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AWS announced to provide their own internal NTP inside VPC at AWS re:Invent 2017. Here is how to do it:

CentOS

sudo yum erase ntp*; sudo yum -y install chrony; sudo service chronyd start

Debian/Ubuntu

sudo apt-get remove -y ntp*; sudo apt-get -y install chrony; sudo service chronyd start; apt-get autoremove

Add an option -o StrictHostKeyChecking=no into a ssh command:

ssh -o StrictHostKeyChecking=no your_username@example.com

OR

export TARGET_HOST='example.com'

if [ -z `ssh-keygen -F ${TARGET_HOST}` ]; then
  ssh-keyscan -H ${TARGET_HOST} >> ~/.ssh/known_hosts
fi

ssh-keygen -R ${TARGET_HOST}
ssh-keyscan -f ~/.ssh/known_hosts ${TARGET_HOST} >>~/.ssh/known_hosts 2>/dev/null

1. Create a file for swap
   ======================

Create a file named /swapfile for 4GB size (e.g. Create a file called swapfile under root (/))

$ dd if=/dev/zero of=/swapfile bs=1024K count=4096

2. Format the swapfile with a mkswap command
   ======================================

$  mkswap /swapfile

1. Confirm the current swap file status
   ==========================

pi@raspberrypi ~ $ swapon -s

Filename                                Type            Size    Used    Priority
/var/swap                               file            102396  0       -1

0. Specification - What I want to do
   =================================
   I would like to input in-out records at my company automagically by using Raspberry PI and detecting my iPhone's bluetooth signal. That is, I want to make bluetooth proximity checker with my Raspberry PI. The specification is:

• Detect Bluetooth signal by Raspberry PI
• When I come into my office, the system records the time to come into the office
• When I leave from my office, the system records the time to leave out from the office
• Integrate Google Sheet to record the time

That is only what I want to do. Think about the spending time to input the in/out time manually. We can reduce our precious 12-20 total hours through a year (Assuming 3-5 min per a day and 240 working days through a year in order to input the records). It can be converted to a half or a day.

Then, OK, Let's see how we can make it!

HTTP/2 has been supported since Apache 2.4.17. I tried sudo a2enmod http2 on this Ubuntu 16.04 host, however it said ERROR: Module http2 does not exist!. Looks I needed to upgrade Apache2 as follows:

sudo add-apt-repository -y ppa:ondrej/apache2
sudo apt-key update
sudo apt-get update
sudo apt-get --only-upgrade install apache2 -y

Select Y (Note that this will overwrite your current configuration; therefore do backup of apache2.conf if necessary.

#!/bin/sh

export DATE_CURRENT=`date +%Y-%m-%d`
export TIME_CURRENT=`date +%Y%m%d%H%M%S`
export PURGE_AFTER_DAYS=30
export PURGE_AFTER=`date -d +${PURGE_AFTER_DAYS}days -u +%Y-%m-%d`
export BACKUP_TAG='backup'
 
# 1-1 Get the list of instances that you want to get backup
INSTANCES=`aws ec2 describe-tags --filters "Name=resource-type,Values=instance" "Name=key,Values=${BACKUP_TAG}" | awk '{print $3}'`

for INSTANCE in ${INSTANCES}; do
 
  BACKUP=`aws ec2 describe-tags --filters "Name=resource-type,Values=instance" "Name=resource-id,Values=${INSTANCE}" "Name=key,Values=${BACKUP_TAG}" | awk '{print $5}'`
 
  # 1-2 Check if the instance is marked as backup or not
  if [ "${BACKUP}" == "true" ]; then
 
    # 1-3 Create a backup
    AMI_ID=`aws ec2 create-image --instance-id ${INSTANCE} --name "${INSTANCE}_${TIME_CURRENT}" --no-reboot`
 
    # 1-4 Create a tag for search for later use
    aws ec2 create-tags --resources ${AMI_ID} --tags Key=PurgeAllow,Value=true Key=PurgeAfter,Value=$PURGE_AFTER
  fi
done
 
# 2-1 Search backups by tag
AMI_PURGE_ALLOWED=`aws ec2 describe-tags --filters "Name=resource-type,Values=image" "Name=key,Values=PurgeAllow" | awk '{print $3}'`
 
for AMI_ID in ${AMI_PURGE_ALLOWED}; do
  PURGE_AFTER_DATE=`aws ec2 describe-tags --filters "Name=resource-type,Values=image" "Name=resource-id,Values=${AMI_ID}" "Name=key,Values=PurgeAfter"  | awk '{print $5}'`
 
  if [ -n ${PURGE_AFTER_DATE} ]; then
    DATE_CURRENT_EPOCH=`date -d ${DATE_CURRENT} +%s`
    PURGE_AFTER_DATE_EPOCH=`date -d ${PURGE_AFTER_DATE} +%s`
 
    if [[ ${PURGE_AFTER_DATE_EPOCH} < ${DATE_CURRENT_EPOCH} ]]; then
      # 2-2 Delete backup if it is marked as backup
      aws ec2 deregister-image --image-id ${AMI_ID}
       
      SNAPSHOT_ID=`aws ec2 describe-images --image-ids ${AMI_ID} | grep EBS | awk '{print $4}'`
      aws ec2 delete-snapshot --snapshot-id ${SNAPSHOT_ID}
    fi
  fi
done

Use System Environment Variable${PIPESTATUS[X]}.

$ exit 1 | exit 2 | exit 3 | exit 0
$ echo ${PIPESTATUS[@]}
1 2 3 0

Also, the exit code of the normal end is 0, and the one of the abnormal end is Greater than 0 in Bash.

The following bash command deletes backup*.tar.gz files in a /var/www/html/backups directory by xargs rm -fr.

find /var/www/html/backups -mtime +2 -name "backup*.tar.gz" | xargs rm -fr

Run the following command:

mount -o remount,rw /