feat(deploy): templatize systemd services on install_dir via Jinja2

Distros reserve /opt for different things (some package managers own it
outright), and a DECNET install that wants to live at /srv/decnet or
/usr/local/decnet had to hand-edit 13 service files post-install.

Converts every deploy/decnet-*.service to a .j2 template keyed on
{{ install_dir }}, rendered by `decnet init` at install time. All other
paths (log_dir, state_dir, runtime_dir, user, group) stay standard —
only install_dir varies.

Changes:
- deploy/decnet-*.service → deploy/decnet-*.service.j2 (13 files).
- decnet init gains --install-dir (default /opt/decnet, preserves
  existing behaviour byte-for-byte). Validates absolute-path at the
  CLI boundary. Threads through useradd --home-dir and the dir-creation
  list so the filesystem layout matches the rendered templates.
- _install_units renders via Jinja2 with StrictUndefined (typo → loud
  error, not a silent broken unit). SHA over rendered output so
  operators with a custom install_dir get idempotent re-runs.
- decnet.target, tmpfiles.d, polkit rule stay static — they don't
  reference install paths.
- 4 new tests: custom install_dir renders into units, default remains
  /opt/decnet, relative paths rejected, second run with same custom
  dir is idempotent.

decnet/cli/init.py

@@ -24,6 +24,7 @@ from pathlib import Path
 from typing import Callable, List
 
 import typer
+from jinja2 import Environment, FileSystemLoader, StrictUndefined
 
 import decnet as _decnet_pkg
 from .gating import _require_master_mode
@@ -99,7 +100,7 @@ def _ensure_group(group: str, *, dry_run: bool) -> str:
         return "ok"
 
 
-def _ensure_user(user: str, group: str, *, dry_run: bool) -> str:
+def _ensure_user(user: str, group: str, install_dir: str, *, dry_run: bool) -> str:
     try:
         pwd.getpwnam(user)
         return f"skip: user {user} already exists"
@@ -108,7 +109,7 @@ def _ensure_user(user: str, group: str, *, dry_run: bool) -> str:
             [
                 "useradd", "--system",
                 "--gid", group,
-                "--home-dir", "/opt/decnet",
+                "--home-dir", install_dir,
                 "--shell", "/usr/sbin/nologin",
                 "--comment", "DECNET honeypot",
                 user,
@@ -177,19 +178,78 @@ def _copy_if_changed(
     return "ok"
 
