Bash While and Until Loops - File Reading, Polling, Counters

While and Until Loops in Bash

While loops repeat a block of commands as long as a condition is true. Until loops repeat until a condition becomes true. Both are essential for reading files, polling services, and implementing retry logic.

Basic While Loop

count=5
while [ $count -gt 0 ]; do
  echo "$count..."
  count=$((count - 1))
done
echo "Done!"

Reading Files Line by Line

The most common use of while loops is reading files:

while IFS= read -r line; do
  echo "Processing: $line"
done < input.txt

IFS= prevents leading/trailing whitespace trimming. -r prevents backslash interpretation.

Processing CSV Data

while IFS=',' read -r name email role; do
  echo "Creating account: $name ($email) - $role"
done < users.csv

Piping to While

# Process command output
ps aux | while read -r user pid cpu mem rest; do
  if (( $(echo "$mem > 10.0" | bc -l) )); then
    echo "High memory: PID $pid ($user) at $mem%"
  fi
done

Until Loop

# Wait for a service to be ready
until curl -sf http://localhost:3000/health > /dev/null 2>&1; do
  echo "Waiting for server..."
  sleep 2
done
echo "Server is up!"

Retry Logic

MAX_RETRIES=5
attempt=0

while [ $attempt -lt $MAX_RETRIES ]; do
  if deploy_application; then
    echo "Deployment successful"
    break
  fi
  attempt=$((attempt + 1))
  echo "Attempt $attempt failed, retrying in 10s..."
  sleep 10
done

if [ $attempt -eq $MAX_RETRIES ]; then
  echo "Deployment failed after $MAX_RETRIES attempts"
  exit 1
fi

Infinite Loop with Break

while true; do
  read -p "Enter command (quit to exit): " cmd
  case "$cmd" in
    quit) break ;;
    *) echo "You entered: $cmd" ;;
  esac
done

Monitoring Pattern

# Monitor disk usage
while true; do
  usage=$(df -h / | awk 'NR==2 {print $5}' | tr -d '%')
  if [ "$usage" -gt 90 ]; then
    echo "ALERT: Disk usage at $usage%"
  fi
  sleep 60
done