Chapter 5

File Permissions

Linux uses a powerful permissions system to control who can read, write, or execute a file. Understanding this is essential for security and collaboration.

Introduction

Every file and folder has an owner and a group.
Each has permissions for the owner, the group, and others.
Permissions control reading, modifying, and executing files.

Reading permissions with `ls -l`

Use ls -l to see permissions. Example line:

-rwxr-xr-- 1 user group 4096 Jan 1 10:00 script.sh

The first 10 characters describe the file type and permissions.

Permission breakdown

The 10-character block is split into file type + 3 permission sets:

Part	Meaning
First character	`-` for file, `d` for directory, `l` for symlink, etc.
Next 3 (rwx)	Permissions for the owner (read, write, execute).
Next 3 (r-x)	Permissions for the group.
Last 3 (r--)	Permissions for others (everyone else).

Permission letters

Each position in rwx can be present or replaced with -:

r (read) – view file contents or list a directory.
w (write) – modify a file or create/delete inside a directory.
x (execute) – run a file as a program or enter a directory.

Changing permissions with `chmod`

You can change permissions using symbolic or numeric modes:

Symbolic: chmod u+x script.sh (add execute for the owner).
Symbolic: chmod g-w file.txt (remove write for group).
Numeric: chmod 755 script.sh (rwx for owner, rx for group and others).
Numeric: chmod 644 notes.txt (rw for owner, r for group and others).

Numeric permission values

Each permission is represented by a number and added together:

Permission set	Value
r (read)	4
w (write)	2
x (execute)	1
rwx	4 + 2 + 1 = 7
rw-	4 + 2 = 6
r-x	4 + 1 = 5
r--	4

Changing owner and group

Sometimes you change who owns a file or which group it belongs to:

chown user file – change file owner (needs sudo).
chown user:group file – change owner and group.
chgrp group file – change only the group.

Practice questions

Question 1

Run ls -l in your home directory and copy one example permission string (like -rw-r--r--). Explain what each part means.

Hint: Focus on owner, group, others and r/w/x flags.

Question 2

Write the exact chmod command you would use to make a script run_backup.sh readable and executable by everyone, but only writable by the owner.

Hint: Think about numeric mode: owner = rwx, group = rx, others = rx.

Quiz

Test your understanding. Click an answer to see if it's correct.

1. In the permission string -rwxr-xr--, what does the x in the first group (rwx) mean?

The group can execute the file.
The owner can execute the file.
Others can execute the file.
No one can execute the file.

The first rwx block refers to the owner of the file.

2. Which numeric permission value corresponds to rwxr-xr-x?

Owner: 7 (rwx), group: 5 (r-x), others: 5 (r-x).

3. Which command adds execute permission for the owner of script.sh without changing anything else?

chmod +x script.sh
chmod u+x script.sh
chmod g+x script.sh
chmod 000 script.sh

u stands for user (owner); +x adds execute permission.

4. What does the w permission mean on a directory?

You can only read files inside it.
You can delete and create files inside that directory.
You can execute all files in it.
You can rename the operating system.

Write permission on a directory means you can modify its contents.

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