What does the 755 permission mean in Linux, and when should it be used?

In Linux, 755 is a common file permission setting used to manage access control for files and directories. It defines what level of interaction the owner, group, and others have with a given file or directory. Understanding this permission model is essential for Linux administrators, developers, and anyone managing shared environments or web server files.

Understanding 755 File Permission in Linux

Linux permissions are represented using a three-digit octal number. Each digit specifies the access level for a specific category of users:

1. First digit: Permissions for the owner (user)
2. Second digit: Permissions for the group
3. Third digit: Permissions for others (everyone else)

The digits correspond to the following values:

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

So, permission 755 breaks down as:

User Type            Binary               Decimal                         Meaning

Owner                   111                           7                             Read, Write, Execute

Group                   101                            5                            Read, Execute

Others                 101                             5                            Read, Execute

This means:

The owner can read, write, and execute the file or directory.
The group and others can read and execute it, but cannot modify it.

755 Permission: Files vs. Directories

For Files:

If a file has 755 permissions, it means:

The owner can read, modify, and execute it.

Others can read and execute, but not change the file.

Example:

  chmod 755 myscript.sh

This allows the script to be executed by anyone, but only modified by the owner.

For Directories:

For a directory, 755 means:

1. The owner can view contents, navigate into the directory, and make changes.
2. Group members and others can view and enter the directory but cannot add or delete files inside it.

This setting is common for directories that need to be accessed by applications, users, or web servers, but where only the admin or owner should make changes.

When to Use 755 Permission

You should use chmod 755 when:

1. You have executable scripts or binaries that should be available to all users.
2. You are deploying web content where the web server (e.g., Apache or Nginx) needs read and execute access.
3. You want to allow users to run a file but restrict them from editing it.

Common scenarios:

• /var/www/html directory for websites is usually set to 755.
• Shared utility scripts in /usr/local/bin often have 755 permissions.

Security Considerations

While 755 is relatively secure for public access to executable files, it can still pose risks if misused. Be cautious of the following:

• Never use 755 for configuration files that contain sensitive information like database credentials.
• Avoid giving execute permissions to files that don't require it.
• Don't use 755 on files that should remain private, like SSH keys or upload folders.

Always follow the principle of least privilege—only grant the permissions that are absolutely necessary for the file to function.

Summary

755 permission in Linux is a standard way to allow public read and execute access while restricting write access to the file or directory owner. It ensures that scripts or directories are accessible system-wide but only modifiable by their creator.

It is widely used in web server configurations, executable scripts, and shared file environments. However, understanding when and where to use it is key to maintaining a secure and functional Linux system.