Loops: Iterating with for and while in Bash

Loops allow scripts to repeat operations over lists of files, lines, or until a condition changes.

1. The for Loop

Iterate over a list of items or files:

#!/bin/bash
# Loop over a list of server names
SERVERS=("web-01" "web-02" "web-03")

for server in "${SERVERS[@]}"; do
  echo "Pinging $server..."
  ping -c 1 "$server" > /dev/null && echo "$server is up" || echo "$server is down"
done

Iterating Over Files

#!/bin/bash
# Process all .log files in the current directory
for logfile in /var/log/*.log; do
  echo "Rotating: $logfile"
  gzip "$logfile"
done

2. The while Loop

Continue looping while a condition remains true:

#!/bin/bash
# Retry a command up to 5 times before giving up
RETRIES=0
MAX_RETRIES=5

while [ $RETRIES -lt $MAX_RETRIES ]; do
  if curl -s https://api.example.com/health | grep -q "ok"; then
    echo "Service is healthy!"
    break
  fi
  
  RETRIES=$((RETRIES + 1))
  echo "Attempt $RETRIES failed. Retrying in 5 seconds..."
  sleep 5
done

if [ $RETRIES -eq $MAX_RETRIES ]; then
  echo "ERROR: Service failed to respond after $MAX_RETRIES attempts."
  exit 1
fi