 
-def _install_units(
-    deploy: Path, systemd_dir: Path, *, force: bool, dry_run: bool
+def _render_template(src: Path, context: dict[str, str]) -> str:
+    """Render a Jinja2 .j2 template with the given context.
+
+    StrictUndefined: a missing context variable is an error, not a
+    silent empty-string substitution — that way a typo in the template
+    fails loudly instead of shipping a broken systemd unit.
+    """
+    env = Environment(
+        loader=FileSystemLoader(str(src.parent)),
+        undefined=StrictUndefined,
+        keep_trailing_newline=True,
+        autoescape=False,  # nosec B701 — rendering systemd INI, not HTML
+    )
+    template = env.get_template(src.name)
+    return template.render(**context)
+
+
+def _write_rendered_if_changed(
+    src: Path, dst: Path, rendered: str, *, mode: int, force: bool, dry_run: bool
 ) -> str:
-    sources = sorted(deploy.glob("decnet-*.service")) + [deploy / "decnet.target"]
+    """Write *rendered* content to *dst* only if it differs from what's there.
+
+    SHA compares rendered-output ↔ on-disk bytes (NOT source-template ↔
+    on-disk) so operators who customise their install_dir get idempotent
+    re-runs instead of every ``decnet init`` rewriting files.
+    """
+    rendered_bytes = rendered.encode("utf-8")
+    if dst.exists() and not force:
+        if hashlib.sha256(dst.read_bytes()).hexdigest() == hashlib.sha256(rendered_bytes).hexdigest():
+            return f"skip: {dst} up to date"
+    if dry_run:
+        console.print(f"  [dim]would render:[/] {src} -> {dst} (mode={oct(mode)})")
+        return "ok"
+    dst.parent.mkdir(parents=True, exist_ok=True)
+    dst.write_bytes(rendered_bytes)
+    try:
+        os.chmod(dst, mode)
+        os.chown(dst, 0, 0)
+    except PermissionError:
+        pass
+    return "ok"
+
+
+def _install_units(
+    deploy: Path, systemd_dir: Path, *, install_dir: str, force: bool, dry_run: bool
+) -> str:
+    """Render decnet-*.service.j2 → systemd_dir/decnet-*.service, and copy
+    the static decnet.target (no templating needed — it has no install
+    path references)."""
+    context = {"install_dir": install_dir}
+    templates = sorted(deploy.glob("decnet-*.service.j2"))
+    static = [deploy / "decnet.target"]
+
     touched = 0
-    for src in sources:
+    for src in templates:
+        rendered = _render_template(src, context)
+        # decnet-api.service.j2 → decnet-api.service
+        dst_name = src.name[: -len(".j2")]
+        result = _write_rendered_if_changed(
+            src, systemd_dir / dst_name, rendered,
+            mode=0o644, force=force, dry_run=dry_run,
+        )
+        if not result.startswith("skip:"):
+            touched += 1
+    for src in static:
         result = _copy_if_changed(
             src, systemd_dir / src.name,
             mode=0o644, force=force, dry_run=dry_run,
         )
         if not result.startswith("skip:"):
             touched += 1
-    total = len(sources)
+    total = len(templates) + len(static)
     if touched == 0:
         return f"skip: {total} unit files up to date"
     return f"ok ({touched}/{total} installed)"
@@ -335,6 +395,14 @@ def register(app: typer.Typer) -> None:
             "decnet", "--group",
             help="Primary group of the DECNET user.",
         ),
+        install_dir: str = typer.Option(
+            "/opt/decnet", "--install-dir",
+            help="Absolute path where DECNET is installed. Default "
+                 "/opt/decnet; distros that reserve /opt can point this "
+                 "at /srv/decnet, /usr/local/decnet, etc. Gets rendered "
+                 "into every systemd unit via Jinja2 and used as the "
+                 "decnet user's home directory.",
+        ),
         prefix: str = typer.Option(
             "", "--prefix", hidden=True,
             help="Filesystem prefix for tests (e.g. tmp_path). Empty = real root.",
@@ -358,6 +426,15 @@ def register(app: typer.Typer) -> None:
             console.print(f"[red]decnet {verb}: must run as root (use sudo)[/]")
             raise typer.Exit(1)
 
+        if not install_dir.startswith("/"):
+            console.print(
+                f"[red]decnet init: --install-dir must be absolute, got {install_dir!r}[/]"
+            )
+            raise typer.Exit(1)
+        # Strip leading slash so pfx-joining works under --prefix test mode
+        # (Path("/").  / "/opt/decnet" == Path("/opt/decnet"), dropping pfx).
+        _install_rel = install_dir.lstrip("/")
+
         required_tools = ("systemctl",) if deinit else (
             "systemctl", "useradd", "groupadd", "systemd-tmpfiles",
         )
@@ -424,9 +501,9 @@ def register(app: typer.Typer) -> None:
                 ),
             )
             _step(
-                f"remove {pfx / 'opt/decnet'}",
+                f"remove {pfx / _install_rel}",
                 lambda: _remove_dir_if_present(
-                    pfx / "opt/decnet", dry_run=dry_run,
+                    pfx / _install_rel, dry_run=dry_run,
                 ),
             )
             if purge:
@@ -468,7 +545,7 @@ def register(app: typer.Typer) -> None:
             raise typer.Exit(1) from exc
 
         dirs = [
-            (pfx / "opt/decnet", 0o755, user, group),
+            (pfx / _install_rel, 0o755, user, group),
             (pfx / "var/lib/decnet", 0o750, user, group),
             (pfx / "var/log/decnet", 0o750, user, group),
             (etc_decnet, 0o755, "root", group),
@@ -486,7 +563,7 @@ def register(app: typer.Typer) -> None:
         )
         _step(
             f"ensure user {user!r}",
-            lambda: _ensure_user(user, group, dry_run=dry_run),
+            lambda: _ensure_user(user, group, install_dir, dry_run=dry_run),
         )
         for path, mode, d_owner, d_group in dirs:
             _step(
@@ -501,7 +578,8 @@ def register(app: typer.Typer) -> None:
         _step(
             "install systemd units",
             lambda: _install_units(
-                deploy, systemd_dir, force=force, dry_run=dry_run,
+                deploy, systemd_dir,
+                install_dir=install_dir, force=force, dry_run=dry_run,
             ),
         )
         _step(

