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Linux/Bash-Script/10-While.md

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+# Bash `while` Loop Guide
+
+A clear, concise, and visually pleasant reference for using `while` loops in Bash.
+
+---
+
+## 1. Overview
+
+The `while` loop in Bash repeatedly executes a block of commands **as long as a condition evaluates to true**. It is a fundamental control structure for iteration when the number of repetitions is not known in advance.
+
+### General form
+
+```bash
+while [ condition ]
+do
+  command1
+  command2
+  ...
+done
+```
+
+* The condition is evaluated before each iteration. If it returns a zero (truthy) exit status, the loop body runs. Otherwise, the loop exits.
+* Square brackets (`[ ... ]`) are a synonym for the `test` command; spacing matters.
+
+---
+
+## 2. Examples
+
+### 2.1 Simple counter
+
+Counts from 1 to 5 and prints the current value.
+
+```bash
+counter=1
+
+while [ $counter -le 5 ]
+do
+  echo "Counter: $counter"
+  ((counter++))
+done
+```
+
+**What it does:**
+
+* Initializes `counter` to 1.
+* The loop continues while `counter` is less than or equal to 5.
+* Prints and increments the counter each iteration.
+
+### 2.2 Waiting for a file to appear
+
+Polls for the existence of `/tmp/myfile.txt` once per second.
+
+```bash
+while [ ! -f /tmp/myfile.txt ]
+do
+  echo "Waiting for file..."
+  sleep 1
+done
+
+echo "File exists!"
+```
+
+**What it does:**
+
+* Checks if the file does **not** exist (`! -f`).
+* Sleeps for one second between checks to avoid busy-waiting.
+* When the file appears, exits the loop and prints confirmation.
+
+### 2.3 Interactive prompt with exit condition
+
+Keeps prompting the user until they type `q`.
+
+```bash
+while true
+do
+  read -p "Type 'q' to quit: " input
+  if [ "$input" = "q" ]; then
+    break
+  fi
+done
+```
+
+**What it does:**
+
+* Uses an infinite loop (`while true`).
+* Prompts the user each time.
+* Exits the loop when the input equals `q` using `break`.
+
+---
+
+## 3. Tips & Best Practices
+
+* **Quote variables** to avoid word splitting and issues when they are empty: `[ "$foo" = "bar" ]`.
+* **Use arithmetic expressions** with `(( ... ))` when dealing with numbers (`((counter++))` is more concise than `counter=$((counter + 1))`).
+* **Avoid busy loops:** if waiting on external state, insert a `sleep` to reduce CPU usage.
+* **Use `set -u` and `set -e`** in scripts to catch undefined variables and stop on errors, but be careful when using them with conditional tests.
+* **Test conditions explicitly:** e.g., use `-f`, `-d`, `-r`, `-s`, etc., for file-related checks.
+
+---
+
+## 4. Common Pitfalls
+
+* Missing spaces inside `[ ]` — `while [$counter -le 5]` is invalid; it must be `[ $counter -le 5 ]`.
+* Forgetting to initialize loop variables, leading to unexpected behavior when using them in tests.
+* Using `read` without handling empty input; consider providing a default or validating.
+* Infinite loops without an exit strategy (e.g., missing `break` or condition never false).
+
+---
+
+## 5. Variations
+
+* **Until loop:** opposite of `while` — runs until a condition becomes true.
+
+  ```bash
+  until [ -f /tmp/myfile.txt ]
+  do
+    echo "Waiting..."
+    sleep 1
+  ```
+
+done
+
+- **Command-based condition:** you can use any command whose exit status matters.
+
+```bash
+while ping -c1 example.com >/dev/null 2>&1
+do
+  echo "Host is reachable"
+  sleep 5
+done
+````
+
+---
+
+## 6. Summary
+
+Use `while` loops when the number of iterations depends on dynamic conditions (user input, file existence, external services). Combine with proper quoting, sleeping, and exit strategies to write robust and readable scripts.
+