🐚 Bash Decision Making – if, elif, else, and [[ ... ]] Conditions Explained

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🧲 Introduction to Bash Conditional Statements – if, elif, else, and [[ ]]

Decision-making is a core part of Bash scripting. The ability to make choices using if, elif, and else statements allows scripts to behave dynamically based on input, file checks, string comparisons, or numeric conditions.

Bash also uses [ ], [[ ]], and (( )) syntaxes to evaluate conditions. Understanding these structures is essential for writing logic-based scripts.

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🎯 In this article, you’ll learn:

• How to write conditional statements using if, elif, and else
• The difference between [ ] and [[ ]] in Bash
• Numeric and string comparisons
• Real-world decision-making examples

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🔍 Basic Syntax: if, elif, else

if condition; then
  # code block
elif another_condition; then
  # alternative block
else
  # fallback block
fi

• then must follow if (can be on a new line or same line with ;)
• Each block ends with fi (if spelled backwards)

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🧪 Example: Basic String Check

name="Vaibhav"

if [[ $name == "Vaibhav" ]]; then
  echo "Name matched!"
else
  echo "Name does not match."
fi

✅ Output:

Name matched!

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🔢 Numeric Comparisons with -eq, -gt, -lt

Operator	Meaning
-eq	Equal to
-ne	Not equal to
-gt	Greater than
-lt	Less than
-ge	Greater or equal
-le	Less or equal

🧪 Example:

num=20

if [ $num -gt 10 ]; then
  echo "Number is greater than 10"
fi

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🔠 String Comparisons

Operator	Meaning
== or =	Equal
!=	Not equal
-z	String is empty
-n	String is not empty

🧪 Example:

text="bash"

if [[ -n "$text" ]]; then
  echo "Text is not empty"
fi

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📁 File Test Conditions

Condition	Checks For
-f file	Regular file exists
-d directory	Directory exists
-e file	File or directory exists
-r file	Read permission
-w file	Write permission
-x file	Execute permission

🧪 Example:

if [[ -f "script.sh" ]]; then
  echo "File exists"
else
  echo "File does not exist"
fi

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🔁 Multiple Conditions – Logical Operators

Operator	Description
&&	Logical AND
`
!	Logical NOT

🧪 Example:

if [[ $age -gt 18 && $age -lt 60 ]]; then
  echo "Adult within working age"
fi

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🔁 Nested if Conditions

if [[ $user == "admin" ]]; then
  if [[ $status == "active" ]]; then
    echo "Admin is active"
  fi
fi

🧠 Nested conditions are useful when checking multiple levels of logic.

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🧱 [ ] vs [[ ]] – What’s the Difference?

Feature	[ ]	[[ ]]
POSIX-compliant	✅ Yes	❌ No
Safer with variables	❌ Limited	✅ Yes
Supports pattern matching	❌ No	✅ Yes
Better error handling	❌ No	✅ Yes

✅ Use [[ ]] when writing modern, safe Bash scripts. Use [ ] for POSIX-compatible minimal scripts.

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📌 Summary – Bash Conditional Statements

Conditional logic is a building block of intelligent Bash scripts. Using if, elif, else, and [[ ... ]] allows your scripts to make decisions, validate inputs, and perform context-based operations.

🔍 Key Takeaways:

• Use if, elif, and else to handle conditions
• Use [[ ]] for safer and enhanced conditions
• Use -eq, -gt, ==, != etc. for numeric/string comparisons
• File test operators like -f or -d are great for validations

⚙️ Real-world Uses:

• Check if files exist before modifying
• Validate user input or arguments
• Branch script logic based on time, user, or status

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❓ FAQ – Bash Conditional Statements

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❓ What is the difference between if [ ] and if [[ ]]?
✅ [ ] is POSIX-compliant but limited. [[ ]] is a Bash enhancement with safer syntax and better string matching.

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❓ Can I use == inside [ ]?
✅ Yes, but it’s better to use = inside [ ] and reserve == for [[ ]].

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❓ How do I check if a string is empty in Bash?
✅ Use:

if [[ -z "$var" ]]; then
  echo "Empty"
fi

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❓ How do I compare numbers in Bash?
✅ Use -eq, -ne, -lt, -gt, etc.:

if [[ $a -gt 5 ]]; then echo "Greater"; fi

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❓ How do I combine two conditions in Bash?
✅ Use && for AND, || for OR:

if [[ $a -gt 0 && $b -lt 100 ]]; then
  echo "Within range"
fi

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