Conditionals

Bash scripting relies on conditional statements to implement logic-based flow control, enabling scripts to make decisions based on runtime evaluations. Conditional constructs like if-else, case, and the test command (also known as [) allow scripts to respond dynamically to system states, user inputs, or environmental variables. These structures are foundational for creating robust, adaptive automation.

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if-else Statements

The if-else construct evaluates a condition and executes code blocks based on whether the condition is true or false. Conditions are typically evaluated using exit statuses (0 for success, non-zero for failure) or comparisons.

Syntax

if [ condition ]; then
    # Code if condition is true
elif [ another_condition ]; then
    # Code if another_condition is true
else
    # Code if all conditions are false
fi

Example: File Existence Check

if [ -f "/path/to/file.txt" ]; then
    echo "File exists."
else
    echo "File not found."
fi

Here, -f checks if the path is a regular file. The script outputs a message based on the result.

Notes

• Always use ; or a newline to terminate the condition.
• Use elif for additional conditional branches.
• Avoid spaces around [ and ] in test expressions.

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case Statements

The case construct matches a variable against multiple patterns, making it ideal for handling discrete options (e.g., command-line flags). Patterns use shell globbing (*, ?, etc.).

Syntax

case $variable in
    pattern1) 
        # Code for pattern1
        ;;
    pattern2)
        # Code for pattern2
        ;;
    *) 
        # Default case
        ;;
esac

Example: Command-Line Option Handling

case "$1" in
    --help|-h)
        echo "Usage: script.sh [OPTIONS]"
        ;;
    --version|-v)
        echo "1.0.0"
        ;;
    *)
        echo "Unknown option: $1"
        ;;
esac

This script processes command-line arguments like --help or -v and defaults to an error message for unknown inputs.

Notes

• Patterns are matched literally unless modified by * or ?.
• Use ;; to terminate each case block.
• The * wildcard matches any unmatched input.

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test Command ( [ )

The test command evaluates expressions for file properties, string comparisons, or arithmetic operations. It is often used inline with if statements.

Common Use Cases

Expression	Description
-f file	True if file is a regular file
-d file	True if file is a directory
-r file	True if file is readable
-w file	True if file is writable
-x file	True if file is executable
str1 == str2	True if strings are equal
num1 -eq num2	True if numbers are equal

Example: Numeric Comparison

if [ 10 -gt 5 ]; then
    echo "10 is greater than 5"
fi

This script outputs "10 is greater than 5" because -gt (greater than) evaluates to true.

Notes

• Use && or || for chaining conditions.
• Avoid spaces around [ and ].
• For string comparisons, use == and ensure variables are quoted to prevent globbing.

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Key takeaways

• Use if-else for branching logic based on conditions.
• Use case for matching variables against multiple patterns.
• Use test (or [) to evaluate file properties, strings, or arithmetic expressions.
• Always quote variables in comparisons to avoid unintended globbing or expansion.
• Prioritize readability by structuring conditions clearly and avoiding nested complexity.