fix: restore persistence functionality

This commit addresses an issue where the switch to using systemd
templates and format specifiers introduced a regression, causing the
persistence functionality to no longer work as expected.

To resolve this issue, we have reverted to an earlier implementation
that manually writes all the units and uses Go's exec.LookPath to
resolve the shell path.

Resolves: #65

bat.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=Persist the battery charging threshold after {{.Event}}
+After={{.Event}}.target
+StartLimitBurst=0
+
+[Service]
+Type=oneshot
+ExecStart={{.Shell}} -c 'echo {{.Threshold}} > /sys/class/power_supply/BAT?/charge_control_end_threshold'
+Restart=on-failure
+RemainAfterExit=true
+
+[Install]
+WantedBy={{.Event}}.target

main_linux.go

@@ -15,6 +15,7 @@ import (
 	"strconv"
 	"strings"
 	"syscall"
+	"text/template"
 	"time"
 
 	"golang.org/x/sys/unix"
@@ -23,10 +24,10 @@ import (
 const (
 	device    = "/sys/class/power_supply/BAT?"
 	threshold = "charge_control_end_threshold"
-	service   = "/etc/systemd/system/bat@.service"
+	services  = "/etc/systemd/system/"
 )
 
-var targets = [...]string{
+var events = [...]string{
 	"hibernate",
 	"hybrid-sleep",
 	"multi-user",
@@ -37,7 +38,7 @@ var targets = [...]string{
 var build, tag string
 
 var (
-	//go:embed bat@.service
+	//go:embed bat.service
 	unit string
 	//go:embed help.fmt
 	help string
@@ -141,16 +142,43 @@ func main() {
 		if err != nil {
 			log.Fatal(err)
 		}
-		tmpl := fmt.Sprintf(unit, current)
-		if err := os.WriteFile(service, []byte(tmpl), 0o644); err != nil {
-			if errors.Is(err, syscall.EACCES) {
-				fmt.Fprintln(os.Stderr, "Permission denied. Try running this command using sudo.")
+
+		shell, err := exec.LookPath("bash")
+		if err != nil {
+			if errors.Is(err, exec.ErrNotFound) {
+				fmt.Fprintln(os.Stderr, "Could not find Bash on your system.")
 				os.Exit(1)
 			}
 			log.Fatal(err)
 		}
-		for _, target := range targets {
-			cmd := exec.Command("systemctl", "enable", fmt.Sprintf("bat@%s.service", target))
+
+		for _, event := range events {
+			tmpl, err := template.New("unit").Parse(unit)
+			if err != nil {
+				log.Fatal(err)
+			}
+
+			name := services + "bat-" + event + ".service"
+			f, err := os.Create(name)
+			if err != nil {
+				if errors.Is(err, syscall.EACCES) {
+					fmt.Fprintln(os.Stderr, "Permission denied. Try running this command using sudo.")
+					os.Exit(1)
+				}
+				log.Fatal(err)
+			}
+			defer f.Close()
+
+			service := struct {
+				Event     string
+				Shell     string
+				Threshold int
+			}{event, shell, current}
+			if err := tmpl.Execute(f, service); err != nil {
+				log.Fatal(err)
+			}
+
+			cmd := exec.Command("systemctl", "enable", name)
 			if err := cmd.Run(); err != nil {
 				log.Fatal(err)
 			}
@@ -202,9 +230,10 @@ func main() {
 			fmt.Fprintln(os.Stdout, "Charging threshold set.\nRun `sudo bat persist` to persist the setting between restarts.")
 		}
 	case "reset":
-		for _, target := range targets {
+		for _, event := range events {
+			name := services + "bat-" + event + ".service"
 			buf := new(bytes.Buffer)
-			cmd := exec.Command("systemctl", "disable", fmt.Sprintf("bat@%s.service", target))
+			cmd := exec.Command("systemctl", "disable", name)
 			cmd.Stderr = buf
 			if err := cmd.Run(); err != nil {
 				switch message := buf.String(); {
@@ -217,9 +246,9 @@ func main() {
 					log.Fatal(err)
 				}
 			}
-		}
-		if err := os.Remove(service); err != nil && !errors.Is(err, syscall.ENOENT) {
-			log.Fatal(err)
+			if err := os.Remove(name); err != nil && !errors.Is(err, syscall.ENOENT) {
+				log.Fatal(err)
+			}
 		}
 		fmt.Fprintln(os.Stdout, "Charging threshold persistence reset.")
 	default: