diff --git a/BUILDING-LINUX.md b/BUILDING-LINUX.md
new file mode 100644
index 0000000..c26fe5d
--- /dev/null
+++ b/BUILDING-LINUX.md
@@ -0,0 +1,112 @@
+# Building OpsLog on Linux
+
+OpsLog is developed on Windows. The Linux build shares every line of the
+frontend and all but a handful of Go files; what differs is listed at the bottom
+of this page.
+
+**It cannot be cross-compiled from Windows.** Wails links against the system
+WebKit on Linux, which needs cgo and the GTK/WebKit headers, so the binary has
+to be produced on a Linux machine (or a container). What *can* be checked from
+Windows — and is, at every release — is that the Go half still compiles:
+
+```bash
+GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build ./...
+GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go vet ./...
+```
+
+## The short way
+
+```bash
+./scripts/linux-setup.sh
+```
+
+It checks everything below, prints the one install command your distribution
+needs if something is missing, and builds when nothing is. The rest of this page
+is what it checks, for when you would rather do it by hand.
+
+## Dependencies
+
+```bash
+# Debian / Ubuntu
+sudo apt install build-essential pkg-config libgtk-3-dev libwebkit2gtk-4.1-dev nodejs npm
+
+# Fedora
+sudo dnf install gcc-c++ pkgconf-pkg-config gtk3-devel webkit2gtk4.1-devel nodejs npm
+
+# Arch
+sudo pacman -S base-devel pkgconf gtk3 webkit2gtk-4.1 nodejs npm
+```
+
+**Go and node do not come from the package manager.** No current distribution
+ships a Go new enough for `go.mod` (Ubuntu 24.04 / Mint 22 have 1.22, Ubuntu
+22.04 / Mint 21 have 1.18), and Ubuntu 22.04 / Mint 21 ship node 12 where Vite
+needs 18. Both are the usual reason a first build fails with an error that
+points somewhere else entirely:
+
+```bash
+# Go, from go.dev
+wget https://go.dev/dl/go1.25.1.linux-amd64.tar.gz
+sudo rm -rf /usr/local/go && sudo tar -C /usr/local -xzf go1.25.1.linux-amd64.tar.gz
+echo 'export PATH=/usr/local/go/bin:$HOME/go/bin:$PATH' >> ~/.profile   # log out and back in
+
+# node 20, only if `node -v` is below 18
+curl -fsSL https://deb.nodesource.com/setup_20.x | sudo -E bash - && sudo apt install nodejs
+```
+
+Then the Wails CLI:
+
+```bash
+go install github.com/wailsapp/wails/v2/cmd/wails@v2.11.0
+wails doctor          # says what is still missing
+```
+
+`libwebkit2gtk-4.0` also works; pass `-tags webkit2_40` to `wails build` if your
+distribution only has the older one.
+
+## Build
+
+```bash
+wails build           # → build/bin/OpsLog
+./build/bin/OpsLog
+```
+
+`wails dev` works the same as on Windows.
+
+## Runtime requirements
+
+- **PulseAudio or PipeWire** for the voice keyer, the QSO recorder and the CW
+  decoder. PipeWire is fine — OpsLog speaks the PulseAudio protocol, which
+  `pipewire-pulse` answers. Without a sound server those three features report
+  "cannot reach the sound server" and everything else works normally.
+- **Serial port access** for CAT, keyers, rotators and amplifiers. Ports appear
+  as `/dev/ttyUSB0`, `/dev/ttyACM0`… and on most distributions belong to the
+  `dialout` group:
+
+  ```bash
+  sudo usermod -aG dialout $USER      # log out and back in
+  ```
+
+  This is the single most common reason a rig that works in WSJT-X shows
+  "permission denied" in OpsLog.
+- **TrustedQSL** (`tqsl`) for LoTW uploads, from your package manager. OpsLog
+  finds it on `PATH`.
+
+## Where OpsLog keeps its data
+
+Next to the binary, in `data/` — the same portable layout as on Windows, so a
+folder in your home directory carries the logbook with it.
+
+If the binary sits somewhere you cannot write (`/usr/bin`, `/opt`), OpsLog uses
+`~/.local/share/OpsLog/data` instead and says so in `startup.log`. The startup
+log itself lives in `~/.cache/OpsLog/startup.log`.
+
+## What is different from the Windows build
+
+| | |
+|---|---|
+| **OmniRig** | Not available — it is Windows COM automation. Use a native backend instead: Icom CI-V (USB and network), Yaesu, Kenwood/Elecraft, FlexRadio, TCI, Xiegu. |
+| **Denkovi USB relay** | Not available — it needs FTDI's `ftd2xx.dll`. The other relay backends work. |
+| **Audio** | PulseAudio/PipeWire instead of WASAPI. Same devices, same fixed 16 kHz mono format. |
+| **Auto-update** | Works, and is simpler: Linux lets a running binary be replaced, so none of the Windows deferred-swap machinery is needed. |
+| **Window placement** | OpsLog cannot read the monitor layout, so a saved window position is always trusted rather than clamped onto a visible screen. |
+| **Single instance** | An `flock` on `$XDG_RUNTIME_DIR/OpsLog/instance.lock` instead of a named mutex. It cannot raise the existing window, only refuse to start a second one. |
diff --git a/app.go b/app.go
index 221942e..2d15511 100644
--- a/app.go
+++ b/app.go
@@ -2035,15 +2035,24 @@ func (a *App) shutdown(ctx context.Context) {
 	applog.Printf("shutdown: teardown done")
 }
 
-// userDataDir returns the OpsLog data directory: always "<exe dir>/data".
+// userDataDir returns the OpsLog data directory: "<exe dir>/data".
 // All data (database, settings, cty.dat, logs) travels with the executable,
 // making OpsLog fully portable for USB sticks and PC migrations.
+//
+// systemInstallDataDir is the one exception, and it never fires on Windows: a
+// Linux build installed system-wide (/usr/bin, /opt) sits in a folder no user
+// may write, so there the data moves to ~/.local/share/OpsLog. See
+// datadir_linux.go for why that is decided by trying rather than by path.
 func userDataDir() (string, error) {
 	exe, err := os.Executable()
 	if err != nil {
 		return "", fmt.Errorf("cannot locate executable: %w", err)
 	}
-	return filepath.Join(filepath.Dir(exe), "data"), nil
+	beside := filepath.Join(filepath.Dir(exe), "data")
+	if alt, ok := systemInstallDataDir(beside); ok {
+		return alt, nil
+	}
+	return beside, nil
 }
 
 // fileExists reports whether path exists and is a regular file.
@@ -19893,7 +19902,7 @@ func tidySerialPorts(ports []string) []string {
 		}
 		key := strings.ToUpper(name)
 		if seen[key] {
-			applog.Printf("serial: %s is claimed by more than one device in the Windows port map — listing it once", name)
+			applog.Printf("serial: %s is claimed by more than one device in the system port map — listing it once", name)
 			continue
 		}
 		seen[key] = true
@@ -19910,11 +19919,41 @@ func tidySerialPorts(ports []string) []string {
 		case okj:
 			return false
 		}
-		return out[i] < out[j]
+		return naturalLess(out[i], out[j])
 	})
 	return out
 }
 
+// naturalLess orders names that end in a number by that number, so
+// /dev/ttyUSB9 comes before /dev/ttyUSB10. The same trap as COM4/COM10 above,
+// met on Linux where the port names are paths rather than COMn — a rig on
+// ttyUSB10 listed between ttyUSB1 and ttyUSB2 is a rig the operator scrolls
+// past.
+func naturalLess(a, b string) bool {
+	pa, na, oka := trailingNumber(a)
+	pb, nb, okb := trailingNumber(b)
+	if oka && okb && pa == pb {
+		return na < nb
+	}
+	return a < b
+}
+
+// trailingNumber splits "name123" into "name" and 123.
+func trailingNumber(s string) (string, int, bool) {
+	i := len(s)
+	for i > 0 && s[i-1] >= '0' && s[i-1] <= '9' {
+		i--
+	}
+	if i == len(s) || i == 0 {
+		return s, 0, false
+	}
+	n, err := strconv.Atoi(s[i:])
+	if err != nil {
+		return s, 0, false
+	}
+	return s[:i], n, true
+}
+
 // comPortNumber extracts n from "COMn", false for any other shape.
 func comPortNumber(s string) (int, bool) {
 	if len(s) <= 3 || !strings.EqualFold(s[:3], "COM") {
diff --git a/autostart.go b/autostart.go
index 4486037..1087e17 100644
--- a/autostart.go
+++ b/autostart.go
@@ -6,10 +6,8 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
-	"strconv"
 	"strings"
 	"sync"
-	"syscall"
 
 	"hamlog/internal/applog"
 
@@ -75,11 +73,8 @@ func (a *App) SaveAutostartPrograms(progs []AutostartProgram) error {
 // BrowseExecutable opens a native file picker for choosing a program to launch.
 func (a *App) BrowseExecutable() (string, error) {
 	return wruntime.OpenFileDialog(a.ctx, wruntime.OpenDialogOptions{
-		Title: "Choose a program to launch on startup",
-		Filters: []wruntime.FileFilter{
-			{DisplayName: "Programs (*.exe;*.bat;*.cmd)", Pattern: "*.exe;*.bat;*.cmd"},
-			{DisplayName: "All files (*.*)", Pattern: "*.*"},
-		},
+		Title:   "Choose a program to launch on startup",
+		Filters: executableFilters(),
 	})
 }
 
@@ -150,9 +145,7 @@ func (a *App) CloseAutostartPrograms() {
 		if name == "" {
 			name = filepath.Base(p.Path)
 		}
-		cmd := exec.Command("taskkill", "/PID", strconv.Itoa(pid))
-		cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true, CreationFlags: 0x08000000}
-		if out, err := cmd.CombinedOutput(); err != nil {
+		if out, err := closeProcess(pid); err != nil {
 			applog.Printf("autostart: could not close %s (pid %d): %v — %s", name, pid, err, strings.TrimSpace(string(out)))
 			continue
 		}
@@ -227,33 +220,3 @@ func splitArgs(s string) []string {
 	}
 	return args
 }
-
-// runningProcessNames returns the set of lowercase executable names currently
-// running, via the Windows `tasklist`. Best effort — on failure the set is
-// empty (we then just attempt to launch, which is acceptable).
-func runningProcessNames() map[string]bool {
-	out := map[string]bool{}
-	cmd := exec.Command("tasklist", "/FO", "CSV", "/NH")
-	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true, CreationFlags: 0x08000000} // CREATE_NO_WINDOW
-	data, err := cmd.Output()
-	if err != nil {
-		applog.Printf("autostart: tasklist failed: %v", err)
-		return out
-	}
-	for _, line := range strings.Split(string(data), "\n") {
-		line = strings.TrimSpace(line)
-		if line == "" {
-			continue
-		}
-		// CSV row: "image.exe","PID",... — take the first quoted field.
-		field := line
-		if i := strings.Index(line[1:], "\""); i >= 0 && strings.HasPrefix(line, "\"") {
-			field = line[1 : i+1]
-		}
-		field = strings.Trim(field, "\"")
-		if field != "" {
-			out[strings.ToLower(field)] = true
-		}
-	}
-	return out
-}
diff --git a/bootlog.go b/bootlog.go
index c612359..db3400f 100644
--- a/bootlog.go
+++ b/bootlog.go
@@ -23,12 +23,9 @@ import (
 	"time"
 )
 
