Permissions

Every file has an owner, a group, and three sets of permissions. This is Linux's security model, and it's simpler than it looks.

Reading ls -l

ls -l notes.txt

-rw-r--r-- 1 paul paul 12 Oct 12 22:57 notes.txt

Breaking down -rw-r--r--:

Part	Meaning
-	File type (- file, d directory, l link)
rw-	Owner (paul) can read and write
r--	Group (paul) can only read
r--	Everyone else can only read

The 1 paul paul after it: link count, owner, group. Then size, date, name.

For a directory, x means "allowed to enter / list":

drwxr-xr-x 2 paul paul 4096 Oct 12 22:57 docs

Changing permissions

Permissions are also written as numbers — each of read/write/execute is worth 4/2/1:

Number	Letters	Meaning
7	rwx	read + write + execute
6	rw-	read + write
5	r-x	read + execute
4	r--	read only
0	---	nothing

chmod 600 notes.txt   # owner read/write, nobody else anything (private)
chmod 644 notes.txt   # owner read/write, everyone else read (normal file)
chmod 755 script.sh   # owner everything, everyone else read+execute (programs)
chmod +x script.sh    # add execute permission (same as the x in 755)

Check: chmod 600 notes.txt && ls -l notes.txt shows -rw------- as the permission string.

The 90% rule: files are 644, directories and programs are 755, secrets are 600. If you learn nothing else, learn that.

Changing ownership

chown paul:paul notes.txt   # change owner and group
chgrp staff notes.txt        # change just the group

You generally only need these when files were created by someone (or something) else — like files unpacked as root.

sudo: borrowing superpowers

Some things need the superuser (root): installing software, editing system files, managing services. Instead of logging in as root (dangerous — no guardrails), you borrow root's power for one command:

sudo apt update

sudo asks for your password (not root's), runs the one command as root, then drops the power. It remembers for a few minutes, so you won't be asked every time in a burst of admin work.

Careful: With sudo, the guardrails come off. sudo rm -rf / would genuinely try to erase the system. Pause before every sudo command and read it once more. If a tutorial tells you to curl something | sudo bash (download a script and run it as root), understand what the script does first — you're handing a stranger the keys.

Check: sudo whoami prints root — proof you borrowed the power — while plain whoami still prints your name.

Next: Finding Things