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Permissions (Just Enough)

Permissions control who can do what with files and directories.

You do not need to become a sysadmin or security expert. For most development work, you just need a gentle understanding of:

  • Read / write / execute
  • Basic ls -l output
  • The idea of chmod
  • Why sudo exists and when to be careful

Read, write, execute (r, w, x)​

In Unix-like systems, each file or directory has three basic kinds of permission:

  • Read (r) — can view the contents
  • Write (w) — can modify or delete
  • Execute (x) — can run (for programs) or access (for directories)

Permissions are defined separately for:

  • Owner — usually the user who created the file
  • Group — a set of users
  • Others — everyone else

Seeing permissions with ls -l​

Use ls -l to view permission details:

ls -l

Example output:

-rw-r--r--  1 alice  staff   1024 Dec  1 09:30 notes.txt
drwxr-xr-x  2 alice  staff     64 Dec  1 10:00 scripts

Break down the first column:

  • -rw-r--r--

Position by position:

  • 1st char:
    • - = regular file
    • d = directory
  • Next 3 (rw-) → permissions for owner
  • Next 3 (r--) → permissions for group
  • Last 3 (r--) → permissions for others

So for notes.txt:

  • Owner: rw- → can read and write
  • Group: r-- → can only read
  • Others: r-- → can only read

For scripts:

  • drwxr-xr-x
  • It’s a directory (d)
  • Owner: rwx → can read, write, and execute
  • Group: r-x → can read and execute
  • Others: r-x → can read and execute

Note: “Execute” on a directory means:

  • You can enter it (cd scripts)
  • You can access files by name inside it

chmod: Changing permissions (conceptual)​

chmod (change mode) changes file permissions.

You’ll most often see it used to make a file executable:

chmod +x script.sh

This:

  • Adds (+) execute permission (x)
  • For owner, group, and others (by default)

Now you can run:

./script.sh

Common patterns:

# Make a script executable only for the owner
chmod u+x script.sh

# Remove write permission for others
chmod o-w file.txt

As a beginner, you don’t need to memorize all the combinations. Just recognize:

  • chmod is how you change permissions
  • ls -l is how you inspect permissions

sudo: Superuser do (handle with care)​

sudo lets you run a command with elevated privileges (usually as the root user).

sudo apt update
sudo apt install python3

This is powerful and potentially dangerous. With sudo you can:

  • Install system packages
  • Change system configuration
  • Delete critical files

Guidelines:

  • Use sudo only when:
    • You understand what the command is doing
    • It genuinely needs system-level access
  • Avoid sudo for:
    • Day-to-day project work in your home directory
    • Random commands from the internet that you don’t understand

If you see a tutorial that prepends every command with sudo, be cautious.

When sudo makes sense​

Examples where sudo is normally appropriate:

  • Installing system packages: 
    sudo apt install git
  • Editing system config: 
    sudo nano /etc/hosts

Examples where you usually don’t want sudo:

  • Installing Python packages for a project: 
    # Better: use a virtual environment, not sudo
    pip install requests
  • Running development servers in your home directory: 
    python manage.py runserver

Connecting permissions to errors​

When you see:

Permission denied

Ask:

  1. What file or directory is it complaining about?
  2. Does ls -l show that:
    • You don’t have write permission?
    • You don’t have execute permission on a directory?
  3. Should this really be a system-level operation?
  4. Is the fix:
    • Change file ownership/permissions (advanced), or
    • Use sudo intentionally, or
    • Simply work in your home directory instead?

Often, the safest and correct solution is:

  • “Don’t put your project in a protected system directory.”

Windows notes​

On Windows:

  • NTFS permissions are different under the hood
  • PowerShell has its own command set for permissions

However, if you use WSL (recommended for this guide), you’ll see the same Unix-style permissions model (ls -l, chmod, etc.) inside your Linux environment.

Summary​

  • Permissions answer: who can do what with a file or directory.
  • ls -l shows you:
    • File type (- or d)
    • Read, write, execute bits for owner, group, and others
  • chmod changes permissions; most commonly you’ll see it to make scripts executable.
  • sudo runs commands with elevated privileges—powerful but risky; use it deliberately, not as a habit.
  • When in doubt, work in your home directory and keep project files there; reserve system-level changes for intentional, well-understood operations.