Understanding chmod 644 and -rw-r–r– in Linux: File Permissions Explained Clearly

1. Introduction

What Are “Access Permissions” in Linux?

In Linux and Unix-like systems, properly managing file and directory access permissions is extremely important. This is essential not only from a security standpoint, but also for maintaining stable system operations.

Many people have probably wondered what symbols like -rw-r--r-- mean when viewing file lists in the terminal.

The Relationship Between chmod and -rw-r–r–

These symbols represent file access permissions using symbolic notation. The chmod command is used to set and modify these permissions. In other words, when you use chmod to assign specific permissions to a file, the result is displayed as symbols like -rw-r--r--.

Purpose of This Article and Intended Audience

This article explains what -rw-r--r-- actually means and how to set or change it using chmod, in a way that is easy for beginners to understand. We also cover security considerations and practical real-world usage examples.

The content is intended for everyone from Linux beginners to intermediate users who are becoming comfortable with terminal operations, with the goal of providing practical knowledge usable in real work environments.

2. What Is chmod? (Basic Understanding)

The Role of the chmod Command

chmod (pronounced “change mode”) is a command used in Linux and Unix-like operating systems to modify file and directory access permissions.
Using this command allows you to flexibly control who can perform which operations (read, write, execute).

Proper permission settings are crucial not only for security, but also for preventing file sharing and execution issues.

Basic Syntax of chmod

chmod [options] [permissions] filename

For example:

chmod 644 sample.txt

This command changes the permissions of sample.txt as follows:

-rw-r--r--  1 user group 1234 Apr 13 20:00 sample.txt

• Owner (user): read and write allowed (rw-)
• Group: read-only (r–)
• Others: read-only (r–)

Two Ways to Specify Permissions

There are two methods for specifying permissions with chmod:

• Numeric mode
• Example: chmod 755 script.sh
• This method assigns permissions using numbers.
• Symbolic mode
• Example: chmod u=rw,g=r,o=r file.txt
• This method uses symbols such as u (user), g (group), o (others), and r/w/x.

Both methods produce the same results. Numeric mode is fast for bulk changes, while symbolic mode allows more flexible control.

Difference Between chmod and chown

A commonly confused command is chown:

• chmod: changes access permissions
• chown: changes file owner and group

Since their purposes differ, it is important to clearly understand the role of each command.

3. Understanding the Meaning of -rw-r–r–

Structure of Permission Symbols

When you run ls -l in Linux, file information is displayed in the following format:

-rw-r--r--  1 user group  1234 Apr 13  2025 sample.txt

The leftmost part, -rw-r--r--, represents the file’s access permissions. Understanding this allows you to know exactly who can do what with the file.

Detailed Breakdown of Each Part

① First character: file type

In the case of -rw-r--r--, the leading - indicates a regular file.

② Characters 2–10: permissions (3 characters × 3 sets)

• Owner (user): rw-
  Read and write are allowed; execute is not.
• Group: r--
  Read-only.
• Others: r--
  Read-only.

This means that only the owner can edit the file, while everyone else can only view it.

Numeric Representation: What 644 Means

Linux permissions can also be expressed using numeric (octal) notation.

The numeric equivalent of -rw-r--r-- is 644.

The numeric values are calculated as follows:

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

Thus, rw- = 4 + 2 = 6, and each r-- = 4, resulting in 644.

Why This Permission Is Commonly Used

-rw-r--r-- (644) is a very common permission for public files and static web server content such as HTML and CSS.

• Only the owner can modify the file, preventing unintended changes
• Other users can read the file, allowing sharing and public access

When used correctly, this setting balances security and usability.

4. How to Set -rw-r–r– Using chmod (Practical Guide)

Changing Permissions with the chmod Command

File and directory access permissions can be freely changed using the chmod command. For example, if you want to set permissions to -rw-r--r--, simply apply permission 644.

Setting Permissions Using Numeric Mode

Use the format chmod 644 filename.

chmod 644 document.txt

This command changes the permissions of document.txt as follows:

-rw-r--r--  1 user group 1234 Apr 13 20:00 document.txt

• Owner (user): read and write allowed (rw-)
• Group: read-only (r–)
• Others: read-only (r–)

Setting Permissions Using Symbolic Mode

If you need more flexible control, symbolic mode is useful.

chmod u=rw,g=r,o=r document.txt

This command means:

• u=rw: grant read and write permissions to the owner
• g=r: grant read-only permission to the group
• o=r: grant read-only permission to others

The result is exactly the same as using numeric mode 644.

Important Notes When Applying Permissions to Directories

Unlike files, directories require execute permission (x) to be accessible.

For example, the following setting may restrict access:

chmod 644 my_folder

With this configuration, read permission is granted, but users cannot enter the directory because execute permission is missing.

Correct example:

chmod 755 my_folder

• Owner: read, write, execute
• Group and others: read and execute

This allows other users to list and access files inside the directory.

Recursively Changing Permissions: The -R Option

If you want to change permissions for many files and subdirectories at once, use the -R (recursive) option.

chmod -R 644 my_folder

This applies permission 644 to all files under my_folder. However, this approach comes with important caveats.