-// bootLogPath is the file, or "" when even LOCALAPPDATA is unavailable.
+// bootLogPath is the file, or "" when no writable folder can be found at all.
 func bootLogPath() string {
-	dir := os.Getenv("LOCALAPPDATA")
-	if strings.TrimSpace(dir) == "" {
-		dir = os.TempDir()
-	}
+	dir := bootLogDir()
 	if dir == "" {
 		return ""
 	}
@@ -136,11 +133,23 @@ func webviewDataPath() string {
 // stuckMarkerPath is written before the window is attempted and removed once it
 // opens, so the NEXT launch can tell that the last one never got there.
 func stuckMarkerPath() string {
-	dir := os.Getenv("LOCALAPPDATA")
-	if strings.TrimSpace(dir) == "" {
-		dir = os.TempDir()
+	return filepath.Join(bootLogDir(), "OpsLog", ".launching")
+}
+
+// bootLogDir is where the breadcrumbs live: %LOCALAPPDATA% on Windows, and on
+// Linux the XDG cache directory (~/.cache) that os.UserCacheDir resolves to.
+//
+// The temp directory is the last resort and not the first, because it is the
+// one place the evidence does not survive: a station that reboots after a
+// failed launch loses exactly the log that would have explained it.
+func bootLogDir() string {
+	if dir := strings.TrimSpace(os.Getenv("LOCALAPPDATA")); dir != "" {
+		return dir
 	}
-	return filepath.Join(dir, "OpsLog", ".launching")
+	if dir, err := os.UserCacheDir(); err == nil && strings.TrimSpace(dir) != "" {
+		return dir
+	}
+	return os.TempDir()
 }
 
 // lastLaunchHung is set at startup from the marker left by the previous run.
diff --git a/datadir_linux.go b/datadir_linux.go
new file mode 100644
index 0000000..ad0ec8c
--- /dev/null
+++ b/datadir_linux.go
@@ -0,0 +1,74 @@
+//go:build linux
+
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+// systemInstallDataDir keeps the portable "data beside the binary" layout where
+// it works, and falls back to the XDG data directory where it cannot.
+//
+// Both halves are needed on Linux, and only on Linux. A tarball or AppImage
+// unpacked into the home directory behaves exactly like the Windows build —
+// the folder travels with the program, which is the whole point of the design.
+// But the ordinary way software arrives here is a package that installs into
+// /usr/bin or /opt, where no user may write, and telling an operator to "move
+// the program somewhere writable" is telling them their distribution installed
+// it wrong. So when the folder beside the binary is read-only, OpsLog keeps its
+// data in ~/.local/share/OpsLog instead and says so in the log.
+//
+// Detection is by TRYING, not by matching path prefixes: /opt, /usr/local and a
+// NFS-mounted home are all writable on some stations and not on others, and the
+// only honest test is whether the write succeeds.
+func systemInstallDataDir(besideExe string) (string, bool) {
+	xdgOnce.Do(func() { xdgDir, xdgUsed = resolveDataDir(besideExe) })
+	return xdgDir, xdgUsed
+}
+
+var (
+	xdgOnce sync.Once
+	xdgDir  string
+	xdgUsed bool
+)
+
+func resolveDataDir(besideExe string) (string, bool) {
+	if writable(besideExe) {
+		return "", false
+	}
+	base := os.Getenv("XDG_DATA_HOME")
+	if base == "" {
+		home, err := os.UserHomeDir()
+		if err != nil || home == "" {
+			return "", false // nowhere better to go; let the caller report the failure
+		}
+		base = filepath.Join(home, ".local", "share")
+	}
+	alt := filepath.Join(base, "OpsLog", "data")
+	if !writable(alt) {
+		return "", false
+	}
+	bootLog("data dir: %s is not writable — keeping the data in %s instead", besideExe, alt)
+	return alt, true
+}
+
+// writable reports whether dir can be created and written to. The probe file is
+// removed again; a leftover in the data folder would be one more thing to
+// explain.
+func writable(dir string) bool {
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return false
+	}
+	probe := filepath.Join(dir, ".writetest")
+	if err := os.WriteFile(probe, []byte("ok"), 0o644); err != nil {
+		return false
+	}
+	_ = os.Remove(probe)
+	return true
+}
+
+// dataDirAdvice is what the operator is told when neither location works — a
+// full disk, or a home directory that is not writable either.
+const dataDirAdvice = "\n\nOpsLog keeps its data next to the program, or in ~/.local/share/OpsLog when that folder belongs to the system. Neither could be written to: check the disk is not full and that your home directory is writable."
diff --git a/datadir_windows.go b/datadir_windows.go
new file mode 100644
index 0000000..ae619b2
--- /dev/null
+++ b/datadir_windows.go
@@ -0,0 +1,16 @@
+//go:build windows
+
+package main
+
+// systemInstallDataDir never diverts on Windows: the data folder is beside the
+// executable, full stop. That is what makes an OpsLog on a USB stick carry its
+// logbook with it, and an operator who copies the folder to a new PC find
+// everything already there.
+//
+// A copy dropped into Program Files is refused the write and told so (see
+// checkDataDirWritable) rather than quietly logging somewhere else, because
+// "where are my QSOs?" is a far worse afternoon than "move this folder".
+func systemInstallDataDir(besideExe string) (string, bool) { return "", false }
+
+// dataDirAdvice is what the operator is told when that folder cannot be written.
+const dataDirAdvice = "\n\nMove OpsLog.exe somewhere your account can write — a folder in Documents, or the desktop — and start it again. Program Files is refused to anything not running as administrator."
diff --git a/go.mod b/go.mod
index d70b9e2..a083bde 100644
--- a/go.mod
+++ b/go.mod
@@ -9,6 +9,7 @@ require (
 	github.com/go-ole/go-ole v1.3.0
 	github.com/go-sql-driver/mysql v1.10.0
 	github.com/gorilla/websocket v1.5.3
+	github.com/jfreymuth/pulse v0.1.3
 	github.com/jlaffaye/ftp v0.2.2
 	github.com/moutend/go-wca v0.3.0
 	github.com/wailsapp/wails/v2 v2.11.0
diff --git a/go.sum b/go.sum
index 1a1d46b..1bfb1cc 100644
--- a/go.sum
+++ b/go.sum
@@ -29,6 +29,8 @@ github.com/hashicorp/golang-lru/v2 v2.0.7 h1:a+bsQ5rvGLjzHuww6tVxozPZFVghXaHOwFs
 github.com/hashicorp/golang-lru/v2 v2.0.7/go.mod h1:QeFd9opnmA6QUJc5vARoKUSoFhyfM2/ZepoAG6RGpeM=
 github.com/jchv/go-winloader v0.0.0-20210711035445-715c2860da7e h1:Q3+PugElBCf4PFpxhErSzU3/PY5sFL5Z6rfv4AbGAck=
 github.com/jchv/go-winloader v0.0.0-20210711035445-715c2860da7e/go.mod h1:alcuEEnZsY1WQsagKhZDsoPCRoOijYqhZvPwLG0kzVs=
+github.com/jfreymuth/pulse v0.1.3 h1:bc5TdxiB8E+2INnFjFWWgyfgXtz2IyNNNCX+Wt/ZD14=
+github.com/jfreymuth/pulse v0.1.3/go.mod h1:cpYspI6YljhkUf1WLXLLDmeaaPFc3CnGLjDZf9dZ4no=
 github.com/jlaffaye/ftp v0.2.2 h1:JwjrXCAIjN9ZYrF1/8qlmHFXDteh9MHYaiEIh/Oqtd8=
 github.com/jlaffaye/ftp v0.2.2/go.mod h1:zuLAKdqFqFvNgkCrH0SC7K1XyUiydS7BFCmmoHUWWg0=
 github.com/labstack/echo/v4 v4.13.3 h1:pwhpCPrTl5qry5HRdM5FwdXnhXSLSY+WE+YQSeCaafY=
diff --git a/internal/audio/device.go b/internal/audio/device.go
new file mode 100644
index 0000000..d940e10
--- /dev/null
+++ b/internal/audio/device.go
@@ -0,0 +1,39 @@
+package audio
+
+// Device is one audio endpoint (a capture input or a render output).
+//
+// ID is whatever the platform calls the endpoint and is PERSISTED in settings:
+// a WASAPI endpoint id on Windows, a PulseAudio source/sink name on Linux.
+// It is opaque to everything above this package, which only ever hands it back.
+type Device struct {
+	ID      string `json:"id"`      // opaque platform endpoint id (persisted)
+	Name    string `json:"name"`    // friendly name shown in dropdowns
+	Default bool   `json:"default"` // is this the system default endpoint
+}
+
+// DeviceName resolves an endpoint id to its friendly name.
+//
+// Diagnostics quote the id that was CONFIGURED, which is a GUID — an operator
+// told "no audio at all from {0.0.1.00000000}.{6a27abfd…}" learns nothing they
+// can act on, while "no audio at all from DAX RX 1 (FlexRadio DAX)" points
+// straight at the DAX panel.
+//
+// Falls back to the id when the endpoint cannot be found, which is itself worth
+// seeing: a device that has disappeared explains an empty recording too.
+func DeviceName(id string) string {
+	if id == "" {
+		return "(none)"
+	}
+	for _, list := range []func() ([]Device, error){ListInputDevices, ListOutputDevices} {
+		devs, err := list()
+		if err != nil {
+			continue
+		}
+		for _, d := range devs {
+			if d.ID == id {
+				return d.Name
+			}
+		}
+	}
+	return id
+}
diff --git a/internal/audio/devices.go b/internal/audio/devices.go
index f7d9cef..28b66b8 100644
--- a/internal/audio/devices.go
+++ b/internal/audio/devices.go
@@ -15,13 +15,6 @@ import (
 	"github.com/moutend/go-wca/pkg/wca"
 )
 
