📜 Bash Script Header & Structure – Understanding #!/bin/bash and Script Layout

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🧲 Introduction to Bash Script Headers – What #!/bin/bash Really Means

Every Bash script begins with a shebang line like #!/bin/bash. This line is more than a comment—it’s an instruction to the operating system about which interpreter should execute the script.

Understanding the structure and conventions of a Bash script ensures your code runs predictably, is portable, and is easier to maintain.

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🎯 In this article, you’ll learn:

• What the #!/bin/bash shebang line does
• How to structure a Bash script properly
• Key conventions for comments, variables, logic blocks
• Example layout for clean and professional Bash scripts

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🚀 What Is #!/bin/bash?

The line #!/bin/bash at the top of a script is called a shebang (or hashbang). It tells the OS to use the Bash interpreter to run the script.

✅ Example:

#!/bin/bash
echo "This is a Bash script"

When you run the script with ./script.sh, the OS sees the shebang and invokes /bin/bash to interpret the code.

💡 Without the shebang, your script may run under /bin/sh or another shell depending on the system default—potentially breaking Bash-specific features.

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🏗️ Recommended Bash Script Structure

Here’s a clean and consistent way to structure your Bash scripts:

#!/bin/bash
# -------------------------------------
# Script Name: myscript.sh
# Description: Does something useful
# Author: Your Name
# Date: 2025-06-14
# -------------------------------------

# ✅ Global Variables
LOG_FILE="/var/log/myscript.log"

# ✅ Functions
log_message() {
  echo "$(date) - $1" >> "$LOG_FILE"
}

# ✅ Main Script Execution
echo "Starting script..."
log_message "Script started"
# Your logic here
log_message "Script ended"
echo "Done."

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🔍 Script Sections Breakdown

Section	Purpose
#!/bin/bash	Specifies Bash interpreter
Comments	Describes script purpose and authorship
Variables	Declares constants, file paths, counters
Functions	Encapsulates reusable logic
Main Body	Core logic flow with function calls and logic

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🧪 Example: Simple Backup Script

#!/bin/bash
# Backup home directory

backup_dir="/backup"
src_dir="/home/$USER"

mkdir -p "$backup_dir"
tar -czf "$backup_dir/home_backup_$(date +%F).tar.gz" "$src_dir"

echo "Backup completed."

✅ Output:

Backup completed.

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🛠️ Making the Script Executable

Step-by-Step:

1. Create a file: nano myscript.sh
2. Add shebang & content, save and exit
3. Make it executable: chmod +x myscript.sh
4. Run it: ./myscript.sh

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⚠️ Shebang Gotchas

Mistake	Fix
Using #!/bin/sh for Bash script	Use #!/bin/bash for Bash-specific syntax
Forgetting shebang	Script may run in wrong shell or not run at all
Adding spaces in shebang	Don’t add spaces: use #!/bin/bash, not #! /bin/bash

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📌 Summary – Bash Script Header & Structure

The #!/bin/bash line is essential for executing scripts reliably in the correct shell. Combine it with a consistent structure—comments, variables, functions, and main logic—for clean, professional, and maintainable Bash scripts.

🔍 Key Takeaways:

• #!/bin/bash sets the script interpreter to Bash
• Structure your script into header, variables, functions, and logic
• Add metadata as comments for clarity and documentation
• Use chmod +x to make scripts executable

⚙️ Real-world Uses:

• Creating startup scripts for automation
• Writing log-rotators, backup tools, or custom CLI utilities
• Packaging scripts as cron jobs or system services

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❓ FAQ – Bash Script Header & Structure

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❓ Is #!/bin/bash mandatory in Bash scripts?
✅ Not strictly, but it ensures your script runs under Bash even if another shell is the system default.

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❓ Can I use another shell instead of /bin/bash?
✅ Yes. You can use /bin/sh, /usr/bin/env bash, or /bin/zsh depending on your needs.

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❓ What happens if I skip the shebang?
✅ If you run with bash script.sh, it works fine. But if you run ./script.sh, the system may default to another shell.

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❓ Where should I define functions in my script?
✅ Define them near the top—after variables and before the main logic—for clarity and reuse.

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❓ How do I run a Bash script line by line for debugging?
✅ Use:

bash -x script.sh

This enables trace mode and shows commands as they execute.

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