notification service initial

version

@@ -6,6 +6,9 @@
 # the local checkout is on, plus the short hash to tell same-day commits apart,
 # e.g. 2026.10.22 (c5f0651). The kernel is shown separately because it comes
 # from Arch, not CoreDesktop.
+#
+# version --check prints the newer version and exits 0 if one is available,
+# exits 1 if this machine is up to date, or 2 if it couldn't check.
 
 COREDESKTOP="$HOME/.local/share/coredesktop"
 
@@ -13,16 +16,40 @@ source "$COREDESKTOP/utils.sh"
 
 if ! git -C "$COREDESKTOP" rev-parse --git-dir &> /dev/null; then
   echo "CoreDesktop isn't installed in $COREDESKTOP."
-  exit 1
+  exit 2
 fi
 
-version="$(git -C "$COREDESKTOP" log -1 --format='%cd (%h)' --date=format:%Y.%m.%d)"
+# Version string for a commit, e.g. 2026.10.22 (c5f0651)
+describe() {
+  git -C "$COREDESKTOP" log -1 --format='%cd (%h)' --date=format:%Y.%m.%d "$1"
+}
+
+# Print the newer version if the remote has commits this checkout doesn't.
+# Returns 1 if up to date, 2 if the remote couldn't be reached.
+newer_version() {
+  GIT_TERMINAL_PROMPT=0 timeout 30 git -C "$COREDESKTOP" fetch --quiet &> /dev/null || return 2
+  local behind
+  behind="$(git -C "$COREDESKTOP" rev-list --count 'HEAD..@{upstream}' 2> /dev/null)" || return 2
+  (( behind > 0 )) || return 1
+  describe '@{upstream}'
+}
+
+if [ "$1" = "--check" ]; then
+  newer_version
+  exit
+fi
+
+version="$(describe HEAD)"
 
 # Local edits to the checkout mean this isn't exactly the version above
 if [ -n "$(git -C "$COREDESKTOP" status --porcelain --untracked-files=no)" ]; then
   version="$version, modified"
 fi
 
+if latest="$(newer_version)"; then
+  version="$version, $latest is available (run update)"
+fi
+
 kernel="$(uname -r)"
 # If the running kernel's modules are gone, a newer kernel was installed
 if [ ! -d "/usr/lib/modules/$kernel" ]; then