deploy/decnet-agent.service.j2

@@ -11,9 +11,9 @@ User=decnet
 Group=decnet
 # docker.sock is group-readable by 'docker'; the agent needs it for compose.
 SupplementaryGroups=docker
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
-ExecStart=/opt/decnet/venv/bin/decnet agent --host 0.0.0.0 --port 8765 --agent-dir /etc/decnet/agent
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
+ExecStart={{ install_dir }}/venv/bin/decnet agent --host 0.0.0.0 --port 8765 --agent-dir /etc/decnet/agent
 
 # MACVLAN/IPVLAN management + scapy raw sockets. Granted via ambient caps so
 # the process starts unprivileged and keeps only these two bits.
@@ -30,8 +30,8 @@ ProtectKernelModules=yes
 ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 LockPersonality=yes
-# /opt/decnet holds release slots + state; the agent reads them and writes its PID.
-ReadWritePaths=/opt/decnet /var/log/decnet
+# {{ install_dir }} holds release slots + state; the agent reads them and writes its PID.
+ReadWritePaths={{ install_dir }} /var/log/decnet
 
 Restart=on-failure
 RestartSec=5

deploy/decnet-api.service.j2

@@ -11,9 +11,9 @@ User=decnet
 Group=decnet
 # docker.sock is group-readable by 'docker'; the API ingester tails container logs.
 SupplementaryGroups=docker
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
-ExecStart=/opt/decnet/venv/bin/decnet api
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
+ExecStart={{ install_dir }}/venv/bin/decnet api
 
 # MACVLAN/IPVLAN setup runs from the API lifespan when the embedded sniffer is on.
 CapabilityBoundingSet=CAP_NET_ADMIN CAP_NET_RAW
@@ -29,7 +29,7 @@ ProtectKernelModules=yes
 ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 LockPersonality=yes
-ReadWritePaths=/opt/decnet /var/log/decnet
+ReadWritePaths={{ install_dir }} /var/log/decnet
 
 Restart=on-failure
 RestartSec=5

deploy/decnet-bus.service.j2

@@ -8,15 +8,15 @@ Wants=network-online.target
 Type=simple
 User=decnet
 Group=decnet
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
 # /run/decnet is created automatically with the RuntimeDirectory= directive
 # below (mode 0755, owned by User/Group) and cleaned up on stop. The bus
 # socket is placed inside it with 0660 perms so only the decnet group can
 # connect.
 RuntimeDirectory=decnet
 RuntimeDirectoryMode=0755
-ExecStart=/opt/decnet/venv/bin/decnet bus \
+ExecStart={{ install_dir }}/venv/bin/decnet bus \
     --socket /run/decnet/bus.sock \
     --group decnet
 

deploy/decnet-collector.service.j2

@@ -11,9 +11,9 @@ User=decnet
 Group=decnet
 # docker.sock is group-readable by 'docker'; the collector tails container logs.
 SupplementaryGroups=docker
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
-ExecStart=/opt/decnet/venv/bin/decnet collect
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
+ExecStart={{ install_dir }}/venv/bin/decnet collect
 
 # No privileged network operations.
 CapabilityBoundingSet=
@@ -29,7 +29,7 @@ ProtectKernelModules=yes
 ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 LockPersonality=yes
-ReadWritePaths=/opt/decnet /var/log/decnet
+ReadWritePaths={{ install_dir }} /var/log/decnet
 
