Overview
This lecture explains how to use if, elif, and else statements in Bash scripting, with syntax, condition types, and practical examples.
Basics of Bash If Statements
- Bash if statements execute code blocks based on specified conditions.
- Basic syntax:
if [ condition ]; then ... fi
- If the condition is true, commands within the block run; otherwise, they're skipped.
Specifying Conditions in Bash
- Single Parentheses ( ): Runs commands in a subprocess and checks exit status for true (0) or false (non-zero).
- Double Parentheses (( )): Performs arithmetic evaluation; true if result ≠ 0, false if result = 0.
- Single Brackets [ ]: Used for test expressions (e.g., string or variable checks).
- Double Brackets [[ ]]: Enhanced test expressions; allows pattern matching and logical operators (&& for AND, || for OR).
Using Logical Operators and Patterns
- Double brackets allow complex expressions with regex patterns and logical operators.
- Example:
[[ "$str" =~ [abc]+[123]+-* ]] checks if a string matches a specific pattern.
- Logical AND (
&&) requires both conditions to be true; OR (||) requires at least one to be true.*
Bash If Else Statements
else block defines actions when the if condition is false.- Syntax:
if [ condition ]; then
statements
else
statements
fi
- Example: Validate username input and give feedback based on match.
Bash If Elif Else Statements
elif allows multiple sequential condition checks.- Syntax:
if [ condition ]; then
statements
elif [ condition ]; then
statements
else
statements
fi
- Used to check several possible values or situations.
Nested If Else Statements
- Nested if blocks allow multiple, layered checks for complex logic.
- Example: Check both username and password for authentication.
- Only executes the inner block if the outer condition is also true.
Key Terms & Definitions
- if statement — Executes code if a specified condition is true.
- elif statement — Tests an additional condition if the prior one was false.
- else statement — Specifies code to run if all previous conditions are false.
- [ ] — Tests expressions (strings, numbers, files).
- [[ ]] — Advanced test operator for pattern matching and logical expressions.
- (( )) — Arithmetic evaluation; true if result is nonzero.
- ( ) — Runs commands in a subprocess for status checking.
Action Items / Next Steps
- Practice writing Bash scripts using if, elif, and else with different condition types.
- Experiment with pattern matching and logical operators using double brackets.
- Review nested if structures and avoid unnecessary complexity.