Set hostname on CentOS on AWS EC2 instance (hostnamectl set-hostname, /etc/hosts, /etc/sysconfig/network, /etc/cloud/cloud.cfg)

setHostname.sh

#!/bin/bash

# # # USAGE: hostname -f ; /bin/bash setHostname.sh new.hostname.to.be.set.in.some.domain ; hostname -f

# # # Per the official but mildly inaccurate AWS EC2 hostname docs
# # #   (https://aws.amazon.com/premiumsupport/knowledge-center/linux-static-hostname-rhel7-centos7/)

setHostname() {
  local fqdn=$1 ;

  # # Probably the least necessary of all.  (Try it: Comment it out and everything still works.)
  updateSysconfigNetwork $fqdn ;

  updateHostsTable $fqdn ;

  sudo hostnamectl set-hostname $fqdn ;
  # /etc/hostname handled by hostnamectl set-hostname

  if [[ -e /etc/cloud/cloud.cfg ]] ; then
    local x=`egrep 'preserve_hostname:  *true' /etc/cloud/cloud.cfg` ;
    [[ $x ]] || echo -e "\npreserve_hostname: true" | sudo tee -a /etc/cloud/cloud.cfg ;
  fi ;
}
updateSysconfigNetwork() {
  local fqdn=$1 ;
  local f=/etc/sysconfig/network ;
  if [[ -e $f ]] ; then
    local tmpf=`mktemp "${TMPDIR:-/tmp}/tmp.XXXXXXXXXX"` ;
    cat $f > $tmpf ;
    cat $tmpf | egrep -v '^\s*(NETWORKIN(G|G_IPV6)|HOSTNAME)[=:]' \
      | sudo tee $f \
    ;
    echo 'NETWORKING=yes'     | sudo tee -a $f ;
    echo 'NETWORKING_IPV6=no' | sudo tee -a $f ;
    echo 'HOSTNAME=$fqdn'     | sudo tee -a $f ;
  fi ;
}
updateHostsTable() {
  local fqdn=$1 ;
  local f=/etc/hosts ;
  if [[ -e $f ]] ; then
    local tmpf=`mktemp "${TMPDIR:-/tmp}/tmp.XXXXXXXXXX"` ;
    cat $f > $tmpf ;
    echo -e \
      "127.0.0.1   $fqdn localhost localhost.localdomain localhost4 localhost4.localdomain4" \
      | sudo tee $f \
    ;
    cat $tmpf | egrep -v '^\s*127.0.0.1\s' \
      | sudo tee -a $f \
    ;
  fi ;
}

setHostname "$@" > /dev/null

#