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Linux File Permissions Explained for Complete Beginners

Understanding Why chmod +x Was Not Enough​

1. Introduction

In Linux, every file and directory has a set of permissions that determine:

  • Who can read the file
  • Who can modify the file
  • Who can execute the file

Understanding permissions is one of the most important skills for every Linux administrator.

In this lesson, we will learn:

  • What file permissions are
  • How to read permissions
  • How chmod works
  • Why a shell script needs both read and execute permissions
  • Why chmod +x alone did not work

2. The Scenario

Suppose we have a shell script:

/tmp/xfusioncorp.sh

Let's check its permissions:

ls -l /tmp/xfusioncorp.sh

Output:

---------- 1 root root 40 Jun 22 18:23 /tmp/xfusioncorp.sh

3. Understanding ls -l

The command:

ls -l

means:

  • ls → List files
  • -l → Show detailed information (long format)

Example:

-rwxr-xr-x 1 root root 40 Jun 22 18:23 file.sh

Every part has a meaning.

4. Breaking Down File Permissions

Example:

-rwxr-xr-x

Let's divide it:

- rwx r-x r-x
│ │   │   │
│ │   │   └── Others
│ │   └────── Group
│ └────────── Owner
└──────────── File type

5. The First Character

The first character tells us the file type.

SymbolMeaning
-Regular file
dDirectory
lSymbolic link
cCharacter device
bBlock device

Example:

-rwxr-xr-x
^

The - means it is a regular file.

6. The Permission Groups

Linux divides permissions into three groups:

Owner (User)​

The person who owns the file.

Group​

Users that belong to the file's group.

Others​

Everyone else on the system.

Example:

-rwxr-xr-x
 └┬┘└┬┘└┬┘
  │   │   │
Owner Group Others

7. The Three Permission Types

Linux has three permissions.

Read (r)​

Value:

4

Allows:

  • Open a file
  • View its contents

Example:

cat file.txt
less file.txt
nano file.txt

Write (w)​

Value:

2

Allows:

  • Modify the file
  • Delete its contents
  • Add new content

Example:

echo "hello" >> file.txt
nano file.txt

Execute (x)​

Value:

1

Allows:

  • Run a program
  • Execute a script

Example:

./script.sh
./program

8. Numeric Permission Values

PermissionValue
r4
w2
x1

We add the numbers.

Example​

rwx​

4 + 2 + 1 = 7

rw-​

4 + 2 = 6

r-x​

4 + 1 = 5

r--​

4 = 4

--x​

1 = 1

9. Understanding chmod

chmod means:

Change Mode

It changes file permissions.

Syntax:

chmod permissions file

10. Symbolic Method

Add execute permission​

chmod +x file

Add read permission​

chmod +r file

Add write permission​

chmod +w file

Remove execute permission​

chmod -x file

11. Who Gets the Permission?

SymbolMeaning
uUser (Owner)
gGroup
oOthers
aAll

Examples:

Owner only​

chmod u+x file

Group only​

chmod g+x file

Others only​

chmod o+x file

Everyone​

chmod a+x file

12. Our File's Initial State

---------- /tmp/xfusioncorp.sh

This means:

Owner  : ---
Group  : ---
Others : ---

Nobody can do anything.

13. Giving Execute Permission

We ran:

sudo chmod a+x /tmp/xfusioncorp.sh

Permissions became:

---x--x--x

This means:

Owner  : execute
Group  : execute
Others : execute

14. Trying to Execute the Script

./xfusioncorp.sh

Error:

/bin/bash: ./xfusioncorp.sh: Permission denied

Why?

15. Understanding How Shell Scripts Work

Suppose the script contains:

#!/bin/bash

echo "Welcome To KodeKloud"

The first line:

#!/bin/bash

is called the:

Shebang

It tells Linux:

"Run this file using /bin/bash."

16. What Happens Internally?

When you run:

./xfusioncorp.sh

Linux does this:

Step 1​

Kernel sees:

#!/bin/bash

Step 2​

Kernel starts:

/bin/bash

Step 3​

Bash tries to read:

xfusioncorp.sh

Step 4​

Bash cannot read the file.

Step 5​

Bash shows:

Permission denied

17. Why Can't Bash Read It?

Because permissions are:

---x--x--x

There is:

No read permission.

Bash needs:

Read permission.

18. The Fix

Give everyone:

  • Read permission
  • Execute permission
sudo chmod a+rx /tmp/xfusioncorp.sh

or

sudo chmod 555 /tmp/xfusioncorp.sh

19. Final Permissions

-r-xr-xr-x

Breaking it down:

Owner  : r-x
Group  : r-x
Others : r-x

Everyone can:

  • Read
  • Execute

Nobody can:

  • Modify

20. Why Does This Work?

Now the sequence is:

Run script
      ↓
Kernel starts Bash
      ↓
Bash reads file
      ↓
Bash executes commands
      ↓
Script runs successfully

21. Verification

./xfusioncorp.sh

Output:

Welcome To KodeKloud

22. Important Concept

Compiled Program​

Example:

./firefox
./nginx

Usually only needs:

--x

because the kernel executes the binary directly.

Shell Script​

Example:

./script.sh
./backup.sh

Needs:

r-x

because the interpreter must read the file.

23. Commands Summary

View permissions​

ls -l file

Add execute permission​

chmod a+x file

Add read and execute permission​

chmod a+rx file

Set permissions numerically​

chmod 555 file

24. Final Takeaway

Remember these two rules:

Binary executable​

Needs: x

Script file​

Needs: r + x

This small difference is one of the most important concepts in Linux permissions and often appears in system administration interviews and certification exams.