Day 29: Stop Writing Linux Scripts Until You Understand Shebang (PATH, Executable Files, and More)

Understanding Shebang, Executable Files, PATH, and How Linux Knows What to Run

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Welcome to Day 29 of the Linux Learning Series.

In ***Day 28,*** we learned about the Linux shell, terminal, kernel, and wrote our first shell scripts. We also learned how to make scripts executable and integrate them with .bashrc.

With that, the basic shell scripting section is now complete.

From this point onward, we are moving into more advanced Linux topics. These topics will help you understand how Linux works behind the scenes and make you more confident as a Linux administrator, developer, or DevOps engineer.

Today, we will learn about one of the most common but often misunderstood concepts in Linux: the Shebang.

Many beginners use it without fully understanding why it exists. By the end of this article, you will know exactly what a shebang does, why it is important, and how Linux uses it when running scripts.

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What is a Shebang?

A shebang is the first line of a script file.

It starts with two special characters:

#!

followed by the path of the interpreter that should execute the script.

Example:

#!/bin/bash

This tells Linux:

“Run this script using the Bash interpreter.”

Without a shebang, Linux may not know which program should interpret the script.

Why is the Shebang Needed?

Imagine you have a script called:

backup.sh

Contents:

echo "Starting backup..."

You make it executable:

chmod +x backup.sh

Now you run:

./backup.sh

How does Linux know whether this file should be executed by:

• Bash
• Python
• Perl
• Ruby
• Another interpreter

The answer is the shebang.

Linux reads the first line and launches the specified interpreter automatically.

Example:

#!/bin/bash

Linux executes:

/bin/bash backup.sh

behind the scenes.

Components of the Shebang

Consider:

#!/bin/bash

It contains two parts.

Part 1: #!

This special combination is called the shebang.

It tells Linux that this file should be processed by an interpreter.

Part 2: Interpreter Path

/bin/bash

This is the full path of the program that should execute the file.

Together:

#!/bin/bash

means:

Use Bash to execute this script.

Understanding Executable Files

Many new Linux users believe every executable file is a compiled program.

This is not true.

Linux can execute:

• Binary programs
• Shell scripts
• Python scripts
• Perl scripts
• Other script files

Example:

file myscript.sh

Output:

ASCII text executable

Even though it is only a text file, Linux can execute it because:

• It has execute permission
• It contains a valid shebang

A Python Example

Let’s create a Python script.

#!/usr/bin/python3

print("Hello Linux")

Make it executable:

chmod +x hello.py

Run:

./hello.py

Output:

Hello Linux

Notice that we did not run:

python3 hello.py

The shebang handled that automatically.

Linux read:

#!/usr/bin/python3

and launched Python for us.

Using a Shebang in Linux

The most common Bash shebang is:

#!/bin/bash

Example:

#!/bin/bash

echo "Welcome to Linux"

Execution:

chmod +x script.sh
./script.sh

Output:

Welcome to Linux

This is the standard approach used in many shell scripts.

A Common Misconception

Many people think:

#!/bin/bash

is mandatory.

It is not.

If you execute the script like this:

bash script.sh

Bash already knows it must run the file.

The shebang becomes important when you run:

./script.sh

because Linux needs to know which interpreter should be used.

Using env

You will often see:

#!/usr/bin/env bash

instead of:

#!/bin/bash

Why?

Because different Linux systems may install Bash in different locations.

Using:

#!/usr/bin/env bash

tells Linux:

Find Bash from the current environment and use it.

This makes scripts more portable.

Example:

#!/usr/bin/env python3

instead of:

#!/usr/bin/python3

This is very common in modern Python projects.

How env Works

Let’s check where Bash exists.

which bash

Output:

/usr/bin/bash

or

/bin/bash

depending on the system.

When Linux sees:

#!/usr/bin/env bash

it first runs:

/usr/bin/env

Then env searches the PATH variable and finds Bash.

This makes the script work across different systems.

Executable Path and Shell Commands

Consider the command:

ls

You do not type:

/bin/ls

every time.

Why?

Because Linux searches directories listed in the PATH variable.

View PATH:

echo $PATH

Example output:

/usr/local/bin:/usr/bin:/bin

Linux searches these directories from left to right.

When it finds the command, it executes it.

Adding Your File to a PATH Location

Suppose you create a script:

hello.sh

Contents:

#!/bin/bash
echo "Hello World"

Make it executable:

chmod +x hello.sh

Move it to:

/usr/local/bin

sudo mv hello.sh /usr/local/bin

Now run:

hello.sh

from anywhere.

Because /usr/local/bin is usually part of PATH, Linux can find it automatically.

Adding a Folder to the PATH List

Instead of moving scripts, you can add your own folder to PATH.

Create a directory:

mkdir ~/scripts

Place your scripts there.

Check the current PATH:

echo $PATH

Add your folder temporarily:

export PATH=$PATH:~/scripts

Now Linux can locate scripts stored in that folder.

For a permanent change, add the line to:

~/.bashrc

or

~/.profile

Example:

export PATH=$PATH:$HOME/scripts

Reload:

source ~/.bashrc

Now the change remains available in future sessions.

Practical Example

Create:

~/scripts/sysinfo

Content:

#!/bin/bash

echo "Current User: $(whoami)"
echo "Hostname: $(hostname)"

Make it executable:

chmod +x ~/scripts/sysinfo

Add the directory to PATH.

Now simply run:

sysinfo

Output:

Current User: pawan
Hostname: linux-server

No need to type the full path.

Some Other Uses of Shebang

Shebang is not limited to Bash.

Python

#!/usr/bin/env python3

Perl

#!/usr/bin/perl

Ruby

#!/usr/bin/ruby

PHP

#!/usr/bin/php

The same concept works across many scripting languages.

Possible Malicious Use

Like many Linux features, shebangs can be used correctly or abused.

Suppose someone creates a script:

#!/bin/bash
rm -rf important_directory

and names it:

update.sh

A user might execute it without checking its contents.

This is why Linux administrators should always inspect unknown scripts before running them.

Useful commands:

cat script.sh

less script.sh

head script.sh

Before making a script executable or running it, review the contents carefully.

The shebang itself is not dangerous. The danger comes from what the script instructs the interpreter to do.

Key Takeaways

• A shebang starts with #!.
• It tells Linux which interpreter should execute a script.
• Common Bash shebang:

#!/bin/bash

• Portable version:

#!/usr/bin/env bash

• Scripts need execute permission to run directly.
• PATH allows Linux to locate commands automatically.
• You can add your own script directories to PATH.
• Shebangs work with Bash, Python, Perl, Ruby, PHP, and many other interpreters.
• Always inspect unknown scripts before executing them.

In the next article, we will continue exploring advanced Linux concepts and learn more about how Linux executes commands and manages processes behind the scenes.