 Restart=on-failure
 RestartSec=5

deploy/decnet-forwarder.service.j2

@@ -10,12 +10,12 @@ Wants=network-online.target
 Type=simple
 User=decnet
 Group=decnet
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
 # Replace <master-host> with the master's LAN address or hostname. The agent
 # cert bundle at /etc/decnet/agent is reused — the forwarder presents the same
 # worker identity when it connects to the master's listener.
-ExecStart=/opt/decnet/venv/bin/decnet forwarder \
+ExecStart={{ install_dir }}/venv/bin/decnet forwarder \
     --log-file /var/log/decnet/decnet.log \
     --master-host ${DECNET_SWARM_MASTER_HOST} \
     --master-port 6514 \

deploy/decnet-listener.service.j2

@@ -8,11 +8,11 @@ Wants=network-online.target
 Type=simple
 User=decnet
 Group=decnet
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
 # Binds 0.0.0.0:6514 so workers across the LAN can connect. 6514 is not a
 # privileged port (≥1024), so no CAP_NET_BIND_SERVICE is required.
-ExecStart=/opt/decnet/venv/bin/decnet listener \
+ExecStart={{ install_dir }}/venv/bin/decnet listener \
     --host 0.0.0.0 --port 6514 \
     --ca-dir /etc/decnet/ca \
     --log-path /var/log/decnet/master.log \

deploy/decnet-mutator.service.j2

@@ -11,9 +11,9 @@ User=decnet
 Group=decnet
 # Mutator recomposes decky services via docker compose.
 SupplementaryGroups=docker
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
-ExecStart=/opt/decnet/venv/bin/decnet mutate --watch
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
+ExecStart={{ install_dir }}/venv/bin/decnet mutate --watch
 
 CapabilityBoundingSet=
 AmbientCapabilities=
@@ -28,7 +28,7 @@ ProtectKernelModules=yes
 ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 LockPersonality=yes
-ReadWritePaths=/opt/decnet /var/log/decnet
+ReadWritePaths={{ install_dir }} /var/log/decnet
 
 Restart=on-failure
 RestartSec=5

deploy/decnet-prober.service.j2

@@ -8,9 +8,9 @@ Wants=network-online.target decnet-bus.service
 Type=simple
 User=decnet
 Group=decnet
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
-ExecStart=/opt/decnet/venv/bin/decnet probe
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
+ExecStart={{ install_dir }}/venv/bin/decnet probe
 
 # TCP connect probes only — no raw sockets required.
 CapabilityBoundingSet=
@@ -26,7 +26,7 @@ ProtectKernelModules=yes
 ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 LockPersonality=yes
-ReadWritePaths=/opt/decnet /var/log/decnet
+ReadWritePaths={{ install_dir }} /var/log/decnet
 
 Restart=on-failure
 RestartSec=5

deploy/decnet-profiler.service.j2

@@ -8,9 +8,9 @@ Wants=network-online.target decnet-bus.service
 Type=simple
 User=decnet
 Group=decnet
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
-ExecStart=/opt/decnet/venv/bin/decnet profiler
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
+ExecStart={{ install_dir }}/venv/bin/decnet profiler
 
 CapabilityBoundingSet=
 AmbientCapabilities=
@@ -25,7 +25,7 @@ ProtectKernelModules=yes
 ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 LockPersonality=yes
-ReadWritePaths=/opt/decnet /var/log/decnet
+ReadWritePaths={{ install_dir }} /var/log/decnet
 
 Restart=on-failure
 RestartSec=5

deploy/decnet-sniffer.service.j2

@@ -8,9 +8,9 @@ Wants=network-online.target decnet-bus.service
 Type=simple
 User=decnet
 Group=decnet
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
-ExecStart=/opt/decnet/venv/bin/decnet sniffer
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
+ExecStart={{ install_dir }}/venv/bin/decnet sniffer
 
 # scapy needs raw packet access on the MACVLAN host interface.
 CapabilityBoundingSet=CAP_NET_RAW
@@ -26,7 +26,7 @@ ProtectKernelModules=yes
 ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 LockPersonality=yes
-ReadWritePaths=/opt/decnet /var/log/decnet
+ReadWritePaths={{ install_dir }} /var/log/decnet
 
