diff --git a/bin/builder_db.ml b/bin/builder_db.ml
index 0232d6c..af5682c 100644
--- a/bin/builder_db.ml
+++ b/bin/builder_db.ml
@@ -320,35 +320,22 @@ let help man_format cmds = function
     then `Help (man_format, Some cmd)
     else `Error (true, "Unknown command: " ^ cmd)
 
-let uname =
-  let cmd = Bos.Cmd.(v "uname" % "-s") in
-  lazy (match Bos.OS.Cmd.(run_out cmd |> out_string |> success) with
-      | Ok s when s = "FreeBSD" -> `FreeBSD
-      | Ok s when s = "Linux" -> `Linux
-      | Ok s -> invalid_arg (Printf.sprintf "OS %s not supported\n" s)
-      | Error (`Msg m) -> invalid_arg m)
-
-let default_datadir =
-  match Lazy.force uname with
-  | `FreeBSD -> "/var/db/builder-web"
-  | `Linux -> "/var/lib/builder-web"
-
 let dbpath =
   let doc = "sqlite3 database path" in
   Cmdliner.Arg.(value &
-                opt non_dir_file (default_datadir ^ "/builder.sqlite3") &
+                opt non_dir_file (Builder_system.default_datadir ^ "/builder.sqlite3") &
                 info ~doc ["dbpath"])
 
 let dbpath_new =
   let doc = "sqlite3 database path" in
   Cmdliner.Arg.(value &
-                opt string (default_datadir ^ "/builder.sqlite3") &
+                opt string (Builder_system.default_datadir ^ "/builder.sqlite3") &
                 info ~doc ["dbpath"])
 
 let datadir =
   let doc = "data directory" in
   Cmdliner.Arg.(value &
-                opt dir default_datadir &
+                opt dir Builder_system.default_datadir &
                 info ~doc ["datadir"])
 
 let jobname =
diff --git a/bin/builder_system.ml b/bin/builder_system.ml
new file mode 100644
index 0000000..551ca84
--- /dev/null
+++ b/bin/builder_system.ml
@@ -0,0 +1,12 @@
+let uname =
+  let cmd = Bos.Cmd.(v "uname" % "-s") in
+  lazy (match Bos.OS.Cmd.(run_out cmd |> out_string |> success) with
+      | Ok s when s = "FreeBSD" -> `FreeBSD
+      | Ok s when s = "Linux" -> `Linux
+      | Ok s -> invalid_arg (Printf.sprintf "OS %s not supported" s)
+      | Error (`Msg m) -> invalid_arg m)
+
+let default_datadir =
+  match Lazy.force uname with
+  | `FreeBSD -> "/var/db/builder-web"
+  | `Linux -> "/var/lib/builder-web"
diff --git a/bin/builder_web_app.ml b/bin/builder_web_app.ml
index 37829bc..7602e87 100644
--- a/bin/builder_web_app.ml
+++ b/bin/builder_web_app.ml
@@ -120,22 +120,9 @@ let ip_port : (Ipaddr.V4.t * int) Arg.converter =
   in
   parse, fun ppf (ip, port) -> Format.fprintf ppf "%a:%d" Ipaddr.V4.pp ip port
 
-let uname =
-  let cmd = Bos.Cmd.(v "uname" % "-s") in
-  lazy (match Bos.OS.Cmd.(run_out cmd |> out_string |> success) with
-      | Ok s when s = "FreeBSD" -> `FreeBSD
-      | Ok s when s = "Linux" -> `Linux
-      | Ok s -> invalid_arg (Printf.sprintf "OS %s not supported" s)
-      | Error (`Msg m) -> invalid_arg m)
-
-let default_datadir =
-  match Lazy.force uname with
-  | `FreeBSD -> "/var/db/builder-web"
-  | `Linux -> "/var/lib/builder-web"
-
 let datadir =
   let doc = "data directory" in
-  Arg.(value & opt dir default_datadir & info [ "d"; "datadir" ] ~doc)
+  Arg.(value & opt dir Builder_system.default_datadir & info [ "d"; "datadir" ] ~doc)
 
