GitLab preinstall scriptlet (using /bin/sh):

gitlab-pre.sh

#!/bin/sh
# GitLab pre-install script

DEST_DIR=/opt/gitlab
NEW_MAJOR_VERSION=10
NEW_MINOR_VERSION=10.8

mkdir -p /var/log/gitlab/reconfigure

skip_migrations_file=/etc/gitlab/skip-auto-migrations
skip_reconfigure_file=/etc/gitlab/skip-auto-reconfigure

config_check() {
if [ -e "${DEST_DIR}/embedded/service/omnibus-ctl/check_config.rb" ] ; then
  ${DEST_DIR}/bin/gitlab-ctl check-config --version=${NEW_MINOR_VERSION}
  if [ $? -ne 0 ]; then
    exit 1
  fi
fi
}

upgrade_check() {
  if [ -e "${DEST_DIR}/version-manifest.json" ] ; then
    OLD_VERSION_STRING=$(grep -i "build_version" ${DEST_DIR}/version-manifest.json | awk -F ': ' '{print $2}' | tr -d '",')

    # Getting the Major and Major.Minor format of existing version string
    OLD_MAJOR_VERSION=$(echo $OLD_VERSION_STRING | awk -F "." '{print $1}')
    OLD_MINOR_VERSION=$(echo $OLD_VERSION_STRING | awk -F "." '{print $1"."$2}')

    # Minimum version from which jumps are permitted to current version. This
    # points to the last minor version from previous series.
    MIN_VERSION=9.5

    # Checking
    # (i) if it is a major version jump
    # (ii) if existing version is less than required minimum version
    if test ${OLD_MAJOR_VERSION} -lt ${NEW_MAJOR_VERSION}; then
      if ! $(echo ${OLD_MINOR_VERSION} | awk -v MIN_VERSION="$MIN_VERSION" '$NF+0 < MIN_VERSION {exit 1}'); then
        notify "It seems you are upgrading from ${OLD_MAJOR_VERSION}.x version series"
        notify "to ${NEW_MAJOR_VERSION}.x series. It is recommended to upgrade"
        notify "to the last minor version in a major version series first before"
        notify "jumping to the next major version."
        notify "Please follow the upgrade documentation at https://docs.gitlab.com/ee/policy/maintenance.html#upgrade-recommendations"
        notify "and upgrade to ${MIN_VERSION} first."
        exit 1
      fi
    fi
  fi
}

pg_check() {
  PG_MIN_VERSION=9.6

  # Fetch the currently running version from the database
  # If this doesn't work, assume this isn't a database node
  if ! running_version=$(${DEST_DIR}/bin/gitlab-psql -d template1 -c 'SHOW server_version' -qt 2>/dev/null); then
    notify
    notify "This node does not appear to be running a database"
    notify "Skipping version check, if you think this is an error exit now"
    notify
    return
  fi

  # Check if PostgreSQL version is less than PG_MIN_VERSION and notify user.
  # In AWK, $NF will return the last column of the version output, which
  # is the version string. By doing $NF+0, we force awk to convert it to a
  # float, so that we can do numerical comparison.
  if ! $(echo ${running_version} | awk -v PG_MIN_VERSION="$PG_MIN_VERSION" '$NF+0 < PG_MIN_VERSION {exit 1}'); then
    notify
    notify "Your version of PostgreSQL is no longer supported. Please upgrade your PostgreSQL version."
    notify "Check https://docs.gitlab.com/omnibus/update/#updating-gitlab-10-0-or-newer for details."
    notify ""
    notify "Upgrade failed. Retry the upgrade after upgrading your PostgreSQL version."
    exit 1
  fi
}

main() {
  if [ -e "${skip_migrations_file}" ] || [ -e "${skip_reconfigure_file}" ] ; then
    # The user wants us to do nothing
    return
  fi

  if [ -d ${DEST_DIR}/service/unicorn ] ; then
    notify "Automatically backing up only the GitLab SQL database (excluding everything else!)"
    if ! ${DEST_DIR}/bin/gitlab-rake gitlab:backup:create SKIP=repositories,uploads,builds,artifacts,lfs,registry,pages ; then
      notify
      notify "Backup failed! If you want to skip this backup, run the following command and"
      notify "try again:"
      notify
      notify "  sudo touch ${skip_migrations_file}"
      notify
      exit 1
    fi
  fi
}

notify() {
  echo "gitlab preinstall: $1"
}

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

case "$1" in
  2)
    # Looks like an RPM upgrade
    upgrade_check
    config_check
    pg_check
    main
    ;;
  upgrade)
    # Looks like a DEB upgrade
    upgrade_check
    config_check
    pg_check
    main
    ;;
  *)
    # This is not an upgrade, nothing to do.
    ;;
esac