Creating Your First Script

Shell scripts are text files containing commands that run automatically. They're essential for automation and repetitive tasks.

What is a Shell Script?

A shell script is:

• A text file with shell commands
• Executable like a program
• Interpreted line by line
• Used for automation

Your First Script

Create a file called hello.sh:

#!/bin/bash
echo "Hello, World!"

The Shebang Line

The first line #!/bin/bash is the shebang (or hashbang):

#!/bin/bash          # Use bash
#!/bin/sh            # Use sh (more portable)
#!/usr/bin/env bash  # Find bash in PATH
#!/usr/bin/python3   # Python script

It tells the system which interpreter to use.

Creating a Script

Step 1: Create the File

touch myscript.sh

Step 2: Add Content

cat > myscript.sh << 'EOF'
#!/bin/bash
echo "Script started"
date
echo "Script finished"
EOF

Step 3: Make Executable

chmod +x myscript.sh

Step 4: Run It

./myscript.sh

Exercise: View a Script

Look at an existing script to understand its structure:

Running Scripts

Multiple ways to run:

./script.sh           # If executable
bash script.sh        # Explicit interpreter
source script.sh      # In current shell
. script.sh           # Same as source

Script Structure

A typical script:

#!/bin/bash
# Description: What this script does
# Author: Your name
# Date: 2024-01-01

# Configuration
NAME="World"

# Main logic
echo "Hello, $NAME!"

# Exit with status
exit 0

Comments

# This is a single line comment

: '
This is a
multi-line comment
'

Exit Codes

Scripts return exit codes:

exit 0    # Success
exit 1    # General error
exit 2    # Misuse of command

Check last exit code:

echo $?   # 0 = success, non-zero = error

Script Example

#!/bin/bash
# Simple backup script

SOURCE="/home/user/documents"
DEST="/backup"
DATE=$(date +%Y%m%d)

echo "Starting backup..."
cp -r "$SOURCE" "$DEST/backup_$DATE"
echo "Backup complete!"

Debugging Scripts

bash -x script.sh     # Print each command
bash -n script.sh     # Check syntax only
set -x                # Enable debug in script
set +x                # Disable debug

Best Practices

1. Always include shebang: #!/bin/bash
2. Add comments: Explain what the script does
3. Use meaningful names: backup_database.sh not script1.sh
4. Handle errors: Check command success
5. Quote variables: "$VAR" not $VAR
6. Test before deploying: Use a safe environment

Key Takeaways

• Scripts start with shebang (#!/bin/bash)
• Make executable with chmod +x
• Run with ./script.sh or bash script.sh
• Use comments to document code
• Exit codes indicate success (0) or failure (non-zero)