-// Device is one audio endpoint (a capture input or a render output).
-type Device struct {
-	ID      string `json:"id"`      // stable WASAPI endpoint id (persisted)
-	Name    string `json:"name"`    // friendly name shown in dropdowns
-	Default bool   `json:"default"` // is this the system default endpoint
-}
-
 // ListInputDevices returns the active capture endpoints — microphones,
 // line-in, and the soundcard input wired to the rig's audio out ("From Radio").
 func ListInputDevices() ([]Device, error) { return listEndpoints(wca.ECapture) }
@@ -101,30 +94,3 @@ func endpointName(dev *wca.IMMDevice, fallback string) string {
 	}
 	return fallback
 }
-
-// DeviceName resolves an endpoint id to its friendly name.
-//
-// Diagnostics quote the id that was CONFIGURED, which is a GUID — an operator
-// told "no audio at all from {0.0.1.00000000}.{6a27abfd…}" learns nothing they
-// can act on, while "no audio at all from DAX RX 1 (FlexRadio DAX)" points
-// straight at the DAX panel.
-//
-// Falls back to the id when the endpoint cannot be found, which is itself worth
-// seeing: a device that has disappeared explains an empty recording too.
-func DeviceName(id string) string {
-	if id == "" {
-		return "(none)"
-	}
-	for _, list := range []func() ([]Device, error){ListInputDevices, ListOutputDevices} {
-		devs, err := list()
-		if err != nil {
-			continue
-		}
-		for _, d := range devs {
-			if d.ID == id {
-				return d.Name
-			}
-		}
-	}
-	return id
-}
diff --git a/internal/audio/devices_linux.go b/internal/audio/devices_linux.go
new file mode 100644
index 0000000..4036d08
--- /dev/null
+++ b/internal/audio/devices_linux.go
@@ -0,0 +1,99 @@
+//go:build linux
+
+package audio
+
+// devices_linux.go — audio endpoints on Linux, through PulseAudio.
+//
+// PulseAudio and not ALSA, for two reasons that both matter here. ALSA's C
+// library needs cgo, and OpsLog is a pure-Go build; and PulseAudio is the API
+// that is actually present on a ham's desktop — PipeWire, which most current
+// distributions ship, answers the PulseAudio protocol through pipewire-pulse,
+// so one client speaks to both. github.com/jfreymuth/pulse implements that
+// protocol in Go over the server's Unix socket, so nothing is linked in.
+//
+// The endpoint id we persist is the sink/source NAME
+// ("alsa_input.usb-Icom_Inc._IC-7610-00.analog-stereo"), never the numeric
+// index: the index is assigned at boot in device-arrival order and moves the
+// moment a rig is plugged in before a headset.
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/jfreymuth/pulse"
+)
+
+// pulseClient opens a short-lived connection to the local sound server. Each
+// call gets its own: the connection is a Unix socket to a server that may be
+// restarted underneath us (a PipeWire update, a user logging the session out
+// and in), and holding one open for the lifetime of the app means every later
+// call fails until OpsLog itself restarts.
+func pulseClient() (*pulse.Client, error) {
+	c, err := pulse.NewClient(pulse.ClientApplicationName("OpsLog"))
+	if err != nil {
+		return nil, fmt.Errorf("cannot reach the sound server (is PulseAudio or PipeWire running?): %w", err)
+	}
+	return c, nil
+}
+
+// ListInputDevices returns the capture sources.
+//
+// Monitor sources (".monitor", what a given output is playing) are kept rather
+// than filtered out. They look like clutter until you meet the operator whose
+// rig audio reaches OpsLog through a virtual cable — on Linux that is a
+// null-sink and its monitor, and hiding it would hide the only device that
+// works for them.
+func ListInputDevices() ([]Device, error) {
+	c, err := pulseClient()
+	if err != nil {
+		return nil, err
+	}
+	defer c.Close()
+
+	srcs, err := c.ListSources()
+	if err != nil {
+		return nil, err
+	}
+	defID := ""
+	if d, err := c.DefaultSource(); err == nil && d != nil {
+		defID = d.ID()
+	}
+	out := make([]Device, 0, len(srcs))
+	for _, s := range srcs {
+		out = append(out, Device{ID: s.ID(), Name: endpointLabel(s.Name(), s.ID()), Default: s.ID() == defID})
+	}
+	return out, nil
+}
+
+// ListOutputDevices returns the render sinks.
+func ListOutputDevices() ([]Device, error) {
+	c, err := pulseClient()
+	if err != nil {
+		return nil, err
+	}
+	defer c.Close()
+
+	sinks, err := c.ListSinks()
+	if err != nil {
+		return nil, err
+	}
+	defID := ""
+	if d, err := c.DefaultSink(); err == nil && d != nil {
+		defID = d.ID()
+	}
+	out := make([]Device, 0, len(sinks))
+	for _, s := range sinks {
+		out = append(out, Device{ID: s.ID(), Name: endpointLabel(s.Name(), s.ID()), Default: s.ID() == defID})
+	}
+	return out, nil
+}
+
+// endpointLabel prefers the server's human description ("USB Audio CODEC
+// Analog Stereo") and falls back to the raw name, which is ugly but still
+// identifies the device — an empty entry in the dropdown identifies nothing.
+func endpointLabel(desc, id string) string {
+	if d := strings.TrimSpace(desc); d != "" {
+		return d
+	}
+	return id
+}
diff --git a/internal/audio/engine.go b/internal/audio/engine.go
index 6d20a7e..bf67931 100644
--- a/internal/audio/engine.go
+++ b/internal/audio/engine.go
@@ -5,7 +5,6 @@ package audio
 import (
 	"fmt"
 	"runtime"
-	"sync"
 	"time"
 	"unsafe"
 
@@ -281,63 +280,6 @@ func playPCM(deviceID string, pcm []byte, rate, ch, bits int, stop <-chan struct
 	}
 }
 
