02-shell-scripting-essential.md

Shell Scripting Essentials

Topics Covered

• Start a script: #!/bin/bash
• Variables and Quotes
• read, select for user input
• Conditional statements: if, else, elif
• Test expressions: [ ], [[ ]], test command

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Key Notes

• #!/bin/bash (called shebang) tells the system to use the bash shell to execute the script.
• Variables are assigned using name=value (no spaces). You access them with $name.
• Quotes:
  • Double quotes " " allow variable expansion.
  • Single quotes ' ' treat everything literally.
• read gets user input.
• select creates simple menus in bash scripts.
• [ ], [[ ]], and test are used to evaluate conditions:
  • [ $a -eq $b ], [[ $a == $b ]], or test $a -eq $b

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Commands Practiced

# Start a basic script
#!/bin/bash
echo "Hello, Linux!"

# Set and print a variable
username="stark"
echo "Hi, $username"

# User input using read
read -p "Enter your age: " age
echo "You are $age years old"

# Conditional if-else
if [ $age -ge 18 ]; then
  echo "Adult"
elif [ $age -eq 17 ]; then
  echo "Almost there!"
else
  echo "Minor"
fi

# Using test command
if test $age -gt 0; then
  echo "Valid age"
fi

# Using select to create a simple menu
select option in Start Stop Exit; do
  echo "You selected $option"
  break
done

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