my-docs/Linux/Bash Script/7-If.md

🐚 Bash if Statement Guide

Conditional statements in Bash allow you to execute code based on specific conditions. Below is a comprehensive guide to if statements, their syntax, comparison operators, and examples.

---

🔹 Basic Syntax

if [[ condition ]]; then
    # commands if condition is true
fi

With else

if [[ condition ]]; then
    # true
else
    # false
fi

With elif

if [[ condition1 ]]; then
    # condition1 is true
elif [[ condition2 ]]; then
    # condition2 is true
else
    # none matched
fi

---

⚙️ Operators

🔢 Numeric Comparison

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

🔤 String Comparison

Operator	Meaning
==	Equal to
!=	Not equal
-z	Empty string
-n	Not empty string

📁 File Tests

Operator	Meaning
-e	File exists
-d	Directory exists
-r	File is readable
-w	File is writable
-x	File is executable

---

🧪 Practical Examples

Example 1: Number Check

read -p "Enter Your Number: " number

if [[ $number -eq 0 ]]; then
    echo "Zero"
elif [[ $number -ge 1 ]]; then
    echo "Positive"
else
    echo "Negative"
fi

---

Example 2: Number Range with Error Handling

read -p "Enter Your Number: " number

if [[ $number -eq 0 ]]; then
    echo "Zero"
elif [[ $number -ge 1 ]]; then
    echo "Positive"
elif [[ $number -le -1 ]]; then
    echo "Negative"
else 
    echo "Error: Invalid input"
    exit 1
fi

---

Example 3: String Check

read -p "Enter Your String: " str

if [[ -n $str ]]; then
    if [[ $str == "hello" ]]; then
        echo "Hi"
    elif [[ $str != "hello" ]]; then
        echo "Ok"
    fi
else
    echo "Nothing to read"
    exit 1
fi

---

Example 4: File Existence

file="script.sh"

if [[ -e $file ]]; then
    echo "File exists"
else
    touch "$file"
    echo "File created"
fi

---

Example 5: File Permission Check

file="script50.sh"

if [[ -e $file ]]; then
    echo "File exists"

    if [[ ! -x $file ]]; then
        chmod +x "$file"
        echo "Executable permission added"
    fi

    if [[ ! -r $file ]]; then
        chmod +r "$file"
        echo "Read permission added"
    fi
else
    touch "$file"
    echo "File created"
fi

---

✅ Tips

• Always quote variables: "$var" to prevent word splitting.
• Use [[ ... ]] for conditional tests (preferred over [ ... ]).
• Prefer read -r to avoid backslash escapes.