-// pcmRing is a thread-safe, latency-bounded FIFO of PCM bytes feeding a live
-// render stream. Producers (a USB-codec capture, or a decoded network audio
-// stream) Push freshly-arrived samples; the render loop Pulls. It is the shared
-// hand-off point between "where the audio comes from" (USB device / UDP 50003)
-// and "where it's heard" (any WASAPI output) — so the transport can be swapped
-// without touching the render side, mirroring the civTransport split on the CAT
-// side. On overflow the oldest audio is dropped to keep latency bounded; on
-// underrun Pull simply returns short and the render loop pads with silence.
-type pcmRing struct {
-	mu  sync.Mutex
-	buf []byte
-	max int // hard cap in bytes (drops oldest beyond this → bounded latency)
-}
-
-// newPCMRing makes a ring whose backlog is capped at maxBytes. Size it from the
-// acceptable latency: bytesPerSec (=32000) worth ≈ 1 s.
-func newPCMRing(maxBytes int) *pcmRing {
-	if maxBytes <= 0 {
-		maxBytes = bytesPerSec // 1 s default
-	}
-	return &pcmRing{max: maxBytes}
-}
-
-// Push appends samples, dropping the oldest audio if the backlog would exceed
-// the cap (a slow/absent consumer never makes the producer block or grow without
-// bound). A short glitch beats runaway latency for live monitoring.
-func (r *pcmRing) Push(p []byte) {
-	if len(p) == 0 {
-		return
-	}
-	r.mu.Lock()
-	r.buf = append(r.buf, p...)
-	if len(r.buf) > r.max {
-		drop := len(r.buf) - r.max
-		r.buf = append(r.buf[:0], r.buf[drop:]...)
-	}
-	r.mu.Unlock()
-}
-
-// pull removes and returns up to maxBytes of queued PCM (a private copy), or nil
-// when empty. The render loop pads any shortfall with silence.
-func (r *pcmRing) pull(maxBytes int) []byte {
-	r.mu.Lock()
-	defer r.mu.Unlock()
-	if len(r.buf) == 0 || maxBytes <= 0 {
-		return nil
-	}
-	n := maxBytes
-	if n > len(r.buf) {
-		n = len(r.buf)
-	}
-	out := make([]byte, n)
-	copy(out, r.buf[:n])
-	r.buf = append(r.buf[:0], r.buf[n:]...)
-	return out
-}
-
 // renderStream continuously renders PCM pulled from src to a device until stop
 // closes — the streaming counterpart to playPCM's fixed buffer. On underrun it
 // writes silence rather than glitching, keeping the WASAPI clock steady so live
diff --git a/internal/audio/engine_linux.go b/internal/audio/engine_linux.go
new file mode 100644
index 0000000..fe55e0a
--- /dev/null
+++ b/internal/audio/engine_linux.go
@@ -0,0 +1,275 @@
+//go:build linux
+
+package audio
+
+// engine_linux.go — the four calls the rest of the package makes into the sound
+// card, implemented on PulseAudio. The Windows half of this pair is engine.go
+// (WASAPI); nothing above these functions knows which one it is talking to.
+//
+// Capture is fixed at 16 kHz mono 16-bit, the format the DVK, the recorder and
+// the CW tap all share (see wav.go). We ask the server for it and let the
+// server resample from whatever the device really runs at — the same division
+// of labour as WASAPI's AUTOCONVERTPCM, and for the same reason: a rig codec
+// that only does 48 kHz must still feed a 16 kHz pipeline, and the sound
+// server's converter filters before it decimates, where a naive one folds the
+// receiver hiss above 8 kHz straight back on top of the voice.
+
+import (
+	"fmt"
+	"io"
+	"time"
+
+	"github.com/jfreymuth/pulse"
+	"github.com/jfreymuth/pulse/proto"
+)
+
+// chunkFrames is how much audio a playback reader hands over at once (20 ms).
+// It bounds how much silence a padded underrun can queue ahead of real audio,
+// which is what keeps live monitoring from drifting seconds behind the rig.
+const chunkFrames = sampleRate / 50
+
+// channelMap describes n channels to the server. Only mono and stereo occur
+// here — capture is always mono, and playback follows the WAV being played.
+func channelMap(n int) proto.ChannelMap {
+	if n >= 2 {
+		return proto.ChannelMap{proto.ChannelLeft, proto.ChannelRight}
+	}
+	return proto.ChannelMap{proto.ChannelMono}
+}
+
+// chunkWriter turns the record stream's byte deliveries into onChunk calls.
+// The server reuses its buffer between deliveries, so every chunk is copied
+// before it leaves: the recorder keeps the slices it is given.
+type chunkWriter struct{ onChunk func([]byte) }
+
+func (w chunkWriter) Write(p []byte) (int, error) {
+	if len(p) > 0 && w.onChunk != nil {
+		cp := make([]byte, len(p))
+		copy(cp, p)
+		w.onChunk(cp)
+	}
+	return len(p), nil
+}
+
+// recordPCM captures from a device into 16 kHz mono 16-bit PCM bytes until the
+// stop channel is closed.
+func recordPCM(deviceID string, stop <-chan struct{}) ([]byte, error) {
+	out := make([]byte, 0, bytesPerSec*4)
+	err := captureStream(deviceID, stop, func(chunk []byte) { out = append(out, chunk...) })
+	return out, err
+}
+
+// captureStream opens a device and calls onChunk with freshly-captured 16 kHz
+// mono 16-bit PCM as it arrives, until stop closes. onChunk receives a private
+// copy it may retain.
+func captureStream(deviceID string, stop <-chan struct{}, onChunk func([]byte)) error {
+	c, err := pulseClient()
+	if err != nil {
+		return err
+	}
+	defer c.Close()
+
+	// Rate and channels first, then latency — the latency option sizes its
+	// buffer from both, so setting it earlier would size it from the defaults.
+	//
+	// 50 ms of fragment: the CW decoder is downstream of this and works on the
+	// chunks as they arrive, so a server-chosen fragment of a quarter of a
+	// second would make it decide about a dit long after the dit was over.
+	opts := []pulse.RecordOption{
+		pulse.RecordSampleRate(sampleRate),
+		pulse.RecordChannels(channelMap(channels)),
+		pulse.RecordLatency(0.05),
+		pulse.RecordMediaName("OpsLog capture"),
+	}
+	// An empty id means "whatever the desktop calls the default", which is also
+	// what an operator who has never opened the audio settings expects.
+	if deviceID != "" {
+		src, err := c.SourceByID(deviceID)
+		if err != nil {
+			return fmt.Errorf("no audio input %q: %w", deviceID, err)
+		}
+		opts = append(opts, pulse.RecordSource(src))
+	}
+	st, err := c.NewRecord(pulse.NewWriter(chunkWriter{onChunk}, proto.FormatInt16LE), opts...)
+	if err != nil {
+		return fmt.Errorf("open capture: %w", err)
+	}
+	defer st.Close()
+
+	st.Start()
+	<-stop
+	st.Stop()
+	return st.Error()
+}
+
+// playPCM plays a fixed buffer to a device and returns when it has been heard
+// (or when stop closes, which cuts it short).
+func playPCM(deviceID string, pcm []byte, rate, ch, bits int, stop <-chan struct{}) error {
+	if len(pcm) == 0 {
+		return nil
+	}
+	format, err := pulseFormat(bits)
+	if err != nil {
+		return err
+	}
+	frameBytes := ch * bits / 8
+	if frameBytes <= 0 || rate <= 0 {
+		return fmt.Errorf("bad audio format")
+	}
+
+	c, err := pulseClient()
+	if err != nil {
+		return err
+	}
+	defer c.Close()
+
+	// The reader hands out the buffer a slice at a time and ends the stream
+	// with EndOfData — the library's own sentinel. io.EOF would work as an end
+	// too, but it is recorded as the stream's error, and a message finishing
+	// normally must not look like a fault in the log.
+	pos := 0
+	read := func(buf []byte) (int, error) {
+		select {
+		case <-stop:
+			return 0, pulse.EndOfData
+		default:
+		}
+		if pos >= len(pcm) {
+			return 0, pulse.EndOfData
+		}
+		n := copy(buf, pcm[pos:])
+		n -= n % frameBytes // never hand the server a partial frame
+		if n == 0 {
+			return 0, pulse.EndOfData
+		}
+		pos += n
+		return n, nil
+	}
+
+	opts := []pulse.PlaybackOption{
+		pulse.PlaybackSampleRate(rate),
+		pulse.PlaybackChannels(channelMap(ch)),
+		pulse.PlaybackLatency(0.1),
+		pulse.PlaybackMediaName("OpsLog playback"),
+	}
+	if deviceID != "" {
+		sink, err := c.SinkByID(deviceID)
+		if err != nil {
+			return fmt.Errorf("no audio output %q: %w", deviceID, err)
+		}
+		opts = append(opts, pulse.PlaybackSink(sink))
+	}
+	st, err := c.NewPlayback(pulse.NewReader(readerFunc(read), format), opts...)
+	if err != nil {
+		return fmt.Errorf("open playback: %w", err)
+	}
+	defer st.Close()
+
+	st.Start()
+
+	// Drain blocks until the server has played everything queued. Waiting on it
+	// in a goroutine keeps stop responsive: a voice message must cut off the
+	// instant the operator unkeys, not at the end of the buffer.
+	drained := make(chan struct{})
+	go func() { st.Drain(); close(drained) }()
+	select {
+	case <-drained:
+	case <-stop:
+		st.Stop()
+	}
+	return st.Error()
+}
+
+// renderStream continuously renders PCM pulled from src to a device until stop
+// closes — the streaming counterpart to playPCM's fixed buffer. On underrun it
+// writes silence rather than glitching, keeping the server's clock steady so
+// live monitor audio flows smoothly even when the source stalls briefly.
+func renderStream(deviceID string, rate, ch, bits int, stop <-chan struct{}, src *pcmRing) error {
+	format, err := pulseFormat(bits)
+	if err != nil {
+		return err
+	}
+	frameBytes := ch * bits / 8
+	if frameBytes <= 0 || rate <= 0 || src == nil {
+		return fmt.Errorf("bad audio format")
+	}
+
+	c, err := pulseClient()
+	if err != nil {
+		return err
+	}
+	defer c.Close()
+
+	// Never return 0 bytes without an error: the library's playback loop would
+	// spin on it. A stalled source therefore yields silence, which is also the
+	// behaviour that keeps the clock running.
+	chunk := chunkFrames * frameBytes
+	read := func(buf []byte) (int, error) {
+		select {
+		case <-stop:
+			return 0, pulse.EndOfData
+		default:
+		}
+		n := len(buf)
+		if n > chunk {
+			n = chunk
+		}
+		n -= n % frameBytes
+		if n == 0 {
+			n = frameBytes
+		}
+		got := copy(buf[:n], src.pull(n))
+		for i := got; i < n; i++ {
+			buf[i] = 0
+		}
+		return n, nil
+	}
+
+	opts := []pulse.PlaybackOption{
+		pulse.PlaybackSampleRate(rate),
+		pulse.PlaybackChannels(channelMap(ch)),
+		pulse.PlaybackLatency(0.1),
+		pulse.PlaybackMediaName("OpsLog monitor"),
+	}
+	if deviceID != "" {
+		sink, err := c.SinkByID(deviceID)
+		if err != nil {
+			return fmt.Errorf("no audio output %q: %w", deviceID, err)
+		}
+		opts = append(opts, pulse.PlaybackSink(sink))
+	}
+	st, err := c.NewPlayback(pulse.NewReader(readerFunc(read), format), opts...)
+	if err != nil {
+		return fmt.Errorf("open monitor: %w", err)
+	}
+	defer st.Close()
+
+	st.Start()
+	<-stop
+	st.Stop()
+	// Give the server a moment to notice the stream stopped before the client
+	// socket goes away, so the last fragment is heard instead of clipped.
+	time.Sleep(20 * time.Millisecond)
+	return st.Error()
+}
+
+// pulseFormat maps a WAV bit depth onto the server's sample formats. 8 and 16
+// bit cover everything OpsLog produces or reads; anything else is refused by
+// name rather than played as noise.
+func pulseFormat(bits int) (byte, error) {
+	switch bits {
+	case 8:
+		return proto.FormatUint8, nil
+	case 16:
+		return proto.FormatInt16LE, nil
+	default:
+		return 0, fmt.Errorf("unsupported sample size %d-bit (8 or 16 expected)", bits)
+	}
+}
+
+// readerFunc adapts a read closure to io.Reader.
+type readerFunc func([]byte) (int, error)
+
+func (f readerFunc) Read(p []byte) (int, error) { return f(p) }
+
+var _ io.Reader = readerFunc(nil)
diff --git a/internal/audio/manager.go b/internal/audio/manager.go
index 2a12108..09189a8 100644
--- a/internal/audio/manager.go
+++ b/internal/audio/manager.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package audio
 
 import (
diff --git a/internal/audio/mp3.go b/internal/audio/mp3.go
index 2ed8f53..09d6cae 100644
--- a/internal/audio/mp3.go
+++ b/internal/audio/mp3.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package audio
 
 import (
diff --git a/internal/audio/recorder.go b/internal/audio/recorder.go
index bd831da..f8d71dc 100644
--- a/internal/audio/recorder.go
+++ b/internal/audio/recorder.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package audio
 
 import (
diff --git a/internal/audio/ring.go b/internal/audio/ring.go
new file mode 100644
index 0000000..b598eb6
--- /dev/null
+++ b/internal/audio/ring.go
@@ -0,0 +1,60 @@
+package audio
+
+import "sync"
+
+// pcmRing is a thread-safe, latency-bounded FIFO of PCM bytes feeding a live
+// render stream. Producers (a USB-codec capture, or a decoded network audio
+// stream) Push freshly-arrived samples; the render loop Pulls. It is the shared
+// hand-off point between "where the audio comes from" (USB device / UDP 50003)
+// and "where it's heard" (any WASAPI output) — so the transport can be swapped
+// without touching the render side, mirroring the civTransport split on the CAT
+// side. On overflow the oldest audio is dropped to keep latency bounded; on
+// underrun Pull simply returns short and the render loop pads with silence.
+type pcmRing struct {
+	mu  sync.Mutex
+	buf []byte
+	max int // hard cap in bytes (drops oldest beyond this → bounded latency)
+}
+
+// newPCMRing makes a ring whose backlog is capped at maxBytes. Size it from the
+// acceptable latency: bytesPerSec (=32000) worth ≈ 1 s.
+func newPCMRing(maxBytes int) *pcmRing {
+	if maxBytes <= 0 {
+		maxBytes = bytesPerSec // 1 s default
+	}
+	return &pcmRing{max: maxBytes}
+}
+
+// Push appends samples, dropping the oldest audio if the backlog would exceed
+// the cap (a slow/absent consumer never makes the producer block or grow without
+// bound). A short glitch beats runaway latency for live monitoring.
+func (r *pcmRing) Push(p []byte) {
+	if len(p) == 0 {
+		return
+	}
+	r.mu.Lock()
+	r.buf = append(r.buf, p...)
+	if len(r.buf) > r.max {
+		drop := len(r.buf) - r.max
+		r.buf = append(r.buf[:0], r.buf[drop:]...)
+	}
+	r.mu.Unlock()
+}
+
+// pull removes and returns up to maxBytes of queued PCM (a private copy), or nil
+// when empty. The render loop pads any shortfall with silence.
+func (r *pcmRing) pull(maxBytes int) []byte {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if len(r.buf) == 0 || maxBytes <= 0 {
+		return nil
+	}
+	n := maxBytes
+	if n > len(r.buf) {
+		n = len(r.buf)
+	}
+	out := make([]byte, n)
+	copy(out, r.buf[:n])
+	r.buf = append(r.buf[:0], r.buf[n:]...)
+	return out
+}
diff --git a/internal/audio/wav.go b/internal/audio/wav.go
index 02e7368..019d283 100644
--- a/internal/audio/wav.go
+++ b/internal/audio/wav.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package audio
 
 import (
diff --git a/internal/cat/flex.go b/internal/cat/flex.go
index c194cd8..dfba69a 100644
--- a/internal/cat/flex.go
+++ b/internal/cat/flex.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package cat
 
 import (
diff --git a/internal/cat/flexdiscover.go b/internal/cat/flexdiscover.go
index 6a58f07..762cb2e 100644
--- a/internal/cat/flexdiscover.go
+++ b/internal/cat/flexdiscover.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package cat
 
 import (
@@ -9,8 +7,6 @@ import (
 	"strconv"
 	"syscall"
 	"time"
-
-	"golang.org/x/sys/windows"
 )
 
