chmod: Numeric Mode

Instead of letters, you can set permissions with a three-digit octal number. Each digit represents one group (user, group, others) and is the sum of:

Permission	Value
Read (r)	4
Write (w)	2
Execute (x)	1
None	0

Common permission numbers

Number	Meaning	String
`755`	owner: rwx, group: r-x, others: r-x	`-rwxr-xr-x`
`644`	owner: rw-, group: r--, others: r--	`-rw-r--r--`
`700`	owner: rwx, group: ---, others: ---	`-rwx------`
`600`	owner: rw-, group: ---, others: ---	`-rw-------`

Using chmod with numbers

chmod 755 script.sh
chmod 644 readme.txt

The numeric form sets all permissions at once: there is no "add" or "remove", you replace the entire set.

When to use which form?

Symbolic (u+x): when you want to change one thing without affecting the rest.
Numeric (755): when you know exactly what the final permissions should be.

Mission: Lock Down the Thruster Script

The engine_control.sh script fires the main thruster and ships with a readme.txt. Station security policy sets strict access levels for both.

Set the permissions on engine_control.sh so the owner has full access, the group can read and run it, and others get nothing at all.
Set the permissions on readme.txt so the owner can read and write it, while the group and others can only read it.
Create a folder called engine-bay and move both files into it.
Display the detailed listing of the folder to confirm the permissions.

Expected result

Inside engine-bay, engine_control.sh is -rwxr-x--- and readme.txt is -rw-r--r--.