 Restart=on-failure
 RestartSec=5

deploy/decnet-swarmctl.service.j2

@@ -8,11 +8,11 @@ Wants=network-online.target
 Type=simple
 User=decnet
 Group=decnet
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
 # Default bind is loopback — the controller is a master-local orchestrator
 # reached by the CLI and the web dashboard, not by workers.
-ExecStart=/opt/decnet/venv/bin/decnet swarmctl --host 127.0.0.1 --port 8770
+ExecStart={{ install_dir }}/venv/bin/decnet swarmctl --host 127.0.0.1 --port 8770
 
 # No special capabilities — the controller issues mTLS certs and talks to
 # workers over TCP on unprivileged ports.
@@ -30,7 +30,7 @@ ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 LockPersonality=yes
 # Reads/writes the CA bundle and the master DB.
-ReadWritePaths=/opt/decnet /var/log/decnet
+ReadWritePaths={{ install_dir }} /var/log/decnet
 ReadOnlyPaths=/etc/decnet
 
 Restart=on-failure

deploy/decnet-updater.service.j2

@@ -10,12 +10,12 @@ Wants=network-online.target
 Type=simple
 User=decnet
 Group=decnet
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
-ExecStart=/opt/decnet/venv/bin/decnet updater \
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
+ExecStart={{ install_dir }}/venv/bin/decnet updater \
     --host 0.0.0.0 --port 8766 \
     --updater-dir /etc/decnet/updater \
-    --install-dir /opt/decnet \
+    --install-dir {{ install_dir }} \
     --agent-dir /etc/decnet/agent
 
 # The updater SIGTERMs the agent and spawns a new one. Same User=decnet means
@@ -37,7 +37,7 @@ ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 LockPersonality=yes
 # Writes release slots, pip installs into venv, manages agent.pid.
-ReadWritePaths=/opt/decnet /var/log/decnet
+ReadWritePaths={{ install_dir }} /var/log/decnet
 
 Restart=on-failure
 RestartSec=5

deploy/decnet-web.service.j2

@@ -8,9 +8,9 @@ Wants=network-online.target
 Type=simple
 User=decnet
 Group=decnet
-WorkingDirectory=/opt/decnet
-EnvironmentFile=-/opt/decnet/.env.local
-ExecStart=/opt/decnet/venv/bin/decnet web
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
+ExecStart={{ install_dir }}/venv/bin/decnet web
 
 # Uncomment if you bind the dashboard to a privileged port (80/443):
 # CapabilityBoundingSet=CAP_NET_BIND_SERVICE
@@ -28,7 +28,7 @@ ProtectKernelModules=yes
 ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 LockPersonality=yes
-ReadWritePaths=/opt/decnet /var/log/decnet
+ReadWritePaths={{ install_dir }} /var/log/decnet
 ReadOnlyPaths=/etc/decnet
 
 Restart=on-failure

tests/cli/test_init.py

@@ -60,12 +60,22 @@ def missing_user_and_group(monkeypatch: Any) -> None:
 
 
 def _seed_deploy(monkeypatch: Any, tmp_path: Path) -> Path:
-    """Point `_deploy_root()` at a faked deploy tree under tmp_path."""
+    """Point `_deploy_root()` at a faked deploy tree under tmp_path.
+
+    Services are Jinja2 templates keyed on ``{{ install_dir }}`` —
+    matching production layout since the refactor that made install
+    path configurable.
+    """
     deploy = tmp_path / "deploy"
     (deploy / "polkit").mkdir(parents=True)
     (deploy / "tmpfiles.d").mkdir()
-    (deploy / "decnet-bus.service").write_text("# bus unit\n")
-    (deploy / "decnet-api.service").write_text("# api unit\n")
+    (deploy / "decnet-bus.service.j2").write_text(
+        "[Service]\nExecStart={{ install_dir }}/venv/bin/decnet bus\n"
+    )
+    (deploy / "decnet-api.service.j2").write_text(
+        "[Service]\nWorkingDirectory={{ install_dir }}\n"
+        "ExecStart={{ install_dir }}/venv/bin/decnet api\n"
+    )
     (deploy / "decnet.target").write_text("# target\n")
     (deploy / "polkit" / "50-decnet-workers.rules").write_text("// rule\n")
     (deploy / "tmpfiles.d" / "decnet.conf").write_text("d /run/decnet\n")
@@ -156,6 +166,105 @@ def test_unit_files_are_installed_then_idempotent(
     assert "unit files up to date" in r2.output
 