 // FlexRadio is one radio found by discovery.
@@ -38,7 +34,7 @@ func DiscoverFlex(timeout time.Duration) ([]FlexRadio, error) {
 		Control: func(_, _ string, c syscall.RawConn) error {
 			var serr error
 			_ = c.Control(func(fd uintptr) {
-				serr = windows.SetsockoptInt(windows.Handle(fd), windows.SOL_SOCKET, windows.SO_REUSEADDR, 1)
+				serr = setSocketReuse(fd)
 			})
 			return serr
 		},
diff --git a/internal/cat/omnirig.go b/internal/cat/omnirig.go
index cc58ff3..02ade56 100644
--- a/internal/cat/omnirig.go
+++ b/internal/cat/omnirig.go
@@ -1,3 +1,5 @@
+//go:build windows
+
 package cat
 
 import (
diff --git a/internal/cat/omnirig_activate32.go b/internal/cat/omnirig_activate32.go
index 5e66974..a02cf97 100644
--- a/internal/cat/omnirig_activate32.go
+++ b/internal/cat/omnirig_activate32.go
@@ -1,3 +1,5 @@
+//go:build windows
+
 package cat
 
 import (
diff --git a/internal/cat/omnirig_elevation_test.go b/internal/cat/omnirig_elevation_test.go
index 81ead6f..ab7c019 100644
--- a/internal/cat/omnirig_elevation_test.go
+++ b/internal/cat/omnirig_elevation_test.go
@@ -1,3 +1,5 @@
+//go:build windows
+
 package cat
 
 import (
diff --git a/internal/cat/omnirig_other.go b/internal/cat/omnirig_other.go
new file mode 100644
index 0000000..fa24328
--- /dev/null
+++ b/internal/cat/omnirig_other.go
@@ -0,0 +1,34 @@
+//go:build !windows
+
+package cat
+
+import "errors"
+
+// OmniRig is COM automation against a Windows-only application, so off Windows
+// there is nothing to talk to. The backend still EXISTS here rather than being
+// compiled out of app.go, because a settings database is portable: an operator
+// who moves a profile from Windows to Linux keeps "omnirig" as their saved CAT
+// backend, and OpsLog must start and say why the rig is silent instead of
+// failing to build or panicking on a nil backend.
+//
+// The fix for those operators is a native backend (Icom, Yaesu, Kenwood/
+// Elecraft, Flex, TCI, Xiegu all speak to the radio directly) or Hamlib.
+type OmniRig struct{ CWLower bool }
+
+var errOmniRigWindowsOnly = errors.New("OmniRig runs only on Windows — pick a native CAT backend (Icom, Yaesu, Kenwood/Elecraft, FlexRadio, TCI, Xiegu) in Settings ▸ CAT")
+
+func NewOmniRig(rigNum int, forceVFO string, cwLower bool) *OmniRig {
+	return &OmniRig{CWLower: cwLower}
+}
+
+func (o *OmniRig) Name() string                 { return "omnirig" }
+func (o *OmniRig) Connect() error               { return errOmniRigWindowsOnly }
+func (o *OmniRig) Disconnect()                  {}
+func (o *OmniRig) ReadState() (RigState, error) { return RigState{}, errOmniRigWindowsOnly }
+func (o *OmniRig) SetFrequency(hz int64) error  { return errOmniRigWindowsOnly }
+func (o *OmniRig) SetMode(mode string) error    { return errOmniRigWindowsOnly }
+func (o *OmniRig) SetPTT(on bool) error         { return errOmniRigWindowsOnly }
+
+// SetCWLower satisfies OmniRigController so the preference push at startup is a
+// no-op here rather than a "backend does not support this" error in the log.
+func (o *OmniRig) SetCWLower(on bool) { o.CWLower = on }
diff --git a/internal/cat/omnirig_vfo_test.go b/internal/cat/omnirig_vfo_test.go
index 671d4a8..ce054eb 100644
--- a/internal/cat/omnirig_vfo_test.go
+++ b/internal/cat/omnirig_vfo_test.go
@@ -1,3 +1,5 @@
+//go:build windows
+
 package cat
 
 import "testing"
diff --git a/internal/cat/sockreuse_unix.go b/internal/cat/sockreuse_unix.go
new file mode 100644
index 0000000..0f16616
--- /dev/null
+++ b/internal/cat/sockreuse_unix.go
@@ -0,0 +1,16 @@
+//go:build !windows
+
+package cat
+
+import "golang.org/x/sys/unix"
+
+// setSocketReuse is the Unix half of the Windows SO_REUSEADDR above. Linux
+// wants SO_REUSEPORT as well before two processes may share a bound UDP port;
+// it is not defined on every Unix, so a refusal there is ignored.
+func setSocketReuse(fd uintptr) error {
+	if err := unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEADDR, 1); err != nil {
+		return err
+	}
+	_ = unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1)
+	return nil
+}
diff --git a/internal/cat/sockreuse_windows.go b/internal/cat/sockreuse_windows.go
new file mode 100644
index 0000000..a6b69f9
--- /dev/null
+++ b/internal/cat/sockreuse_windows.go
@@ -0,0 +1,14 @@
+//go:build windows
+
+package cat
+
+import "golang.org/x/sys/windows"
+
+// setSocketReuse enables SO_REUSEADDR before bind, so discovery can listen on
+// :4992 while SmartSDR is already listening for the same radio broadcast.
+// Without it the second bind fails with WSAEADDRINUSE and the operator has to
+// type the radio's IP by hand.
+func setSocketReuse(fd uintptr) error {
+	return windows.SetsockoptInt(windows.Handle(fd),
+		windows.SOL_SOCKET, windows.SO_REUSEADDR, 1)
+}
diff --git a/internal/cat/tci.go b/internal/cat/tci.go
index 09b6b9b..b6cad58 100644
--- a/internal/cat/tci.go
+++ b/internal/cat/tci.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package cat
 
 import (
diff --git a/internal/cat/tci_audio.go b/internal/cat/tci_audio.go
index 662117f..5490cf5 100644
--- a/internal/cat/tci_audio.go
+++ b/internal/cat/tci_audio.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package cat
 
 // TCI audio — receiving the radio's audio over the same WebSocket that carries
diff --git a/internal/cat/tci_manager.go b/internal/cat/tci_manager.go
index 36273dc..d69807c 100644
--- a/internal/cat/tci_manager.go
+++ b/internal/cat/tci_manager.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package cat
 
 import "fmt"
diff --git a/internal/cat/tci_panel.go b/internal/cat/tci_panel.go
index 06e2160..1a1d877 100644
--- a/internal/cat/tci_panel.go
+++ b/internal/cat/tci_panel.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package cat
 
 // The TCI control panel: what the radio already tells us, gathered up.
diff --git a/internal/cat/tci_ptt_test.go b/internal/cat/tci_ptt_test.go
index b338d30..56b2718 100644
--- a/internal/cat/tci_ptt_test.go
+++ b/internal/cat/tci_ptt_test.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package cat
 
 import "testing"
diff --git a/internal/cat/tci_spot_test.go b/internal/cat/tci_spot_test.go
index f716399..71b9c88 100644
--- a/internal/cat/tci_spot_test.go
+++ b/internal/cat/tci_spot_test.go
@@ -1,5 +1,3 @@
-//go:build windows
-
 package cat
 
 import "testing"
diff --git a/internal/extsvc/configured.go b/internal/extsvc/configured.go
index b35c23a..2cdee08 100644
--- a/internal/extsvc/configured.go
+++ b/internal/extsvc/configured.go
@@ -69,7 +69,7 @@ func Configured(svc Service, cfg ExternalServices) error {
 			return missing("Cloudlog / Wavelog", need...)
 		}
 	case ServiceLoTW:
-		add(set(cfg.LoTW.TQSLPath), "the path to tqsl.exe")
+		add(set(cfg.LoTW.TQSLPath), "the path to TQSL")
 		add(set(cfg.LoTW.StationLocation), "the TQSL station location")
 		if len(need) > 0 {
 			return missing("LoTW", need...)
diff --git a/internal/extsvc/lotw.go b/internal/extsvc/lotw.go
index 5113f79..b7b1a24 100644
--- a/internal/extsvc/lotw.go
+++ b/internal/extsvc/lotw.go
@@ -297,29 +297,6 @@ func ListStationLocations(stationDataPath string) ([]StationLocation, error) {
 	return out, nil
 }
 
