Introduction

The Confusion of File Permissions

If you've ever deployed a website, managed a Linux server, or written a bash script, you've inevitably run into a "Permission Denied" error.

The immediate, panicky reaction for many developers is to run:

chmod 777 myfile.txt

Do not do this. Setting permissions to 777 means literally anyone with access to the system can read, modify, or execute your file. If that file is an executable script or a configuration file containing database passwords, you have just created a massive security vulnerability.

How Chmod Works

Linux file permissions are divided into three groups of users:

1. Owner (u): The user who created the file.
2. Group (g): Users who are part of the file's assigned group.
3. Public/Others (o): Everyone else on the system.

For each of these groups, there are three types of permissions:

• Read (r): View the file contents (Value: 4)
• Write (w): Modify or delete the file (Value: 2)
• Execute (x): Run the file as a script/program (Value: 1)

Octal vs Symbolic Notation

Octal (Numeric)

By adding the values together, you get a single digit between 0 and 7 for each group.

• Read + Write = 4 + 2 = 6
• Read + Execute = 4 + 1 = 5
• Read + Write + Execute = 4 + 2 + 1 = 7

So, chmod 644 means:

• Owner gets 6 (Read/Write)
• Group gets 4 (Read Only)
• Public gets 4 (Read Only)

Symbolic

Symbolic notation uses letters. To set the exact same permissions as 644, you would write:

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

Standard Permission Presets

• 644: Standard for files (Owner can read/write, everyone else can read).
• 755: Standard for directories and executable scripts (Owner can read/write/execute, everyone else can read/execute).
• 600: Private files, like SSH keys (Only the owner can read/write).
• 400: Read-only private files.