Bash — Loops

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tags:

• text-parsing
• linux

• sysadmin

  Bash Loops — built-in shell constructs (for, while, until) for iterating over files, numbers, arrays, and command output. These are not standalone programs but fundamental parts of Bash (Bourne-Again SHell), the default shell on most Linux systems. Loops are essential for scripting automation, batch processing, retry logic, and monitoring. For complex orchestration, consider dedicated tools like ansible or xargs --parallel.

🔄 FOR Loops

Over Files

for f in *.log; do echo "$f"; done             # Iterate .log files / Перебрать .log файлы
for f in *.{yml,yaml}; do echo "$f"; done      # Multiple extensions / Несколько расширений
for f in /var/log/*.log; do : > "$f"; done     # Truncate all logs / Очистить все логи
for f in $(find . -name "*.txt"); do echo "$f"; done  # With find / С find
for f in *.conf; do cp "$f" "$f.bak"; done     # Backup all configs / Бэкап всех конфигов

Over Numbers

for i in {1..5}; do echo $i; done              # Range 1-5 / Диапазон 1-5
for i in {1..10..2}; do echo $i; done          # Step by 2 / Шаг 2
for i in {10..1}; do echo $i; done             # Reverse / Обратный порядок
for ((i=1; i<=5; i++)); do echo $i; done       # C-style loop / Цикл в стиле C
for ((i=0; i<10; i+=2)); do echo $i; done      # C-style with step / С шагом

Over Arrays

arr=("apple" "banana" "cherry")
for item in "${arr[@]}"; do echo "$item"; done # Iterate array / Перебрать массив
for i in "${!arr[@]}"; do echo "$i: ${arr[$i]}"; done  # With indices / С индексами

Over Command Output

for user in $(cut -d: -f1 /etc/passwd); do echo "$user"; done  # All users / Все пользователи
for ip in $(cat ips.txt); do ping -c1 "$ip"; done  # Ping all IPs / Пинговать все IP
for pod in $(kubectl get pods -o name); do kubectl describe "$pod"; done  # K8s pods / Поды Kubernetes

⏳ WHILE Loops

Basic While

i=1
while [ $i -le 5 ]; do
  echo $i
  ((i++))
done                                           # Counter 1-5 / Счётчик 1-5

Read from File

while read -r line; do
  echo "$line"
done < file.txt                                # Read file lines / Читать построчно

Infinite Loops

while :; do
  date
  sleep 5
done                                           # Tick every 5s / Каждые 5 секунд

while true; do
  echo "Running..."
  sleep 10
done                                           # Alternative infinite / Альтернативный бесконечный

Conditional While

while pgrep -x "nginx" > /dev/null; do
  echo "Nginx running"
  sleep 5
done                                           # While process exists / Пока процесс существует

while ! ping -c1 8.8.8.8 &>/dev/null; do
  echo "Waiting for network..."
  sleep 2
done                                           # Wait for network / Ждать сети

🔁 UNTIL Loops

Basic Until

i=1
until [ $i -gt 5 ]; do
  echo $i
  ((i++))
done                                           # Run until condition true / Пока условие ЛОЖНО

Wait for Condition

until curl -sf http://localhost:8080/health > /dev/null; do
  echo "Waiting for service..."
  sleep 2
done                                           # Wait for service / Ждать сервис

until [ -f /tmp/ready ]; do
  echo "Waiting for file..."
  sleep 1
done                                           # Wait for file / Ждать файл

🎮 Loop Control

Break

for i in {1..10}; do
  if [ $i -eq 5 ]; then
    break
  fi
  echo $i
done                                           # Exit loop at 5 / Выход на 5

Continue

for i in {1..10}; do
  if [ $((i % 2)) -eq 0 ]; then
    continue
  fi
  echo $i
done                                           # Skip even numbers / Пропустить чётные

Nested Loops

for i in {1..3}; do
  for j in {1..3}; do
    echo "$i,$j"
  done
done                                           # Nested iteration / Вложенный перебор

📖 Reading Files

Read Lines

while IFS= read -r line; do
  echo "$line"
done < file.txt                                # Preserve whitespace / Сохранить пробелы

Read CSV

while IFS=',' read -r col1 col2 col3; do
  echo "Col1: $col1, Col2: $col2"
done < data.csv                                # Parse CSV / Парсить CSV

Process Substitution

while IFS= read -r f; do
  echo "$f"
done < <(ls -1)                                # No subshell variable loss / Без потери переменных