-// DefaultTQSLPath returns the usual tqsl.exe install path on Windows, or ""
-// if not found.
-func DefaultTQSLPath() string {
-	for _, p := range []string{
-		`C:\Program Files (x86)\TrustedQSL\tqsl.exe`,
-		`C:\Program Files\TrustedQSL\tqsl.exe`,
-	} {
-		if fileExists(p) {
-			return p
-		}
-	}
-	return ""
-}
-
-// DefaultStationDataPath returns TQSL's station_data location (%APPDATA%\
-// TrustedQSL\station_data on Windows), or "" if APPDATA isn't set.
-func DefaultStationDataPath() string {
-	if appData := os.Getenv("APPDATA"); appData != "" {
-		return filepath.Join(appData, "TrustedQSL", "station_data")
-	}
-	return ""
-}
-
 func fileExists(p string) bool {
 	info, err := os.Stat(p)
 	return err == nil && !info.IsDir()
@@ -375,7 +352,7 @@ func UploadLoTW(ctx context.Context, cfg ServiceConfig, tempDir, adifRecord stri
 	case tqsl == "":
 		return UploadResult{}, fmt.Errorf("lotw: TQSL path not set")
 	case !fileExists(tqsl):
-		return UploadResult{}, fmt.Errorf("lotw: tqsl.exe not found at %q", tqsl)
+		return UploadResult{}, fmt.Errorf("lotw: TQSL not found at %q", tqsl)
 	case loc == "":
 		return UploadResult{}, fmt.Errorf("lotw: station location not set")
 	case strings.TrimSpace(adifRecord) == "":
@@ -515,7 +492,7 @@ func TestLoTW(cfg ServiceConfig, stationDataPath string) (string, error) {
 	tqsl := strings.TrimSpace(cfg.TQSLPath)
 	loc := strings.TrimSpace(cfg.StationLocation)
 	if tqsl == "" || !fileExists(tqsl) {
-		return "", fmt.Errorf("lotw: tqsl.exe not found (set the TQSL path)")
+		return "", fmt.Errorf("lotw: TQSL not found (set the TQSL path)")
 	}
 	if loc == "" {
 		return "", fmt.Errorf("lotw: pick a station location")
diff --git a/internal/extsvc/tqslpath_unix.go b/internal/extsvc/tqslpath_unix.go
new file mode 100644
index 0000000..6c5b67b
--- /dev/null
+++ b/internal/extsvc/tqslpath_unix.go
@@ -0,0 +1,66 @@
+//go:build !windows
+
+package extsvc
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+)
+
+// DefaultTQSLPath finds TrustedQSL, or returns "" so the operator can point at
+// it by hand.
+//
+// PATH is asked FIRST, unlike the Windows side where two fixed install folders
+// are the whole story. On Linux tqsl comes from the distribution's package
+// manager, a Flatpak or a self-built copy, and each puts it somewhere
+// different; whichever one the operator installed is the one their shell finds.
+// The fixed list below is only for a desktop session that started without a
+// useful PATH.
+func DefaultTQSLPath() string {
+	if p, err := exec.LookPath("tqsl"); err == nil && fileExists(p) {
+		return p
+	}
+	candidates := []string{
+		"/usr/bin/tqsl",
+		"/usr/local/bin/tqsl",
+		"/var/lib/flatpak/exports/bin/org.arrl.tqsl",
+		filepath.Join(os.Getenv("HOME"), ".local/share/flatpak/exports/bin/org.arrl.tqsl"),
+	}
+	if runtime.GOOS == "darwin" {
+		candidates = append(candidates, "/Applications/TrustedQSL/tqsl.app/Contents/MacOS/tqsl")
+	}
+	for _, p := range candidates {
+		if fileExists(p) {
+			return p
+		}
+	}
+	return ""
+}
+
+// DefaultStationDataPath returns TQSL's station_data location.
+//
+// ~/.tqsl is where the Unix build of TrustedQSL keeps its configuration. A
+// Flatpak install redirects it into the sandbox
+// (~/.var/app/org.arrl.tqsl/data/tqsl), so that is tried too — an operator on a
+// Flatpak TQSL otherwise sees an empty station-location list with nothing to
+// explain it.
+func DefaultStationDataPath() string {
+	home, err := os.UserHomeDir()
+	if err != nil || home == "" {
+		return ""
+	}
+	for _, p := range []string{
+		filepath.Join(home, ".tqsl", "station_data"),
+		filepath.Join(home, ".var", "app", "org.arrl.tqsl", "data", "tqsl", "station_data"),
+	} {
+		if fileExists(p) {
+			return p
+		}
+	}
+	// Nothing found: name the ordinary location anyway. The settings field then
+	// shows the path TQSL would create on its first run, which is a better
+	// starting point for the operator than an empty box.
+	return filepath.Join(home, ".tqsl", "station_data")
+}
diff --git a/internal/extsvc/tqslpath_windows.go b/internal/extsvc/tqslpath_windows.go
new file mode 100644
index 0000000..f3e6940
--- /dev/null
+++ b/internal/extsvc/tqslpath_windows.go
@@ -0,0 +1,30 @@
+//go:build windows
+
+package extsvc
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// DefaultTQSLPath returns the usual tqsl.exe install path, or "" if not found.
+func DefaultTQSLPath() string {
+	for _, p := range []string{
+		`C:\Program Files (x86)\TrustedQSL\tqsl.exe`,
+		`C:\Program Files\TrustedQSL\tqsl.exe`,
+	} {
+		if fileExists(p) {
+			return p
+		}
+	}
+	return ""
+}
+
+// DefaultStationDataPath returns TQSL's station_data location
+// (%APPDATA%\TrustedQSL\station_data), or "" if APPDATA isn't set.
+func DefaultStationDataPath() string {
+	if appData := os.Getenv("APPDATA"); appData != "" {
+		return filepath.Join(appData, "TrustedQSL", "station_data")
+	}
+	return ""
+}
diff --git a/main.go b/main.go
index 87b326b..56e3117 100644
--- a/main.go
+++ b/main.go
@@ -162,8 +162,7 @@ func main() {
 	// OpsLog had was inside the folder it could not create.
 	if err := checkDataDirWritable(); err != nil {
 		bootLog("FATAL %v", err)
-		fatalBox("OpsLog", "OpsLog cannot write next to its own program file.\n\n"+err.Error()+
-			"\n\nMove OpsLog.exe somewhere your account can write — a folder in Documents, or the desktop — and start it again. Program Files is refused to anything not running as administrator.")
+		fatalBox("OpsLog", "OpsLog cannot write next to its own program file.\n\n"+err.Error()+dataDirAdvice)
 		return
 	}
 	if postUpdate {
diff --git a/proc_linux.go b/proc_linux.go
new file mode 100644
index 0000000..f99227d
--- /dev/null
+++ b/proc_linux.go
@@ -0,0 +1,79 @@
+//go:build linux
+
+package main
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"syscall"
+
+	"hamlog/internal/applog"
+
+	wruntime "github.com/wailsapp/wails/v2/pkg/runtime"
+)
+
+// hideConsole has nothing to hide on Linux: a child started from a GUI process
+// inherits no console, so no window can flash.
+func hideConsole(cmd *exec.Cmd) {}
+
+// runningProcessNames returns the set of lowercase executable names currently
+// running, read straight from /proc rather than by shelling out to `ps` — the
+// output format of `ps` varies between distributions and busybox, /proc does
+// not.
+//
+// Two names are recorded per process: the kernel's comm (truncated to 15
+// characters, which is why it cannot be the only one — "gridtracker-bin" and
+// "wsjtx-improved" both hit the limit) and the basename of the real executable
+// behind /proc/<pid>/exe. Either may be what the operator configured.
+func runningProcessNames() map[string]bool {
+	out := map[string]bool{}
+	entries, err := os.ReadDir("/proc")
+	if err != nil {
+		applog.Printf("autostart: cannot read /proc: %v", err)
+		return out
+	}
+	for _, e := range entries {
+		if !e.IsDir() {
+			continue
+		}
+		pid := e.Name()
+		if _, err := strconv.Atoi(pid); err != nil {
+			continue // not a process directory
+		}
+		if comm, err := os.ReadFile("/proc/" + pid + "/comm"); err == nil {
+			if n := strings.ToLower(strings.TrimSpace(string(comm))); n != "" {
+				out[n] = true
+			}
+		}
+		// The exe symlink is unreadable for processes owned by another user;
+		// that is expected and not worth a log line.
+		if exe, err := os.Readlink("/proc/" + pid + "/exe"); err == nil {
+			if n := strings.ToLower(filepath.Base(strings.TrimSuffix(exe, " (deleted)"))); n != "" {
+				out[n] = true
+			}
+		}
+	}
+	return out
+}
+
+// closeProcess asks the process to close. SIGTERM is the Unix equivalent of the
+// polite WM_CLOSE used on Windows: WSJT-X and friends get to save their state
+// instead of being shot with SIGKILL.
+func closeProcess(pid int) ([]byte, error) {
+	p, err := os.FindProcess(pid)
+	if err != nil {
+		return nil, err
+	}
+	return nil, p.Signal(syscall.SIGTERM)
+}
+
+// executableFilters is what the "choose a program" dialog offers. Nothing to
+// filter on here: a Linux program is a file with the executable bit, and its
+// name carries no extension — wsjtx, gridtracker, flrig. An *.exe filter would
+// show the operator an empty folder.
+func executableFilters() []wruntime.FileFilter {
+	return []wruntime.FileFilter{{DisplayName: "All files", Pattern: "*"}}
+}
diff --git a/proc_windows.go b/proc_windows.go
new file mode 100644
index 0000000..f68ad96
--- /dev/null
+++ b/proc_windows.go
@@ -0,0 +1,68 @@
+//go:build windows
+
+package main
+
+import (
+	"os/exec"
+	"strconv"
+	"strings"
+	"syscall"
+
+	"hamlog/internal/applog"
+
+	wruntime "github.com/wailsapp/wails/v2/pkg/runtime"
+)
+
+// hideConsole keeps a helper process from flashing a console window. OpsLog is
+// a GUI application, and every `tasklist`/`taskkill`/`powershell` it runs would
+// otherwise blink a black box in front of the operator mid-QSO.
+func hideConsole(cmd *exec.Cmd) {
+	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true, CreationFlags: 0x08000000} // CREATE_NO_WINDOW
+}
+
+// runningProcessNames returns the set of lowercase executable names currently
+// running, via the Windows `tasklist`. Best effort — on failure the set is
+// empty (we then just attempt to launch, which is acceptable).
+func runningProcessNames() map[string]bool {
+	out := map[string]bool{}
+	cmd := exec.Command("tasklist", "/FO", "CSV", "/NH")
+	hideConsole(cmd)
+	data, err := cmd.Output()
+	if err != nil {
+		applog.Printf("autostart: tasklist failed: %v", err)
+		return out
+	}
+	for _, line := range strings.Split(string(data), "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		// CSV row: "image.exe","PID",... — take the first quoted field.
+		field := line
+		if i := strings.Index(line[1:], "\""); i >= 0 && strings.HasPrefix(line, "\"") {
+			field = line[1 : i+1]
+		}
+		field = strings.Trim(field, "\"")
+		if field != "" {
+			out[strings.ToLower(field)] = true
+		}
+	}
+	return out
+}
+
+// closeProcess asks the process to close. `taskkill` without /F sends WM_CLOSE,
+// so WSJT-X and friends get to save their state instead of being shot.
+func closeProcess(pid int) ([]byte, error) {
+	cmd := exec.Command("taskkill", "/PID", strconv.Itoa(pid))
+	hideConsole(cmd)
+	return cmd.CombinedOutput()
+}
+
+// executableFilters is what the "choose a program" dialog offers. Windows knows
+// a program by its extension.
+func executableFilters() []wruntime.FileFilter {
+	return []wruntime.FileFilter{
+		{DisplayName: "Programs (*.exe;*.bat;*.cmd)", Pattern: "*.exe;*.bat;*.cmd"},
+		{DisplayName: "All files (*.*)", Pattern: "*.*"},
+	}
+}
diff --git a/screenbounds_other.go b/screenbounds_other.go
index 6f0e2ef..16fa0fd 100644
--- a/screenbounds_other.go
+++ b/screenbounds_other.go
@@ -12,3 +12,16 @@ func onSomeMonitorImpl(x, y, w, h int) bool { return true }
 func clampToVisible(x, y, w, h int) (int, int, bool) { return x, y, false }
 