 let port =
   let doc = "port" in
diff --git a/bin/dune b/bin/dune
index 2eee321..04a7bfc 100644
--- a/bin/dune
+++ b/bin/dune
@@ -1,11 +1,16 @@
+(library
+ (name builder_system)
+ (modules builder_system)
+ (libraries bos))
+
 (executable
  (public_name builder-web)
  (name builder_web_app)
  (modules builder_web_app)
- (libraries builder_web mirage-crypto-rng.unix cmdliner logs.cli metrics metrics-lwt metrics-influx metrics-rusage ipaddr ipaddr.unix http_status_metrics))
+ (libraries builder_web builder_system mirage-crypto-rng.unix cmdliner logs.cli metrics metrics-lwt metrics-influx metrics-rusage ipaddr ipaddr.unix http_status_metrics))
 
 (executable
  (public_name builder-db)
  (name builder_db)
  (modules builder_db)
- (libraries builder_db caqti.blocking uri bos fmt logs logs.cli logs.fmt cmdliner mirage-crypto-rng.unix))
+ (libraries builder_db builder_system caqti.blocking uri bos fmt logs logs.cli logs.fmt cmdliner mirage-crypto-rng.unix))
diff --git a/bin/migrations/builder_migrations.ml b/bin/migrations/builder_migrations.ml
index ab83d49..e1b713d 100644
--- a/bin/migrations/builder_migrations.ml
+++ b/bin/migrations/builder_migrations.ml
@@ -59,23 +59,10 @@ let help man_format migrations = function
     then `Help (man_format, Some migration)
     else `Error (true, "Unknown migration: " ^ migration)
 
-let uname =
-  let cmd = Bos.Cmd.(v "uname" % "-s") in
-  lazy (match Bos.OS.Cmd.(run_out cmd |> out_string |> success) with
-      | Ok s when s = "FreeBSD" -> `FreeBSD
-      | Ok s when s = "Linux" -> `Linux
-      | Ok s -> invalid_arg (Printf.sprintf "OS %s not supported" s)
-      | Error (`Msg m) -> invalid_arg m)
-
-let default_datadir =
-  match Lazy.force uname with
-  | `FreeBSD -> "/var/db/builder-web"
-  | `Linux -> "/var/lib/builder-web"
-
 let datadir =
   let doc = "data directory containing builder.sqlite3 and data files" in
   Cmdliner.Arg.(value &
-                opt dir default_datadir &
+                opt dir Builder_system.default_datadir &
                 info ~doc ["datadir"])
 
 let setup_log =
diff --git a/bin/migrations/dune b/bin/migrations/dune
index 9150bfb..b27283f 100644
--- a/bin/migrations/dune
+++ b/bin/migrations/dune
@@ -1,4 +1,4 @@
 (executable
  (public_name builder-migrations)
  (name builder_migrations)
- (libraries builder_db caqti caqti-driver-sqlite3 caqti.blocking cmdliner logs logs.cli logs.fmt opam-format bos duration))
+ (libraries builder_system builder_db caqti caqti-driver-sqlite3 caqti.blocking cmdliner logs logs.cli logs.fmt opam-format bos duration))
diff --git a/packaging/debian/builder-web.service b/packaging/debian/builder-web.service
index 7ac1d3c..2835d92 100644
--- a/packaging/debian/builder-web.service
+++ b/packaging/debian/builder-web.service
@@ -6,7 +6,7 @@ After=syslog.target network.target
 Type=simple
 User=builder
 Group=builder
-ExecStart=/usr/bin/builder-web --datadir /var/lib/builder-web/
+ExecStart=/usr/bin/builder-web
 
 [Install]
 WantedBy=multi-user.target