All

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

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

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

$ mkswap /swapfile

Confirm the current swap file status
==========================

pi@raspberrypi ~ $ swapon -s

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

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.
*** apache2.conf (Y/I/N/O/D/Z) [default=N] ?

#!/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 /