If-elif-else Statements in Bash

Bash scripting is powerful but can be confusing for newcomers, especially when it comes to conditional statements. One common pitfall is incorrectly using comparison operators, which leads to unexpected behavior in your scripts.

The Problem: Incorrect Comparison Operators

The core issue in the original question stems from using the wrong comparison operator. In bash, -eq is specifically for numeric comparisons, not string comparisons.

# This will NOT work as expected for string comparison
if [[ $aaa -eq "bbb" ]]; then
    echo "bbb"
fi

Solution: Use the Right Operators and Proper Quoting

For string comparisons in bash, you should use = or == (they're equivalent in bash) and always quote your variables to prevent unexpected behavior.

Basic if-elif-else Structure

aaa="xxx"
if [[ "$aaa" == "bbb" ]]; then
   echo "bbb"
elif [[ "$aaa" == "ccc" ]]; then
   echo "ccc"
else
   echo "something else"
fi

Single Bracket Syntax (POSIX-compliant)

if [ "$aaa" = "bbb" ]; then
    echo "bbb"
elif [ "$aaa" = "ccc" ]; then
    echo "ccc"
else
    echo "something else"
fi

TIP

While [[ (double brackets) is generally preferred for bash scripting as it's more robust and feature-rich, [ (single bracket) is POSIX-compliant and works across different shells.

When to Use Different Comparison Operators

String Comparison

# Use = or == with strings
if [[ "$var" == "value" ]]; then
    echo "String matches"
fi

Numeric Comparison

# Use -eq, -ne, -lt, -gt with numbers
if [[ $num -eq 10 ]]; then
    echo "Number is 10"
fi

Alternative: Case Statements

For multiple string comparisons, a case statement is often more readable and efficient:

case "$aaa" in
    "bbb")
        echo "bbb"
        ;;
    "ccc")
        echo "ccc"
        ;;
    *)
        echo "something else"
        ;;
esac

Always Quote Variables

Unquoted variables can cause unexpected behavior when they contain spaces or special characters. Always wrap variables in quotes:

# ✅ Correct
if [ "$my_var" = "some value" ]; then

# ❌ Risky - may break with spaces or special characters
if [ $my_var = "some value" ]; then

Common Pitfalls and Solutions

1. Empty Variable Handling

# Safe comparison that handles empty variables
if [ "${aaa:-}" = "bbb" ]; then
    echo "bbb"
fi

2. Testing for Empty Strings

# Check if variable is empty
if [ -z "$aaa" ]; then
    echo "Variable is empty"
fi

# Check if variable is not empty
if [ -n "$aaa" ]; then
    echo "Variable has content"
fi

3. Pattern Matching with Double Brackets

# Pattern matching (only works with [[ ]])
if [[ "$filename" == *.txt ]]; then
    echo "Text file detected"
fi

Best Practices Summary

1. Use = or == for string comparisons
2. Use -eq, -ne, -lt, -gt for numeric comparisons
3. Always quote variables to prevent word splitting
4. Prefer [[ ]] over [ ] in bash scripts for better features
5. Consider case statements for multiple string comparisons
6. Test your scripts with various inputs including empty values

By following these guidelines, you'll avoid the common pitfalls of bash conditional statements and write more robust shell scripts.