upgrade-postgres.nix

upgrade-postgres.nix

{ pkgs ? import <nixpkgs> {}
, old ? "postgresql_9_6"
, new ? "postgresql_12"
}:

let
  oldPkg = pkgs."${old}";
  newPkg = pkgs."${new}";
in

pkgs.writeShellScript "upgrade-pg-cluster" ''
  set -euo pipefail
  set -x
  export OLDDATA="/var/lib/postgresql/${oldPkg.psqlSchema}"
  export NEWDATA="/var/lib/postgresql/${newPkg.psqlSchema}"
  export OLDBIN="${oldPkg}/bin"
  export NEWBIN="${newPkg}/bin"

  if [ ! -e "$OLDDATA/PG_VERSION" ]; then
    echo "Old data dir invalid"
    exit 1
  fi

  install -d -m 0700 -o postgres -g postgres "$NEWDATA"
  cd "$NEWDATA"
  sudo -u postgres $NEWBIN/initdb -D "$NEWDATA"

  systemctl stop postgresql    # old one

  sudo -u postgres $NEWBIN/pg_upgrade \
    --old-datadir "$OLDDATA" --new-datadir "$NEWDATA" \
    --old-bindir $OLDBIN --new-bindir $NEWBIN \
    "$@"
''

Like the manual, but adjusted to be a simple shell script.