 func logMonitorLayout() {}
+
+// monitorRects cannot be answered off Windows: Wails exposes no monitor
+// geometry, and X11/Wayland disagree about whether a client may even ask. The
+// empty list is the "cannot tell" the callers above already handle by trusting
+// the operator's saved position.
+func monitorRects() []screenRect { return nil }
+
+// describeMonitors still has to say something in the log — a window nobody can
+// see is reported as "OpsLog did not start", and the log line is the first
+// thing looked at.
+func describeMonitors(rects []screenRect) string {
+	return "monitor layout unavailable on this platform"
+}
diff --git a/scripts/linux-setup.sh b/scripts/linux-setup.sh
new file mode 100755
index 0000000..9864f48
--- /dev/null
+++ b/scripts/linux-setup.sh
@@ -0,0 +1,154 @@
+#!/usr/bin/env bash
+# linux-setup.sh — check what a Linux build of OpsLog needs, then build it.
+#
+# Run it from the repository root:   ./scripts/linux-setup.sh
+#
+# It never installs anything itself. It prints the one command your distribution
+# needs and stops, because a script that runs sudo on somebody else's machine is
+# a script nobody should run. Once the dependencies are in, run it again and it
+# builds.
+set -u
+
+red()  { printf '\033[31m%s\033[0m\n' "$*"; }
+grn()  { printf '\033[32m%s\033[0m\n' "$*"; }
+ylw()  { printf '\033[33m%s\033[0m\n' "$*"; }
+head_() { printf '\n\033[1m== %s\033[0m\n' "$*"; }
+
+missing=0
+note() { red "  MISSING: $*"; missing=1; }
+ok()   { grn "  ok: $*"; }
+
+head_ "Distribution"
+if [ -r /etc/os-release ]; then
+	# shellcheck disable=SC1091
+	. /etc/os-release
+	echo "  ${PRETTY_NAME:-unknown}"
+	family="${ID_LIKE:-$ID}"
+else
+	echo "  unknown (no /etc/os-release)"
+	family=""
+fi
+
+head_ "Build dependencies"
+have() { command -v "$1" >/dev/null 2>&1; }
+pkg()  { pkg-config --exists "$1" 2>/dev/null; }
+
+have gcc || have cc || note "a C compiler (Wails links against the system WebKit, so cgo is required here)"
+have pkg-config || note "pkg-config"
+pkg gtk+-3.0 || note "GTK 3 development headers"
+
+# Wails 2.10+ builds against webkit2gtk-4.1; Debian 12 and older still ship 4.0,
+# which works with an extra build tag. Detect which one is present rather than
+# telling the operator to guess.
+webkit_tags=""
+if pkg webkit2gtk-4.1; then
+	ok "webkit2gtk-4.1"
+elif pkg webkit2gtk-4.0; then
+	ok "webkit2gtk-4.0 (older — will build with -tags webkit2_40)"
+	webkit_tags="-tags webkit2_40"
+else
+	note "webkit2gtk development headers (4.1 preferred, 4.0 accepted)"
+fi
+
+# Node, and its VERSION — this is the trap on an LTS base. Ubuntu 22.04 (so
+# Linux Mint 21) ships node 12, and Vite needs 18. The build fails deep inside
+# the frontend with a syntax error that says nothing about the real cause, so
+# catch it here where it can be named.
+if have node; then
+	nodemaj=$(node -p 'process.versions.node.split(".")[0]' 2>/dev/null || echo 0)
+	if [ "${nodemaj:-0}" -ge 18 ]; then
+		ok "node $(node -v)"
+	else
+		note "node 18 or newer (you have $(node -v) — the distribution package is too old for Vite)"
+		ylw "      curl -fsSL https://deb.nodesource.com/setup_20.x | sudo -E bash - && sudo apt install nodejs"
+		ylw "      or use nvm:  https://github.com/nvm-sh/nvm"
+	fi
+else
+	note "node 18+"
+fi
+have npm || note "npm"
+
+head_ "Go"
+if have go; then
+	gov=$(go version | awk '{print $3}')
+	ok "$gov"
+	# 1.25 is what go.mod asks for. No distribution ships it yet — Ubuntu 22.04
+	# (Mint 21) has 1.18, Ubuntu 24.04 (Mint 22) has 1.22 — so `apt install
+	# golang-go` gets you a Go that cannot build this repository at all.
+	if ! go version | grep -Eq 'go1\.(2[5-9]|[3-9][0-9])'; then
+		note "go 1.25+ — $gov is too old for go.mod, and no apt package is new enough"
+		ylw "      wget https://go.dev/dl/go1.25.1.linux-amd64.tar.gz"
+		ylw "      sudo rm -rf /usr/local/go && sudo tar -C /usr/local -xzf go1.25.1.linux-amd64.tar.gz"
+		ylw "      echo 'export PATH=/usr/local/go/bin:\$HOME/go/bin:\$PATH' >> ~/.profile   # then log out and back in"
+	fi
+else
+	note "go 1.25+ — from https://go.dev/dl/, NOT from apt (no distribution ships 1.25 yet)"
+fi
+
+head_ "Wails CLI"
+WAILS="$(command -v wails || true)"
+[ -z "$WAILS" ] && [ -x "$HOME/go/bin/wails" ] && WAILS="$HOME/go/bin/wails"
+if [ -n "$WAILS" ]; then
+	ok "$($WAILS version 2>/dev/null | head -1)"
+else
+	note "the wails CLI:  go install github.com/wailsapp/wails/v2/cmd/wails@v2.11.0"
+	ylw "  (then make sure ~/go/bin is on your PATH)"
+fi
+
+if [ "$missing" -ne 0 ]; then
+	head_ "Install these first"
+	case "$family" in
+		*debian*|*ubuntu*)
+			echo "  sudo apt install build-essential pkg-config libgtk-3-dev libwebkit2gtk-4.1-dev"
+			echo "  (on Debian 12 and older: libwebkit2gtk-4.0-dev)"
+			echo
+			echo "  Go and node are NOT in that line on purpose — the apt versions are too"
+			echo "  old on every current Ubuntu/Mint base. See the notes above for both." ;;
+		*fedora*|*rhel*)
+			echo "  sudo dnf install gcc-c++ pkgconf-pkg-config gtk3-devel webkit2gtk4.1-devel nodejs npm" ;;
+		*arch*)
+			echo "  sudo pacman -S base-devel pkgconf gtk3 webkit2gtk-4.1 nodejs npm" ;;
+		*suse*)
+			echo "  sudo zypper install -t pattern devel_basis && sudo zypper install pkg-config gtk3-devel webkit2gtk3-soup2-devel nodejs npm" ;;
+		*)
+			echo "  a C compiler, pkg-config, GTK 3 and webkit2gtk development packages, node and npm" ;;
+	esac
+	echo
+	echo "Then run this script again."
+	exit 1
+fi
+
+head_ "Serial ports"
+# The single most common reason a rig that works in WSJT-X refuses to open here.
+if id -nG | tr ' ' '\n' | grep -qx 'dialout\|uucp'; then
+	ok "you are in the serial group"
+else
+	ylw "  You are NOT in the 'dialout' group (or 'uucp' on Arch/Fedora)."
+	ylw "  CAT, keyers, rotators and amplifiers will fail with \"permission denied\"."
+	ylw "      sudo usermod -aG dialout \$USER      # then log out and back in"
+fi
+
+head_ "Sound server"
+if have pactl && pactl info >/dev/null 2>&1; then
+	ok "$(pactl info | sed -n 's/^Server Name: //p')"
+else
+	ylw "  No PulseAudio/PipeWire server answered. The voice keyer, the QSO"
+	ylw "  recorder and the CW decoder will report they cannot reach it."
+	ylw "  Everything else works without one."
+fi
+
+head_ "Building"
+echo "  wails build $webkit_tags"
+# shellcheck disable=SC2086
+"$WAILS" build $webkit_tags || { red "Build failed."; exit 1; }
+
+head_ "Done"
+grn "  ./build/bin/OpsLog"
+echo
+echo "  First run creates build/bin/data/ beside the binary — a fresh, empty"
+echo "  logbook. Do NOT copy config.json over from Windows: it holds a Windows"
+echo "  path. Point OpsLog at your real logbook from Settings ▸ Database."
+echo
+echo "  If the window never opens, look at:"
+echo "    ~/.cache/OpsLog/startup.log       (before the window)"
+echo "    build/bin/data/opslog.log         (after it)"
diff --git a/serialports_test.go b/serialports_test.go
index f64010e..760413a 100644
--- a/serialports_test.go
+++ b/serialports_test.go
@@ -44,3 +44,13 @@ func TestTidySerialPortsIgnoresCase(t *testing.T) {
 		t.Errorf("got %v, want [COM3]", got)
 	}
 }
+
+// On Linux the ports are paths, and the same trap is waiting there:
+// /dev/ttyUSB10 sorted lexically lands between USB1 and USB2.
+func TestTidySerialPortsSortsUnixNamesNaturally(t *testing.T) {
+	got := tidySerialPorts([]string{"/dev/ttyUSB10", "/dev/ttyUSB2", "/dev/ttyACM0", "/dev/ttyUSB1"})
+	want := []string{"/dev/ttyACM0", "/dev/ttyUSB1", "/dev/ttyUSB2", "/dev/ttyUSB10"}
+	if strings.Join(got, ",") != strings.Join(want, ",") {
+		t.Errorf("got %v, want %v", got, want)
+	}
+}
diff --git a/singleinstance_linux.go b/singleinstance_linux.go
new file mode 100644
index 0000000..4afe8a6
--- /dev/null
+++ b/singleinstance_linux.go
@@ -0,0 +1,99 @@
+//go:build linux && !bindings
+
+// NB the !bindings tag: Wails generates the TypeScript bindings by BUILDING AND
+// RUNNING this binary. With the guard active, a normal OpsLog already running on
+// the dev machine holds the lock, the generator's process exits instantly, and
+// no bindings are produced. Excluding the guard from that build keeps generation
+// working while shipping builds still get it.
+
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strconv"
+	"syscall"
+	"time"
+
+	"hamlog/internal/applog"
+)
+
+// The Linux half of the single-instance guard. Windows uses a named mutex; here
+// it is an advisory lock (flock) held on a file for as long as the process
+// lives.
+//
+// A lock and not a pid file, because a pid file is wrong exactly when it
+// matters: OpsLog killed by the OOM killer, or crashing on a bad rig response,
+// leaves its pid behind and every later launch refuses to start. The kernel
+// drops an flock when the process ends however it ends, so there is no stale
+// state to clean up and no "delete this file to start again" for the operator
+// to discover.
+var instanceLock *os.File
+
+// instanceLockPath prefers XDG_RUNTIME_DIR (/run/user/1000) — per user, and
+// emptied when the session ends, which is what a runtime lock wants. The cache
+// directory is the fallback for the sessions that do not set it (a bare TTY, an
+// ssh -X login).
+func instanceLockPath() string {
+	dir := os.Getenv("XDG_RUNTIME_DIR")
+	if dir == "" {
+		dir = bootLogDir()
+	}
+	dir = filepath.Join(dir, "OpsLog")
+	if err := os.MkdirAll(dir, 0o700); err != nil {
+		return ""
+	}
+	return filepath.Join(dir, "instance.lock")
+}
+
+// acquireSingleInstance reports whether this process now owns the instance
+// lock. Safe to call repeatedly: the retry loop in acquireInstance does, and a
+// second flock on a second descriptor of the same file would conflict with the
+// one we already hold.
+func acquireSingleInstance() bool {
+	if instanceLock != nil {
+		return true
+	}
+	path := instanceLockPath()
+	if path == "" {
+		applog.Printf("single-instance: no writable folder for the lock — the guard is off for this run")
+		return true // fail open: refusing to start is worse than a possible duplicate
+	}
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		applog.Printf("single-instance: cannot open %s (%v) — the guard is off for this run", path, err)
+		return true
+	}
+	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
+		_ = f.Close()
+		return false // another OpsLog holds it
+	}
+	// The pid is written for the operator's benefit, not ours — it is what a
+	// "which process is holding this?" question needs. The lock itself is the
+	// kernel's, and does not depend on the contents.
+	_ = f.Truncate(0)
+	_, _ = f.WriteString(strconv.Itoa(os.Getpid()) + "\n")
+	_ = f.Sync()
+	instanceLock = f // deliberately never closed: closing releases the lock
+	return true
+}
+
+// waitForProcessExit waits for pid to disappear, up to timeout, and reports
+// whether it did. Signal 0 asks the kernel "does this process exist?" without
+// touching it.
+//
+// EPERM means it exists and belongs to somebody else — still running, as far as
+// the caller is concerned. Only ESRCH is gone.
+func waitForProcessExit(pid int, timeout time.Duration) bool {
+	if pid <= 0 {
+		return true
+	}
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		if err := syscall.Kill(pid, 0); err == syscall.ESRCH {
+			return true
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+	return syscall.Kill(pid, 0) == syscall.ESRCH
+}
diff --git a/singleinstance_other.go b/singleinstance_other.go
index 027866d..ef27fa1 100644
--- a/singleinstance_other.go
+++ b/singleinstance_other.go
@@ -1,4 +1,4 @@
-//go:build !windows || bindings
+//go:build (!windows && !linux) || bindings
 
