Shebang and Script Execution

The shebang line is the first and most important line in any script. It tells the operating system how to execute your script.

What is a Shebang?

The shebang (also called hashbang) is a special character sequence at the very beginning of a script:

#!/bin/bash

It consists of:

• #! - The magic number that identifies this as a script
• /bin/bash - The path to the interpreter

Why is the Shebang Important?

Without a shebang, the system doesn't know which interpreter to use:

# With shebang - system knows to use bash
#!/bin/bash
echo "This runs with bash"

# Without shebang - behavior is undefined
echo "Which shell runs this?"

Common Shebang Lines

Shebang	Interpreter	Use Case
#!/bin/bash	Bash	Most common, bash-specific features
#!/bin/sh	POSIX shell	Maximum portability
#!/usr/bin/env bash	Bash via env	Flexible path lookup
#!/usr/bin/env python3	Python 3	Python scripts
#!/usr/bin/env node	Node.js	JavaScript scripts

The env Trick

Using #!/usr/bin/env bash instead of #!/bin/bash has advantages:

#!/usr/bin/env bash
# Finds bash wherever it's installed
# More portable across different systems

This is useful because:

• Bash might be at /bin/bash, /usr/bin/bash, or elsewhere
• env searches PATH to find the interpreter
• Works across different Unix/Linux distributions

Shebang Rules

Critical rules:

1. Must be the very first line (no blank lines before it)
2. Must start at column 1 (no spaces before #!)
3. Path must be absolute or use env

# WRONG - space before shebang
 #!/bin/bash

# WRONG - blank line before shebang

#!/bin/bash

# CORRECT
#!/bin/bash
# Rest of script follows

Exercise: Identify Shebang

Check what shebang the backup script uses:

How Script Execution Works

When you run a script, here's what happens:

# Step 1: You execute the script
./myscript.sh

# Step 2: System reads the shebang
#!/bin/bash

# Step 3: System actually runs:
/bin/bash ./myscript.sh

The shebang transforms your command into an interpreter call.

Ways to Execute Scripts

There are multiple ways to run a script:

# Method 1: Direct execution (requires execute permission)
./script.sh

# Method 2: Explicit interpreter (ignores shebang)
bash script.sh

# Method 3: Source (runs in current shell)
source script.sh
# or
. script.sh

Differences Between Methods

Method	New Shell	Inherits Variables	Needs +x
./script.sh	Yes	No	Yes
bash script.sh	Yes	No	No
source script.sh	No	Yes	No

Sourcing vs Executing

Executing creates a new shell process:

# script.sh
#!/bin/bash
MY_VAR="set in script"

# Running it
./script.sh
echo $MY_VAR  # Empty! Variable was set in child shell

Sourcing runs in the current shell:

source script.sh
echo $MY_VAR  # "set in script" - variable persists!

When to Use Each Method

Scenario	Recommended Method
Normal script execution	./script.sh
Testing without permissions	bash script.sh
Setting environment variables	source script.sh
Loading functions/aliases	source ~/.bashrc

Exercise: Execute a Script

Create and execute a simple script:

Key Takeaways

• The shebang (#!/bin/bash) must be the first line of every script
• It tells the system which interpreter to use
• Use #!/usr/bin/env bash for maximum portability
• ./script.sh executes in a new shell (requires +x permission)
• bash script.sh executes in a new shell (no permission needed)
• source script.sh runs in the current shell (variables persist)