5. Checking Permissions with the ls -l Command

How to View File Permissions

To check file and directory permissions in Linux, use the standard ls -l command. This displays detailed information such as permissions, owner, file size, and last modified date.

Basic Usage

ls -l

The output looks like this:

-rw-r--r--  1 naoya devs  2048 Apr 13 20:00 index.html

Each column has the following meaning:

How to Read the Permission Field

Focus on the -rw-r--r-- portion. It can be broken down as follows:

• First character: file type
• -: regular file
• d: directory
• l: symbolic link
• Remaining nine characters: permissions (3 × 3 sets)
• Owner: rw- → read and write
• Group: r-- → read-only
• Others: r-- → read-only

This tells you that only the owner can edit the file, while others can only read it.

Checking a Specific File

To check permissions for a specific file, specify the file name:

ls -l index.html

This is useful when working in directories containing many files.

Checking Directory Permissions

Directories can also be checked using ls -l. To view the directory itself rather than its contents, use the -d option.

ls -ld my_folder

This displays permission information for my_folder only.

Safe Permission Management

After modifying permissions with chmod, it is recommended to always verify the result using ls -l.

Incorrect settings can lead to security issues or unexpected behavior.

You can also combine it with pipes for filtering:

ls -l | grep '.sh'

This filters and displays only shell script files.

6. Checking and Changing Permissions Using GUI Tools (Beginner-Friendly)

An Option for Those Uncomfortable with the Command Line

Linux is often operated via the terminal and command line, but many beginners feel that text-based interfaces are difficult or error-prone.
For such users, file permissions can also be checked and modified using GUI (Graphical User Interface) tools.

Using the Ubuntu File Manager (Nautilus)

Linux distributions such as Ubuntu include a default file manager called Nautilus, similar to Windows Explorer.

How to check permissions:

1. Right-click the target file or directory
2. Select “Properties”
3. Open the “Permissions” tab (labeling may vary by version)

What you can do there:

• View and change owner and group (administrator privileges required)
• Toggle read, write, and execute permissions
• Changes are applied immediately

Instead of symbolic notations like rw- or r--, permissions are shown in plain-language descriptions such as “Read-only” or “Read and write”, making them easy to understand for beginners.

WinSCP (SFTP Client for Windows Users)

When connecting from Windows to a remote Linux server, WinSCP allows permission management through a GUI.

Steps:

1. Log in to the server using WinSCP
2. Right-click a file and select “Properties”
3. Change permission values (e.g., 644) or use checkboxes
4. Click “OK” to apply changes

Advantages:

• Permissions can be viewed in both numeric and symbolic formats
• Recursive permission changes are supported via GUI

Notes:

• Files requiring root privileges may not be modifiable without sudo permissions

WinSCP - Free SFTP and FTP client

WinSCP

WinSCP is a popular free file manager for Windows supporting…

FileZilla (Cross-Platform SFTP Client)

The cross-platform SFTP client FileZilla, available for macOS and Linux, also allows permission changes.

Steps:

1. Connect to the server
2. Right-click the target file
3. Select “File Permissions”
4. Adjust permissions using checkboxes or numeric values and click “OK”

Advantages and Caveats of GUI-Based Operations

Advantages:

• Less prone to mistakes for beginners
• Changes can be visually confirmed immediately
• No need to memorize command syntax

Caveats:

• Some server environments do not allow permission changes via GUI tools
• Displayed values may not always reflect actual applied permissions, so verification with ls -l is recommended

FileZilla - The free FTP solution

FileZilla - The free FTP solution for both client and server…

7. Important Notes and Troubleshooting for chmod

It Still Doesn’t Work—Why?

Even after changing permissions using chmod, files may not behave as expected. In most cases, this is due to misconfigured permissions or misunderstandings.
Below are common real-world issues and their solutions.

“Permission denied” Errors

Symptom:

bash: ./script.sh: Permission denied

Cause:

• The file does not have execute permission (x)

Solution:

chmod +x script.sh

Or using numeric mode:

chmod 755 script.sh

Note that having read or write permission does not imply execution permission.

Directories Without Execute Permission Cannot Be Accessed

Applying chmod 644 to a directory may prevent access to its contents.

Reason:

• Execute permission (x) on directories is required to enter and list their contents.

Example:

chmod 644 my_folder
ls my_folder

This may result in an error.

Fix:

chmod 755 my_folder

Pitfalls of Recursive Changes (-R)

While chmod -R is convenient, it applies the same permissions to both files and directories, which can cause unexpected issues.

Common mistake:

chmod -R 644 /var/www/html

This removes execute permissions from directories, making them inaccessible.

Correct approach:

# Apply execute permissions to directories
find /var/www/html -type d -exec chmod 755 {} \;

# Apply file permissions
find /var/www/html -type f -exec chmod 644 {} \;

Owner and Group Mismatch

Permissions alone are not enough; file ownership and group settings also matter.
For example, if a web server user such as apache needs to read a file, but the owner is a regular user and others lack read permission, the file will not be served correctly.

