Removes write permission from group

# Sets others to read-only

3. The Basics of chmod Commands

Checking Current File Permissions

To see the permissions of a file, use:

ls -l filename

This command lists the permissions in the format -rwxr-xr–, where each part represents different permissions for the owner, group, and others.

Using chmod in Numeric Mode

The octal (numeric) format is efficient and widely used. Here are some examples of common permission values:

• 744: The owner has full access; the group and others can only read.
• 600: The owner has read and write; the group and others have no permissions.

chmod 744 config.ini

chmod 600 private_data.txt

Using chmod in Symbolic Mode

Symbolic mode provides flexibility to add or remove specific permissions without altering other existing permissions.

Examples:

Add Execute for User:

chmod u+x script.sh

Remove Write for Group:

chmod g-w notes.txt

4. chmod Command Options and Flags

Common chmod Options

-c: Outputs only when changes are made.

chmod -c 644 file.txt

-v: Verbose mode, showing detailed output for each file.

chmod -v 755 program.sh

-R: Recursive change for directories, applying the command to all files within.

chmod -R 755 /home/user/docs/

Advanced chmod Options

Sticky Bit (+t)

The sticky bit is often used for shared directories, ensuring that only file owners can delete files:

chmod +t /var/public

SUID (+s)

SUID (Set User ID) allows users to execute a file with the file owner’s permissions:

chmod u+s executable

SGID (+s)

SGID (Set Group ID) confirms that new files in a directory inherit the group of the directory:

chmod g+s shared_folder

5. Advanced chmod Commands for Specialized Use Cases

Setting Special Permissions

SUID: Useful for files that need to run with elevated privileges, such as /bin/passwd.

chmod 4755 /bin/passwd

SGID: Often set on directories shared by groups, like a team project directory.

chmod 2755 /var/shared

Sticky Bit: Used in directories like /tmp where files should only be deletable by their owners.

chmod 1777 /tmp

Changing Permissions on Multiple Files and Directories

To apply permissions to all files in a directory:

chmod -R 644 /path/to/folder

Secure Permissions for Scripts and Executables

For scripts, it’s common to restrict permissions to 700 or 755 for security:

chmod 700 start_script.sh

chmod 755 run_program.sh

6. chmod and Security: Best Practices for Permission Management

Common Security Risks

Improper permissions can lead to data exposure or accidental changes:

• Setting permissions too permissively (e.g., 777) can expose files to anyone.
• Setting scripts with execute permissions for all can introduce security vulnerabilities.

Guidelines for Secure Permissions

• System Configuration Files: Restrict to 644 to prevent unauthorized changes.
• Executable Scripts: Limit execution permissions to the owner or trusted group members.
• Shared Directories: Use sticky bits to prevent unauthorized deletions.

chmod for Linux Security

Setting permissions carefully with chmod helps secure files, preventing unauthorized access and modifications. Regularly audit permissions on critical files using tools like find and ls.

7. chmod Permissions in Practice: Real-World Scenarios and Examples

chmod in Multi-User Environments

In shared directories, use chmod with sticky and SGID bits to organize files:

chmod 2775 team_shared/

chmod +t team_shared/

chmod in Development and Production Environments

For development, scripts often need to execute permissions (700 or 755):

chmod 755 deploy.sh

Examples and Tutorials

1. Secure Permissions for Config Files:

chmod 644 /etc/config.conf

2. Setting Up a Shared Group Folder:

chmod 2775 /project_folder

3. Limit Permissions for Sensitive Files:

chmod 600 private_notes.txt

8. Summary of chmod Commands and Cheat Sheet

Quick Reference for chmod Commands

• chmod 755 filename: Owner full access, group, and others read/execute.
• chmod 644 filename: Owner read/write, group and others read-only.
• chmod +x filename: Adds execution permission.

Quick Tips for chmod Mastery

• Use numeric codes for common permissions.
• Use symbolic mode for fine adjustments.
• Remember -R for recursive changes.

9. FAQs on chmod and File Permissions

1. How do I make a file executable?

chmod +x filename

2. How can I set permissions for all files in a directory?

Add -R for recursive permissions:

chmod -R 644 /path/to/directory

3. What’s the difference between 755 and 777?

• 755: The owner has all permissions; the group and others can read and execute.
• 777: Full access for all, which is typically insecure.

4. How can I remove execute permissions for a script?

To prevent execution, remove the x permission:

chmod -x script.sh

By mastering chmod, you increase Linux security, managing access for multiple users while keeping sensitive data secure. In short, this guide will give you a good foundation on which to build your permission management.

About the writer

Vinayak Baranwal wrote this article. Use the provided link to connect with Vinayak on LinkedIn for more insightful content or collaboration opportunities.