Shell Scripting — Loops: for & while

# ── Basic list ────────────────────────────────────────────
for fruit in apple banana cherry mango; do
  echo "  Fruit: ${fruit}"
done

# ── Loop over array ───────────────────────────────────────
servers=("web-01" "web-02" "db-01" "cache-01")
for srv in "${servers[@]}"; do
  echo "  Pinging ${srv}..."
  ping -c1 -W1 "${srv}" >/dev/null 2>&1 \
    && echo "    ✔ ${srv} reachable" \
    || echo "    ✘ ${srv} unreachable"
done

# ── Loop over command output ──────────────────────────────
for user in $(awk -F: '$3 >= 1000 {print $1}' /etc/passwd); do
  echo "  System user: ${user}"
done

# ── Loop over files matching pattern ─────────────────────
for f in /var/log/*.log; do
  [[ -f "${f}" ]] || continue   # skip if no match (glob safety)
  echo "  Log: ${f}  ($(wc -l < "${f}") lines)"
done

# ── Brace expansion range ─────────────────────────────────
for i in {1..5}; do
  echo "  Attempt ${i}"
done

# With step
for i in {0..100..10}; do    # 0 10 20 ... 100
  echo "  ${i}%"
done

# Countdown
for i in {10..1}; do
  printf "\r  Restarting in %2d seconds..." "${i}"
  sleep 1
done

# ── C-style for (most flexible) ───────────────────────────
for (( i=1; i<=5; i++ )); do
  echo "  i = ${i}"
done

# C-style with custom step
for (( i=0; i<=100; i+=25 )); do
  echo "  Progress: ${i}%"
done

# ── seq — useful when range is a variable ─────────────────
MAX=5
for i in $(seq 1 "${MAX}"); do
  echo "  Step ${i} of ${MAX}"
done

# NOTE: brace expansion {1..N} doesn't expand variables
# Use $(seq 1 $N) or C-style for when N is a variable

# ── Basic while ───────────────────────────────────────────
count=1
while (( count <= 5 )); do
  echo "  Count: ${count}"
  (( count++ ))
done

# ── Retry loop with backoff ───────────────────────────────
MAX_RETRIES=5
attempt=1

while (( attempt <= MAX_RETRIES )); do
  echo "  Attempt ${attempt}/${MAX_RETRIES}..."
  if curl -sf "http://api.example.com/health" >/dev/null; then
    echo "  ✔ Service healthy"
    break
  fi
  echo "  ✘ Not ready — waiting $((attempt * 2))s"
  sleep $(( attempt * 2 ))
  (( attempt++ ))
done

(( attempt > MAX_RETRIES )) && {
  echo "ERROR: Service did not become healthy" >&2
  exit 1
}

# ── Infinite loop with break ──────────────────────────────
while true; do
  load=$(awk '{print $1}' /proc/loadavg)
  echo "  Load: ${load}"
  (( $(echo "${load} > 2.0" | bc -l) )) && {
    echo "  HIGH LOAD — alerting"
    break
  }
  sleep 5
done

# ── Basic file reading ────────────────────────────────────
while IFS= read -r line; do
  echo "  → ${line}"
done < /etc/hosts

# ── Parse CSV with IFS ────────────────────────────────────
while IFS=',' read -r host port service; do
  [[ "${host}" == "#"* ]] && continue   # skip comments
  [[ -z "${host}" ]] && continue         # skip blank lines
  printf "  Checking %-20s %-6s %s\n" "${host}" "${port}" "${service}"
  nc -z "${host}" "${port}" 2>/dev/null \
    && echo "    ✔ UP" || echo "    ✘ DOWN"
done < services.csv

# ── Read from command output ──────────────────────────────
df -h | while IFS= read -r line; do
  echo "  ${line}"
done

# ── Process substitution (avoids subshell — vars persist) ─
total=0
while IFS=',' read -r name amount; do
  (( total += amount ))
done < <(grep '^item' sales.csv)
echo "Total: ${total}"   # Variable IS accessible here

# ── break — exit the loop ─────────────────────────────────
for i in {1..10}; do
  (( i == 5 )) && break
  echo "  i=${i}"
done
# Prints 1 2 3 4 then stops

# ── continue — skip to next iteration ─────────────────────
for i in {1..10}; do
  (( i % 2 == 0 )) && continue   # skip even numbers
  echo "  odd: ${i}"
done

# ── break N — break out of N nested loops ─────────────────
for host in host1 host2 host3; do
  for port in 80 443 8080; do
    if nc -z "${host}" "${port}" >/dev/null 2>&1; then
      echo "  First open port: ${host}:${port}"
      break 2   # break out of BOTH loops
    fi
  done
done

# ── Real-world: find first available backup slot ──────────
for slot in {1..7}; do
  backup_dir="/backups/slot_${slot}"
  [[ -d "${backup_dir}" ]] && continue
  mkdir -p "${backup_dir}"
  echo "  Using backup slot ${slot}"
  break
done

#!/usr/bin/env bash
# rotate_logs.sh — Archive logs older than N days

set -euo pipefail

LOG_DIR="/var/log/myapp"
ARCHIVE_DIR="/var/log/myapp/archive"
DAYS_OLD=7
rotated=0
errors=0

mkdir -p "${ARCHIVE_DIR}"

echo "Rotating logs older than ${DAYS_OLD} days..."

while IFS= read -r -d '' logfile; do
  filename=$(basename "${logfile}")
  archive_name="${ARCHIVE_DIR}/${filename%.log}_$(date +%Y%m%d).log.gz"

  echo "  Archiving: ${filename}"
  if gzip -c "${logfile}" > "${archive_name}"; then
    rm -f "${logfile}"
    echo "  ✔ Archived to $(basename "${archive_name}")"
    (( rotated++ ))
  else
    echo "  ✘ Failed to archive ${filename}" >&2
    (( errors++ ))
  fi
done < <(find "${LOG_DIR}" -maxdepth 1 -name "*.log" \
          -mtime +"${DAYS_OLD}" -print0)

echo "Done — rotated: ${rotated}, errors: ${errors}"
(( errors > 0 )) && exit 1 || exit 0