diff --git a/install.sh b/install.sh
index 4d112b0..16e8923 100755
--- a/install.sh
+++ b/install.sh
@@ -95,7 +95,6 @@ linux_packages() {
   # $1: build | app | hands | server
   case $PM in
     apt-get)
-      $SUDO apt-get update -qq || warn "apt-get update 有錯誤，繼續試試看"
       case $1 in
         server) set -- curl ca-certificates git ;;
         build) set -- build-essential pkg-config git curl ca-certificates libasound2-dev ;;
@@ -106,8 +105,19 @@ linux_packages() {
              libxkbcommon-x11-0 "libgl1|libgl1-mesa-glx" xdg-utils libnotify-bin libsecret-tools espeak-ng zenity \
              xclip wl-clipboard ffmpeg fonts-noto-cjk ;;
       esac
+      # A fresh system has no package lists yet: fetch them first, or nothing can be picked.
+      if ! ls /var/lib/apt/lists/*_Packages >/dev/null 2>&1; then
+        $SUDO apt-get update -qq || warn "apt-get update 有錯誤，繼續試試看"
+        UPDATED=1
+      fi
       pkgs=$(apt_pick "$@" | tr '\n' ' ')
-      info "apt-get install $pkgs"
+      # Already there (an update): nothing to install, no sudo password needed.
+      missing=""
+      for p in $pkgs; do dpkg -s "$p" >/dev/null 2>&1 || missing="$missing $p"; done
+      if [ -z "$missing" ]; then info "都已經裝好了"; return 0; fi
+      pkgs=$missing
+      [ -n "${UPDATED:-}" ] || $SUDO apt-get update -qq || warn "apt-get update 有錯誤，繼續試試看"
+      info "apt-get install$pkgs"
       $SUDO env DEBIAN_FRONTEND=noninteractive apt-get install -y -qq $pkgs >/dev/null || die "安裝系統套件失敗。" "請看上面的錯誤，或手動執行：sudo apt-get install $pkgs"
       ;;
     dnf)