v0.7.3: work out the user bus instead of asking for it by hand

A shell entered via `su` keeps the ORIGINAL user's XDG_RUNTIME_DIR, so
`systemctl --user` hunts for the bus in someone else's directory and finds
nothing — even when this user's manager is running perfectly. v0.7.2 detected
that and told the operator to export the right path. Typing /run/user/100
instead of /run/user/1000 cost a whole run.

The runtime dir is /run/user/<uid> by definition. The script now derives it,
retries the bus check with it, and installs the units — then tells the operator
to export it for the REST of their shell, because a child process cannot fix
its parent's environment.

The failure message also stopped guessing. It now distinguishes the two real
cases: /run/user/<uid> missing (no logind session — enable-linger, and no fresh
login is needed, contrary to what v0.7.1 said) versus present but unanswered
(the manager itself is the problem — go look at user@<uid>.service).

Four branches fired in a sandbox with three stub systemctls: bus answers
directly; bus answers only with the corrected dir; runtime dir absent; runtime
dir present but manager silent. All exit 0, and the middle one installs units
that the previous version would have skipped.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

install.sh

@@ -79,8 +79,31 @@ say "systemd user units"
 # "Failed to connect to user scope bus". Ask the real question — can we talk to
 # the user manager — instead of asking whether the binary exists.
 USER_BUS=""
-if command -v systemctl >/dev/null 2>&1 && systemctl --user show-environment >/dev/null 2>&1; then
+BUS_FIXED_UP=""
+if command -v systemctl >/dev/null 2>&1; then
+  if systemctl --user show-environment >/dev/null 2>&1; then
     USER_BUS="yes"
+  else
+    # A shell entered via `su` keeps the ORIGINAL user's XDG_RUNTIME_DIR, so
+    # `systemctl --user` looks for the bus in someone else's directory and finds
+    # nothing — even when this user's manager is running perfectly well. The
+    # runtime dir is /run/user/<uid> by definition, so work it out instead of
+    # asking someone to export it by hand and get the digits right.
+    _own_runtime="/run/user/$(id -u)"
+    if [[ -d "$_own_runtime" && "${XDG_RUNTIME_DIR:-}" != "$_own_runtime" ]]; then
+      export XDG_RUNTIME_DIR="$_own_runtime"
+      if systemctl --user show-environment >/dev/null 2>&1; then
+        USER_BUS="yes"
+        BUS_FIXED_UP="yes"
+      fi
+    fi
+  fi
+fi
+
+if [[ -n "$BUS_FIXED_UP" ]]; then
+  ok "user bus found at $XDG_RUNTIME_DIR (the inherited XDG_RUNTIME_DIR pointed elsewhere)"
+  warn "this shell still has the old value. For later commands in THIS shell:"
+  warn "  export XDG_RUNTIME_DIR=$XDG_RUNTIME_DIR"
 fi
 
 if [[ -z "$USER_BUS" ]]; then
@@ -89,8 +112,13 @@ if [[ -z "$USER_BUS" ]]; then
     warn "reason: systemctl is not on PATH."
   else
     warn "reason: systemctl --user cannot reach a user manager (XDG_RUNTIME_DIR=${XDG_RUNTIME_DIR:-unset})."
-    warn "common causes: an LXC without logind, a shell entered via su, or no lingering."
+    if [[ ! -d "/run/user/$(id -u)" ]]; then
-    warn "  sudo loginctl enable-linger $USER   # then log in fresh and re-run"
+      warn "/run/user/$(id -u) does not exist — $USER has no logind session at all."
+      warn "  sudo loginctl enable-linger $USER   # then re-run; no fresh login needed"
+    else
+      warn "/run/user/$(id -u) exists but the manager there did not answer."
+      warn "  systemctl status user@$(id -u).service   # start here"
+    fi
   fi
   warn "until then, run the three by hand from $REPO:"
   warn "  $VENV/bin/socialmag-openbridge &"