Solutions:

• Change owner: sudo chown www-data:www-data index.html
• Grant group access: chmod 640 index.html (if group is appropriate)

Why chmod 777 Is Dangerous

Some users attempt to fix issues by setting permissions to 777, but this is extremely dangerous.

Reasons:

• Anyone can read, write, and execute the file
• High risk of tampering or malicious script execution

Recommended best practices:

• Files: 644 or 600
• Directories: 755 or 700
• Scripts/Binaries: 755 (as needed)

8. Practical Use Cases and Common Scenarios

How Are chmod Permissions Used in Real-World Environments?

The chmod command and permissions such as -rw-r--r-- are frequently used in real server operations and development environments.
Below are several representative scenarios illustrating how permissions are applied in practice.

1. Managing HTML Files on a Web Server

Scenario:
When publishing HTML files on a web server such as Apache or Nginx, the typical permission is -rw-r--r-- (644).

chmod 644 index.html

Why 644?

• The owner (site administrator) needs write access
• The web server user (e.g., www-data) only needs read access
• Write access for others is unnecessary and unsafe

This configuration enables secure publication of web content using the principle of least privilege.

2. Granting Execute Permission to Script Files

Scenario:
You created a shell script backup.sh, but execution fails with a “Permission denied” error.

chmod 755 backup.sh

Meaning of this setting:

• Owner: read, write, execute (rwx)
• Group and others: read and execute (rx)

This allows others to run the script while restricting editing to the owner.

3. Restricting Access to Sensitive Files

Scenario:
You manage a file secrets.txt containing API keys or passwords and want to prevent anyone else from accessing it.

chmod 600 secrets.txt

Meaning:

• Owner: read and write only
• Group and others: no access

This is a critical security best practice. Sensitive files should always be managed with 600 or 400 (read-only).

4. Securely Sharing a Directory

Scenario:
You create a shared directory shared_folder accessed by multiple developers.

chmod 770 shared_folder

Meaning:

• Owner and group: full permissions (rwx)
• Others: no access

By assigning the development team as the group, you enable secure and efficient collaboration. Group membership can be adjusted using chgrp.

5. Avoiding Problems Caused by Misconfiguration

Scenario:
You accidentally run chmod -R 777 ., making all files writable by anyone.

Recommended precautions:

• Always use find to differentiate between files and directories
• Check permissions beforehand with ls -l
• Test changes in a staging environment first

9. Frequently Asked Questions (FAQ)

Q1. What is the difference between chmod 644 and -rw-r--r--?

A. They represent the same permission; only the notation differs.

• chmod 644: numeric permission specification
• -rw-r--r--: symbolic representation shown by ls -l

Q2. What happens if a file has no execute permission?

A. Executing scripts or binaries will fail.

bash: ./script.sh: Permission denied

Fix:

chmod +x script.sh

Q3. Is it safe to set 644 on directories?

A. Generally, no.

Directories require execute permission (x) to be accessible.

Correct setting:

chmod 755 directory_name

Q4. Are there any risks when using chmod -R?

A. Yes. The same permissions are applied to both files and directories.

chmod -R 644 my_project/

This removes execute permissions from directories.

Safer approach:

find my_project/ -type d -exec chmod 755 {} \;
find my_project/ -type f -exec chmod 644 {} \;

Q5. I keep getting “Permission denied.” What should I check?

A. Verify the following:

• The file has execute permission if it is being executed
• You are the owner or have write permission
• The directory has execute permission
• You have sudo privileges if required

Q6. I want to share a file but prevent editing. What should I do?

A. Use permissions 644 or 444.

chmod 644 share.txt

• 644: others can read only
• 444: everyone has read-only access

Q7. I accidentally changed permissions. How can I restore them?

A. Compare with a correctly configured file.

ls -l /path/to/correct_file

Reapply permissions using chmod. Ideally, document default permissions for your project.

10. Summary

Why Understanding chmod and Permissions Matters

This article explained the chmod command and the meaning of symbolic permissions such as -rw-r--r--, which are widely used in Linux and Unix-like systems.

These concepts are essential for ensuring system security, operational stability, and accurate file sharing.

Key Takeaways

• chmod modifies file and directory access permissions
• -rw-r--r-- means the owner can read and write, others can only read
• Numeric (644) and symbolic (-rw-r--r--) representations are equivalent
• Permissions can be set using numeric or symbolic modes
• ls -l is used to verify permissions
• GUI tools such as Nautilus, WinSCP, and FileZilla can also manage permissions
• Incorrect permissions often cause “Permission denied” errors
• There is always a reason behind commonly used permission patterns

The “Best” Permission Depends on Context

There is no universal permission setting that fits all cases.
Permissions should always be chosen based on purpose, users, and required security level.

Final Advice

While permissions may seem complex at first, mastering chmod enables you to prevent issues proactively and operate systems securely.

Always ask yourself:
Who should access this file, and for what purpose?
Keeping this perspective will help you build safe and reliable Linux environments.