Mastering Different Ways to Run Bash Scripts

Understanding the various methods to execute Bash scripts is crucial for effective shell scripting. Each approach has its specific use cases, permissions requirements, and behavioral differences. Let’s explore the most common and practical ways to run your scripts.

The Standard Method: Execution Permission

The most common and recommended approach involves two simple steps:

Step 1: Set Execution Permissions

chmod +x scriptname.sh

Step 2: Execute Using Relative or Absolute Path

Relative path (when in the same directory):
```
./scriptname.sh
```
Absolute path (from anywhere):
```
/home/user/scripts/scriptname.sh
```

Important Note About PATH

By default, running just the script name (scriptname.sh) will result in a “command not found” error. This happens because your shell doesn’t search the current directory for executables. You have two solutions:

Use ./ prefix (recommended for security)
Add script directory to PATH (for frequently used scripts)

Method 2: Explicit Interpreter Call

You can bypass execution permissions by explicitly calling the interpreter:

bash scriptname.sh
python3 scriptname.sh

Key Characteristics:

No execution permission required
Overrides shebang directive - the interpreter you specify takes precedence
Useful for testing different interpreters or debugging

Example:

# Remove execution permission
chmod -x scriptname.sh

# Still executable via explicit interpreter
bash scriptname.sh

Method 3: Source Command

The source command (or its shorthand .) executes scripts in the current shell environment:

source scriptname.sh
# OR
. scriptname.sh

Critical Differences from Other Methods:

Aspect	`./script.sh`	`source script.sh`
Execution Environment	New sub-shell	Current shell
Permissions Required	Yes	No
Variable Scope	Isolated	Shared with current session
Exit Behavior	Returns to parent shell	Stays in current shell

Practical Implications:

Environment setup: Use source for scripts that modify environment variables
Function definitions: Functions defined in sourced scripts become available in current session
Configuration files: .bashrc, .profile are sourced, not executed

Choosing the Right Method

When to Use Each Approach:

Use ./script.sh (with execution permission):

General-purpose scripts
Standalone applications
When you want isolation from current shell
This is the recommended default method

Use bash script.sh:

Testing scripts without changing permissions
Debugging or development
When you need to override the shebang

Use source script.sh:

Loading environment variables
Defining functions for current session
Configuration scripts
When you need the script to affect your current shell

Security Considerations

Always prefer ./script.sh over adding . to your PATH. This prevents:

Accidental execution of malicious scripts
Confusion about which script is running
Security vulnerabilities from unknown script locations

Summary of Best Practices

Default choice: Use chmod +x and ./script.sh
For environment changes: Use source or . script.sh
For testing: Use bash script.sh without changing permissions
Avoid: Adding current directory to PATH for security reasons

Section 1 Complete! 🎉

Congratulations on completing the first section of your Bash scripting journey! You’ve learned:

✅ The difference between shells and scripts
✅ How to create your first shell script
✅ Understanding and using shebangs
✅ Adding comments for clarity
✅ Multiple ways to execute scripts

What’s Next?

You’re now ready to dive deeper into more advanced topics:

Variables and parameters
Shell expansions
User input handling
And much more!

These skills will transform you from a beginner into a proficient Bash scripter. Keep practicing, and I’ll see you in the next section!