What does 'chmod 755' mean?

In Linux, chmod 755 is a shorthand way to set specific permissions using Numeric (Octal) Notation. Each digit represents a different category of user:

1. 7 (First digit): Permissions for the Owner (User).
2. 5 (Second digit): Permissions for the Group.
3. 5 (Third digit): Permissions for Others (everyone else).

How the numbers are calculated

Each number is a sum of three basic permission values:

• 4 = Read (r)
• 2 = Write (w)
• 1 = Execute (x)
• 0 = No permission

Breaking down "755":

• 7 (4+2+1): The Owner can Read, Write, and Execute. (Full control)
• 5 (4+0+1): The Group can Read and Execute, but cannot Write (cannot edit or delete).
• 5 (4+0+1): Others can Read and Execute, but cannot Write.

What does this look like in ls -l?

If you apply chmod 755 to a file, the permission string will look like this:
-rwxr-xr-x

When is it used?

• For Directories: 755 is the standard permission. It allows the owner to manage files inside, while allowing everyone else to enter the directory and see the files.
• For Scripts: It is commonly used for scripts (like a .sh file) that you want everyone to be able to run, but only you to be able to edit.

Quick Tip: If you wanted a file to be private so only you could see it, you would use chmod 700!