update lpic: if statment doc

Linux/Bash Script/7-If.md

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+
+# 🐚 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
+
+```bash
+if [[ condition ]]; then
+    # commands if condition is true
+fi
+```
+
+### With `else`
+
+```bash
+if [[ condition ]]; then
+    # true
+else
+    # false
+fi
+```
+
+### With `elif`
+
+```bash
+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
+
+```bash
+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
+
+```bash
+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
+
+```bash
+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
+
+```bash
+file="script.sh"
+
+if [[ -e $file ]]; then
+    echo "File exists"
+else
+    touch "$file"
+    echo "File created"
+fi
+```
+
+---
+
+### Example 5: File Permission Check
+
+```bash
+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.
+