 package main
 
diff --git a/update.go b/update.go
index ce89e5e..7a4d338 100644
--- a/update.go
+++ b/update.go
@@ -11,7 +11,6 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
-	"syscall"
 	"time"
 
 	wruntime "github.com/wailsapp/wails/v2/pkg/runtime"
@@ -209,8 +208,11 @@ func (a *App) DownloadAndApplyUpdate(url string) error {
 	// Otherwise Windows SmartScreen wants to prompt "are you sure you want to open
 	// this?" — but since we launch the exe programmatically that prompt never shows,
 	// and the launch is silently blocked. This is exactly why the relaunch failed.
-	_ = os.Remove(exe + ":Zone.Identifier")
-	applog.Printf("update: installed new exe, scheduling relaunch")
+	clearDownloadMark(exe)
+	if err := makeExecutable(exe); err != nil {
+		applog.Printf("update: could not restore the executable bit on %s: %v", filepath.Base(exe), err)
+	}
+	applog.Printf("update: installed new build, scheduling relaunch")
 
 	// THE NEW EXE STARTS ITSELF. No helper, no script.
 	//
@@ -235,7 +237,7 @@ func (a *App) DownloadAndApplyUpdate(url string) error {
 	// helper never did: it waited for the pid, however long it took.
 	cmd := exec.Command(exe, "--post-update", "--wait-pid", strconv.Itoa(os.Getpid()))
 	cmd.Dir = dir
-	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true, CreationFlags: 0x08000000} // CREATE_NO_WINDOW
+	hideConsole(cmd)
 	if err := cmd.Start(); err != nil {
 		return fmt.Errorf("schedule relaunch: %w", err)
 	}
@@ -250,52 +252,6 @@ func (a *App) DownloadAndApplyUpdate(url string) error {
 	return nil
 }
 
-// scheduleDeferredSwap hands the exe swap to a detached helper that runs AFTER
-// this process is gone.
-//
-// The fallback for when the running image cannot be renamed at all. Once OpsLog
-// has exited its exe is an ordinary file again, so the move that was refused a
-// moment earlier succeeds — and the helper keeps trying for ten seconds, because
-// an antivirus that was holding the file usually lets go a beat after the
-// process dies rather than instantly.
-//
-// OpsLog is restarted either way. If the move failed, that starts the OLD build
-// — the update simply has not applied — and the operator keeps a working logger
-// instead of having it vanish mid-session, which for someone in a QSO is worse
-// than an update that waits. Only a successful swap passes --post-update, so a
-// failure leaves the .new file in place for the next attempt rather than having
-// the cleanup delete the download.
-// The LAST resort still needs a helper that outlives this process: nothing else
-// can move a file over an image that is still running. It stays PowerShell —
-// there is no smaller tool on a stock Windows that can wait for a pid and then
-// move a file — but it is reached only when the rename above failed, which is
-// rare, and never on the ordinary update path (see the relaunch there for why
-// that matters to Defender).
-func (a *App) scheduleDeferredSwap(exe, pending string) error {
-	// Clear the "downloaded from the internet" mark before it becomes the exe —
-	// SmartScreen silently blocks a programmatic launch of a marked file, and the
-	// mark follows the file across the move.
-	_ = os.Remove(pending + ":Zone.Identifier")
-
-	q := func(s string) string { return strings.ReplaceAll(s, "'", "''") }
-	ps := fmt.Sprintf(
-		"Wait-Process -Id %d -ErrorAction SilentlyContinue; "+
-			"$ok=$false; "+
-			"for ($i=0; $i -lt 40; $i++) { "+
-			"try { Move-Item -LiteralPath '%s' -Destination '%s' -Force -ErrorAction Stop; $ok=$true; break } "+
-			"catch { Start-Sleep -Milliseconds 250 } }; "+
-			"if ($ok) { Start-Process -FilePath '%s' -ArgumentList '--post-update' } "+
-			"else { Start-Process -FilePath '%s' }",
-		os.Getpid(), q(pending), q(exe), q(exe), q(exe))
-	cmd := exec.Command("powershell", "-NoProfile", "-WindowStyle", "Hidden", "-Command", ps)
-	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true, CreationFlags: 0x08000000} // CREATE_NO_WINDOW
-	if err := cmd.Start(); err != nil {
-		return fmt.Errorf("schedule the update swap: %w", err)
-	}
-	applog.Printf("update: swap scheduled for after exit (%s → %s)", filepath.Base(pending), filepath.Base(exe))
-	return nil
-}
-
 // downloadWithProgress streams url into dest, emitting "update:progress" (0-100).
 func (a *App) downloadWithProgress(url, dest string) error {
 	client := &http.Client{Timeout: 10 * time.Minute}
diff --git a/updateswap_linux.go b/updateswap_linux.go
new file mode 100644
index 0000000..646acc3
--- /dev/null
+++ b/updateswap_linux.go
@@ -0,0 +1,42 @@
+//go:build linux
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"hamlog/internal/applog"
+)
+
+// clearDownloadMark has nothing to clear on Linux: there is no
+// mark-of-the-web, and no SmartScreen to refuse a programmatic launch.
+func clearDownloadMark(path string) {}
+
+// makeExecutable restores the executable bit. A binary downloaded over HTTP
+// arrives 0644 — on Windows the extension decides and this is a no-op, but here
+// a freshly installed OpsLog that nothing can exec is a dead station.
+func makeExecutable(path string) error { return os.Chmod(path, 0o755) }
+
+// scheduleDeferredSwap is the fallback for when the running binary could not be
+// renamed out of the way — the path Windows needs a detached PowerShell helper
+// for, because nothing there can move a file over a running image.
+//
+// On Linux it should never be reached. A rename only touches the directory
+// entry, and the running process holds the inode, so replacing the binary of a
+// live process is ordinary and the staging rename in DownloadAndApplyUpdate
+// succeeds. If it did fail, the cause was the filesystem (read-only mount, no
+// write permission on the directory, a full disk) and no helper would get past
+// it either — so do the honest thing: try the move once more now, and say
+// plainly what is wrong if it still refuses.
+func (a *App) scheduleDeferredSwap(exe, pending string) error {
+	if err := os.Rename(pending, exe); err != nil {
+		return fmt.Errorf("install the new build: %w (the folder %s must be writable)", err, filepath.Dir(exe))
+	}
+	if err := makeExecutable(exe); err != nil {
+		return fmt.Errorf("make the new build executable: %w", err)
+	}
+	applog.Printf("update: installed %s over %s after the staging rename failed", filepath.Base(pending), filepath.Base(exe))
+	return nil
+}
diff --git a/updateswap_windows.go b/updateswap_windows.go
new file mode 100644
index 0000000..03ac254
--- /dev/null
+++ b/updateswap_windows.go
@@ -0,0 +1,69 @@
+//go:build windows
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+
+	"hamlog/internal/applog"
+)
+
+// clearDownloadMark strips the NTFS "downloaded from the internet" stream.
+// Otherwise Windows SmartScreen wants to prompt "are you sure you want to open
+// this?" — but since we launch the exe programmatically that prompt never
+// shows, and the launch is silently blocked. This is exactly why the relaunch
+// used to fail after an update.
+func clearDownloadMark(path string) { _ = os.Remove(path + ":Zone.Identifier") }
+
+// makeExecutable is a no-op on Windows, where the extension decides.
+func makeExecutable(path string) error { return nil }
+
+// scheduleDeferredSwap hands the exe swap to a detached helper that runs AFTER
+// this process is gone.
+//
+// The fallback for when the running image cannot be renamed at all. Once OpsLog
+// has exited its exe is an ordinary file again, so the move that was refused a
+// moment earlier succeeds — and the helper keeps trying for ten seconds, because
+// an antivirus that was holding the file usually lets go a beat after the
+// process dies rather than instantly.
+//
+// OpsLog is restarted either way. If the move failed, that starts the OLD build
+// — the update simply has not applied — and the operator keeps a working logger
+// instead of having it vanish mid-session, which for someone in a QSO is worse
+// than an update that waits. Only a successful swap passes --post-update, so a
+// failure leaves the .new file in place for the next attempt rather than having
+// the cleanup delete the download.
+// The LAST resort still needs a helper that outlives this process: nothing else
+// can move a file over an image that is still running. It stays PowerShell —
+// there is no smaller tool on a stock Windows that can wait for a pid and then
+// move a file — but it is reached only when the rename above failed, which is
+// rare, and never on the ordinary update path (see the relaunch there for why
+// that matters to Defender).
+func (a *App) scheduleDeferredSwap(exe, pending string) error {
+	// Clear the "downloaded from the internet" mark before it becomes the exe —
+	// SmartScreen silently blocks a programmatic launch of a marked file, and the
+	// mark follows the file across the move.
+	_ = os.Remove(pending + ":Zone.Identifier")
+
+	q := func(s string) string { return strings.ReplaceAll(s, "'", "''") }
+	ps := fmt.Sprintf(
+		"Wait-Process -Id %d -ErrorAction SilentlyContinue; "+
+			"$ok=$false; "+
+			"for ($i=0; $i -lt 40; $i++) { "+
+			"try { Move-Item -LiteralPath '%s' -Destination '%s' -Force -ErrorAction Stop; $ok=$true; break } "+
+			"catch { Start-Sleep -Milliseconds 250 } }; "+
+			"if ($ok) { Start-Process -FilePath '%s' -ArgumentList '--post-update' } "+
+			"else { Start-Process -FilePath '%s' }",
+		os.Getpid(), q(pending), q(exe), q(exe), q(exe))
+	cmd := exec.Command("powershell", "-NoProfile", "-WindowStyle", "Hidden", "-Command", ps)
+	hideConsole(cmd)
+	if err := cmd.Start(); err != nil {
+		return fmt.Errorf("schedule the update swap: %w", err)
+	}
+	applog.Printf("update: swap scheduled for after exit (%s → %s)", filepath.Base(pending), filepath.Base(exe))
+	return nil
+}