GitLab postinstall scriptlet (using /bin/sh):

gitlab-post.sh

#!/bin/sh
#
# Perform necessary gitlab setup steps
# after package is installed.
#
DEST_DIR=/opt/gitlab

notify()
{
  echo "gitlab: $1"
}
create_config_template()
{
  # Create a minimal gitlab.rb template if /etc/gitlab/gitlab.rb does not exist.
  if ! [ -e /etc/gitlab/gitlab.rb ] ; then
    mkdir -p /etc/gitlab
    cp "${DEST_DIR}/etc/gitlab.rb.template" /etc/gitlab/gitlab.rb
    sed -i 's!GENERATED_EXTERNAL_URL!'$EXTERNAL_URL'!g' /etc/gitlab/gitlab.rb
    chmod 600 /etc/gitlab/gitlab.rb
  else
    EXTERNAL_URL=$(awk '/^external_url/ { print $2 }' /etc/gitlab/gitlab.rb | tr -d "'\"")
  fi
}

fix_directory_permissions()
{
  if [ -x /usr/bin/dpkg-query ] ; then
    # We are in the land of .deb packages. We should fix package directory owners
    # because of the faulty 7.2.0 / 7.2.1 .deb packages.
    /usr/bin/dpkg-query -L gitlab-ce gitlab-ee 2>/dev/null | while read f ; do
    if [ -d "$f" ] ; then
      # This directory may have been created when installing omnibus-gitlab
      # 7.2.0 / 7.2.1, so it could have the wrong owner.
      chown root:root "$f"
    fi
  done
fi
}

print_upgrade_info()
{
  # Moving towards GitLab 11 we're deprecating skip-auto-migrations in favour of
  # skip-auto-reconfigure.  In the meantime check for both and populate skip_file.
  if [ -e "/etc/gitlab/skip-auto-migrations" ]; then
    skip_file="/etc/gitlab/skip-auto-migrations"
  elif [ -e "/etc/gitlab/skip-auto-reconfigure" ]; then
    skip_file="/etc/gitlab/skip-auto-reconfigure"
  else
    skip_file=""
  fi

  # As of 9.0, we're updating PostgreSQL by default.
  # But, if /etc/gitlab/skip-auto-migrations exists we won't do anything.
  # Here we'll warn users with out of date databases.
  if [ -n "${skip_file}" ]
  then
    local version=$(cat /var/opt/gitlab/postgresql/data/PG_VERSION 2>/dev/null || true)
    local new_version=$(awk '$1 == "postgresql_new" {print $2}' /opt/gitlab/version-manifest.txt)
    if [ -n "${new_version}" -a "${new_version#*$version}" = "${new_version}" ]
    then
      echo
      notify "WARNING:"
      notify "GitLab now ships with a newer version of PostgreSQL (${new_version}) by default"
      notify "We did not upgrade since ${skip_file} exists"
      notify "To manually upgrade, RUN THE FOLLOWING COMMAND:"
      echo
      echo "sudo gitlab-ctl pg-upgrade"
      echo
      notify "In the future, we will be removing old versions of PostgreSQL from the omnibus package"
      notify "Hosts which have not upgraded to the new version will begin to fail then"
      notify "For more details, please see:"
      notify "https://docs.gitlab.com/omnibus/settings/database.html#upgrade-packaged-postgresql-server"
      notify
    fi
  fi
}

if [ -n "${GITLAB_DEBUG}" ] ; then
  notify "debug: arguments: $@"
fi

check_if_ec2()
{
  if [ -f /sys/hypervisor/uuid ] && [ `head -c 3 /sys/hypervisor/uuid` = 'ec2' ]; then
    return 0
  else
    return 1
  fi
}

get_ec2_address()
{
  url=$1
  # Try collecting fqdn if it is set correctly
  fqdn=$(/opt/gitlab/embedded/bin/curl -s ${url})
  if [ -n "${fqdn}" ]; then
    # Checking if curl returned an XML message
    word="<?xml"
    if ! $(test "${fqdn#*$word}" != "$fqdn"); then
        EXTERNAL_URL="http://${fqdn}"
    fi
  fi
}

get_details_from_ec2()
{
  get_ec2_address "http://169.254.169.254/latest/meta-data/public-hostname"
  if [ -z "${EXTERNAL_URL}" ]; then
    get_ec2_address "http://169.254.169.254/latest/meta-data/public-ipv4"
  fi
}

set_protocol()
{
  # Checking if EXTERNAL_URL starts with http:// or https://
  if ! $(echo ${EXTERNAL_URL} | awk '$0 !~ /^http[s]?:\/\// {exit 1}'); then
    EXTERNAL_URL="http://${EXTERNAL_URL}"
  fi
}

if [ -z "${EXTERNAL_URL}" ]; then
  check_if_ec2
  if [ $? -eq 0 ]  ; then
    get_details_from_ec2
  fi
else
    set_protocol
fi

if [ -z "${EXTERNAL_URL}" ]; then
  EXTERNAL_URL="http://gitlab.example.com"
fi


${DEST_DIR}/embedded/bin/symlink_ctl_cmds ${DEST_DIR}
create_config_template
fix_directory_permissions
print_upgrade_info

case "$1" in
  configure)
    # Looks like a DEB install. We don't know if it is a fresh install or an
    # upgrade.
    EXTERNAL_URL=${EXTERNAL_URL} ${DEST_DIR}/bin/gitlab-ctl upgrade
    ;;
  *)
    # No op.
    ;;
esac