Permissions and sudo

Welcome back.

In the previous lesson, you learned how to read files and search inside them.

Now we enter the part where Linux starts saying:

“Permission denied.”

Do not take it personally.

Linux is not angry.

Linux is just protecting the system from chaos, mistakes, and sleepy fingers with administrator power.

What You’ll Learn

In this lesson, you’ll learn:

• what file permissions are;
• how to read permission symbols like rwx;
• what users, groups, and others mean;
• how to check your current user with whoami;
• how to use chmod;
• what sudo does;
• why you should not use sudo like hot sauce.

The Mission

Your mission is simple:

Understand why some files can be read, changed, or executed — and why some files politely refuse your requests like a serious security guard at the door.

Check Who You Are

First, check your current user:

whoami

You may see something like:

viktor

This means you are working as the user viktor.

You are not root.

That is good.

Being root all the time is like driving a tank to buy bread.

Possible, but unnecessary and slightly worrying.

Create a Practice File

Go to your practice folder:

cd ~/terminal-practice

If it does not exist, create it:

mkdir -p ~/terminal-practice
cd ~/terminal-practice

Create a file:

touch permissions-demo.txt

Now list it with details:

ls -l permissions-demo.txt

You may see something like:

-rw-r--r-- 1 viktor viktor 0 May 2 10:00 permissions-demo.txt

This looks scary at first.

But it is not magic.

It is just information wearing a serious jacket.

Understanding Permission Symbols

Look at this part:

-rw-r--r--

The first character tells the file type:

- means regular file
d means directory

Then permissions are grouped like this:

rw- r-- r--

These groups mean:

user  group  others

So:

rw-   the owner can read and write
r--   the group can only read
r--   others can only read

Linux is basically saying:

“The owner can edit this. Everyone else can look, but keep their hands in their pockets.”

What r, w, and x Mean

Permissions use three letters:

r = read
w = write
x = execute

For files:

• r means you can read the file;
• w means you can change the file;
• x means you can run the file as a program or script.

For directories:

• r means you can list the directory;
• w means you can create or delete files inside;
• x means you can enter the directory.

Yes, x on a directory means “enter”.

Linux likes to keep life interesting.

Add Text to the File

Write something into the file:

echo "Permissions are important." > permissions-demo.txt

Read it:

cat permissions-demo.txt

You should see:

Permissions are important.

So far, everything works.

The file belongs to you, and you can write to it.

Remove Write Permission

Now remove your own write permission:

chmod u-w permissions-demo.txt

Check:

ls -l permissions-demo.txt

You may see:

-r--r--r-- 1 viktor viktor 28 May 2 10:00 permissions-demo.txt

Now try to write to it again:

echo "Trying to write again." > permissions-demo.txt

You may get:

Permission denied

Good.

Linux just protected the file from changes.

Even from you.

A little rude, but technically correct.

Add Write Permission Back

Give yourself write permission again:

chmod u+w permissions-demo.txt

Now try:

echo "Writing works again." > permissions-demo.txt
cat permissions-demo.txt

You should see:

Writing works again.

Congratulations.

You removed a permission, broke your own access, and fixed it.

This is basically Linux training in miniature.

Making a Script Executable

Create a small script:

echo 'echo "Hello from my script!"' > hello.sh

Try to run it:

./hello.sh

You may see:

Permission denied

Why?

Because the file is not executable yet.

Add execute permission:

chmod u+x hello.sh

Now run it:

./hello.sh

You should see:

Hello from my script!

That is what x does.

It tells Linux:

“This file is allowed to run.”

Numeric Permissions

You may see commands like this:

chmod 755 hello.sh

This is numeric permission style.

The numbers mean:

7 = read + write + execute
5 = read + execute
5 = read + execute

So 755 means:

owner can read, write, execute
group can read and execute
others can read and execute

Another common one:

chmod 644 permissions-demo.txt

This means:

owner can read and write
group can read
others can read

For now, remember:

• 644 is common for normal files;
• 755 is common for scripts and directories.

Do not memorize everything today.

Your brain is not a USB stick.

What Is sudo?

sudo means “run this command with administrator privileges”.

Example:

sudo pacman -S htop

On Ubuntu or Debian:

sudo apt install htop

On Fedora:

sudo dnf install htop

sudo is used when a command needs higher privileges.

For example:

• installing software;
• changing system files;
• restarting services;
• editing protected configuration.

sudo is powerful.

Do not use it automatically.

Using sudo everywhere is like opening every door with a chainsaw.

Effective, but not elegant.

Common Permission Error

You may see:

Permission denied

This usually means:

• you do not have permission to read the file;
• you do not have permission to write to the file;
• you do not have permission to execute the file;
• you are trying to change something owned by another user or by root.

Before using sudo, ask:

Do I really need administrator power here?

Sometimes the answer is yes.

Sometimes the answer is: “No, Viktor, you are just in the wrong folder.”

Be Careful with sudo

This is dangerous:

sudo rm -r /some/system/folder

This can delete important system files.

Linux will do exactly what you ask.

Not what you meant.

That is the difference between a computer and a wise friend.

Practice

Try this:

cd ~/terminal-practice
touch permission-test.txt
ls -l permission-test.txt
chmod u-w permission-test.txt
ls -l permission-test.txt
chmod u+w permission-test.txt
ls -l permission-test.txt
echo 'echo "Script works!"' > test-script.sh
chmod u+x test-script.sh
./test-script.sh

Then answer:

1. What does whoami show?
2. What does chmod u-w do?
3. What does chmod u+w do?
4. What does chmod u+x do?
5. What does sudo allow you to do?

Mini Challenge

Create a script called my-info.sh.

It should print:

Hello from Linux!
Today I am learning permissions.

Then:

1. Create the file.
2. Try to run it.
3. Add execute permission.
4. Run it again.
5. Check permissions with ls -l.

Use only terminal commands.

No mouse.

The mouse has now filed a complaint with HR.

Summary

Today you learned:

• Linux uses permissions to protect files and directories;
• r means read;
• w means write;
• x means execute;
• permissions are divided into user, group, and others;
• ls -l shows permissions;
• chmod changes permissions;
• sudo runs commands with administrator privileges;
• sudo should be used carefully.

Permissions may look strange at first, but they are one of the reasons Linux is powerful and safe.

Linux says “no” not because it hates you.

It says “no” because sometimes you are one command away from creating a very educational disaster.

Next Lesson

In the next lesson, we’ll learn how to install software from the terminal using package managers.

That is where Linux becomes very convenient.

And where Arch Linux users begin to smile mysteriously.