Read with Timeout

while read -t 5 -r line; do
  echo "$line"
done < <(some_command)                         # 5s timeout / Таймаут 5с

🌟 Real-World Examples

System Administration

# Rotate logs
for log in /var/log/app/*.log; do
  mv "$log" "$log.old"
  touch "$log"
done

# Monitor services
while :; do
  if ! systemctl is-active --quiet nginx; then
    echo "Nginx down, restarting..."
    systemctl restart nginx
  fi
  sleep 60
done

# Cleanup old files
for f in $(find /tmp -name "*.tmp" -mtime +7); do
  echo "Deleting $f"
  rm "$f"
done

Network Operations

# Ping sweep
for i in {1..254}; do
  {
    ip="192.168.1.$i"
    ping -c1 -W1 "$ip" &>/dev/null && echo "$ip is up"
  } &
done
wait

# Port scan
for port in {20..30}; do
  timeout 1 bash -c "echo > /dev/tcp/<HOST>/$port" 2>/dev/null && echo "Port $port open"
done

# Check URLs
while IFS= read -r url; do
  status=$(curl -o /dev/null -s -w "%{http_code}" "$url")
  echo "$url: $status"
done < urls.txt

File Processing

# Batch rename
for f in *.jpeg; do
  mv "$f" "${f%.jpeg}.jpg"
done

# Convert images
for img in *.png; do
  convert "$img" "${img%.png}.webp"
done

# Extract archives
for archive in *.tar.gz; do
  tar -xzf "$archive" -C /dest/
done

Database Operations

# Backup all databases
for db in $(mysql -Ne "SHOW DATABASES" | grep -v "information_schema\|performance_schema\|mysql"); do
  mysqldump "$db" | gzip > "/backup/${db}-$(date +%Y%m%d).sql.gz"
done

# Check DB connections
while IFS= read -r host; do
  mysql -h "$host" -e "SELECT 1" &>/dev/null && echo "$host: OK" || echo "$host: FAIL"
done < db_hosts.txt

Kubernetes / Docker

[!CAUTION] Deleting pods and containers is destructive. Make sure you are targeting the correct namespace/resources. Удаление подов и контейнеров — деструктивная операция. Убедитесь, что вы работаете с нужным namespace/ресурсами.

# Restart all pods
for pod in $(kubectl get pods -o name -n <NAMESPACE>); do
  kubectl delete "$pod" -n <NAMESPACE>
done

# Cleanup old containers
for container in $(docker ps -aq); do
  docker rm -f "$container"
done

# Check pod status
while :; do
  kubectl get pods -o wide
  sleep 10
done

Parallel Processing

# Process files in parallel
for f in *.log; do
  {
    gzip "$f"
    echo "$f compressed"
  } &
done
wait                                           # Wait for all background jobs / Ждать все фоновые задачи

# Limited parallelism
max_jobs=4
for f in *.txt; do
  while [ $(jobs -r | wc -l) -ge $max_jobs ]; do
    sleep 0.1
  done
  {
    process_file "$f"
  } &
done
wait

Monitoring & Alerting

# Watch disk space
while :; do
  usage=$(df -h / | awk 'NR==2 {print $5}' | sed 's/%//')
  if [ "$usage" -gt 90 ]; then
    echo "Disk usage critical: ${usage}%"
  fi
  sleep 300
done

# Monitor log for errors
tail -f /var/log/app.log | while read -r line; do
  if echo "$line" | grep -q "ERROR"; then
    echo "Error detected: $line"
  fi
done

Advanced Patterns

# Counter with timeout
timeout=10
counter=0
while [ $counter -lt $timeout ]; do
  if check_condition; then
    echo "Success!"
    break
  fi
  ((counter++))
  sleep 1
done

# Retry logic
max_retries=3
retry=0
until some_command || [ $retry -eq $max_retries ]; do
  ((retry++))
  echo "Retry $retry/$max_retries..."
  sleep 5
done

# Interactive menu
options=("Start" "Stop" "Restart" "Exit")
while :; do
  for i in "${!options[@]}"; do
    echo "$((i+1)). ${options[$i]}"
  done
  read -p "Select option: " choice
  case $choice in
    4) break ;;
    *) echo "Processing ${options[$((choice-1))]}" ;;
  esac
done

📚 Documentation

• Bash Reference Manual — Looping Constructs
• Bash Reference Manual (full)
• Advanced Bash-Scripting Guide — Loops
• help for / help while / help until (Bash built-in help)