 
+def test_install_dir_renders_into_service_units(
+    monkeypatch: Any, tmp_path: Path, subprocess_calls: List[List[str]],
+    no_missing_tools: None, missing_user_and_group: None,
+) -> None:
+    """`--install-dir /srv/decnet` must land in the rendered service
+    files. Regression guard for the Jinja2 templating refactor."""
+    _seed_deploy(monkeypatch, tmp_path)
+    prefix = tmp_path / "root"
+    r = runner.invoke(
+        app,
+        [
+            "init", "--no-start",
+            "--prefix", str(prefix),
+            "--install-dir", "/srv/decnet",
+        ],
+    )
+    assert r.exit_code == 0, r.output
+
+    api_unit = prefix / "etc/systemd/system" / "decnet-api.service"
+    bus_unit = prefix / "etc/systemd/system" / "decnet-bus.service"
+    assert api_unit.is_file()
+    api_text = api_unit.read_text()
+    assert "/srv/decnet" in api_text
+    assert "/opt/decnet" not in api_text
+    assert "{{" not in api_text, "unrendered Jinja tag leaked through"
+    assert "/srv/decnet" in bus_unit.read_text()
+
+    # useradd --home-dir must match the install_dir override too.
+    useradds = [c for c in subprocess_calls if c and c[0] == "useradd"]
+    assert useradds, "expected useradd call"
+    assert "/srv/decnet" in useradds[0]
+    assert "/opt/decnet" not in useradds[0]
+
+    # And /srv/decnet on disk should be the dir we created.
+    assert (prefix / "srv/decnet").is_dir()
+    assert not (prefix / "opt/decnet").exists()
+
+
+def test_install_dir_defaults_to_opt(
+    monkeypatch: Any, tmp_path: Path, subprocess_calls: List[List[str]],
+    no_missing_tools: None, present_user_and_group: None,
+) -> None:
+    """Default --install-dir is /opt/decnet — existing installs remain
+    byte-identical with no explicit flag."""
+    _seed_deploy(monkeypatch, tmp_path)
+    prefix = tmp_path / "root"
+    r = runner.invoke(app, ["init", "--no-start", "--prefix", str(prefix)])
+    assert r.exit_code == 0, r.output
+    api_unit = prefix / "etc/systemd/system" / "decnet-api.service"
+    assert "/opt/decnet" in api_unit.read_text()
+
+
+def test_install_dir_rejects_relative_path(
+    monkeypatch: Any, tmp_path: Path,
+    no_missing_tools: None, missing_user_and_group: None,
+) -> None:
+    """Relative install_dir would break every absolute path in a
+    rendered service. Reject at the CLI boundary with a clear message."""
+    _seed_deploy(monkeypatch, tmp_path)
+    r = runner.invoke(
+        app,
+        [
+            "init", "--no-start",
+            "--prefix", str(tmp_path / "root"),
+            "--install-dir", "relative/path",
+        ],
+    )
+    assert r.exit_code == 1
+    assert "must be absolute" in r.output
+
+
+def test_install_dir_custom_idempotent_second_run(
+    monkeypatch: Any, tmp_path: Path, subprocess_calls: List[List[str]],
+    no_missing_tools: None, present_user_and_group: None,
+) -> None:
+    """Rendering the same templates twice with the same context must
+    produce byte-identical output — second run SKIPs, no churn."""
+    _seed_deploy(monkeypatch, tmp_path)
+    prefix = tmp_path / "root"
+    runner.invoke(
+        app,
+        [
+            "init", "--no-start",
+            "--prefix", str(prefix),
+            "--install-dir", "/srv/decnet",
+        ],
+    )
+    r2 = runner.invoke(
+        app,
+        [
+            "init", "--no-start",
+            "--prefix", str(prefix),
+            "--install-dir", "/srv/decnet",
+        ],
+    )
+    assert r2.exit_code == 0, r2.output
+    assert "unit files up to date" in r2.output
+
+
 def test_force_overwrites_existing_units(
     monkeypatch: Any, tmp_path: Path, subprocess_calls: List[List[str]],
     no_missing_tools: None, present_user_and_group: None,