Ditch fmt, log, path/filepath, and strings for ~17KB more sav…

…ings

```console
$ stat --format '% 11n %s' gosu-before gosu-after
gosu-before 1495254
 gosu-after 1478001
```

main.go

@@ -1,13 +1,10 @@
 package main // import "github.com/tianon/gosu"
 
 import (
- "fmt"
- "log"
+ "io"
  "os"
  "os/exec"
- "path/filepath"
  "runtime"
- "strings"
  "syscall"
 )
 
@@ -18,67 +15,74 @@ func init() {
 }
 
 func version() string {
- return fmt.Sprintf(`%s (%s on %s/%s; %s)`, Version, runtime.Version(), runtime.GOOS, runtime.GOARCH, runtime.Compiler)
+ // 1.17 (go1.18.2 on linux/amd64; gc)
+ return Version + ` (` + runtime.Version() + ` on ` + runtime.GOOS + `/` + runtime.GOARCH + `; ` + runtime.Compiler + `)`
 }
 
 func usage() string {
+ self := os.Args[0]
+ v := version()
  t := `
-Usage: {{ .Self }} user-spec command [args]
- eg: {{ .Self }} tianon bash
- {{ .Self }} nobody:root bash -c 'whoami && id'
- {{ .Self }} 1000:1 id
+Usage: ` + self + ` user-spec command [args]
+ eg: ` + self + ` tianon bash
+ ` + self + ` nobody:root bash -c 'whoami && id'
+ ` + self + ` 1000:1 id
 
-{{ .Self }} version: {{ .Version }}
-{{ .Self }} license: Apache-2.0 (full text at https://github.com/tianon/gosu)
+` + self + ` version: ` + v + `
+` + self + ` license: Apache-2.0 (full text at https://github.com/tianon/gosu)
 `
- t = strings.ReplaceAll(t, "{{ .Self }}", filepath.Base(os.Args[0]))
- t = strings.ReplaceAll(t, "{{ .Version }}", version())
  return t[1:]
 }
 
-func main() {
- log.SetFlags(0) // no timestamps on our logs
+func exit(code int, w io.Writer, ss ...string) {
+ for i, s := range ss {
+ if i > 0 {
+ w.Write([]byte{' '})
+ }
+ w.Write([]byte(s))
+ }
+ w.Write([]byte{'\n'})
+ os.Exit(code)
+}
 
+func main() {
  if ok := os.Getenv("GOSU_PLEASE_LET_ME_BE_COMPLETELY_INSECURE_I_GET_TO_KEEP_ALL_THE_PIECES"); ok != "I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the dark near the Tannhäuser Gate. All those moments will be lost in time, like tears in rain. Time to die." {
  if fi, err := os.Stat("/proc/self/exe"); err != nil {
- log.Fatalf("error: %v", err)
- } else if fi.Mode()&os.ModeSetuid != 0 {
+ exit(1, os.Stderr, "error:", err.Error())
+ } else if mode := fi.Mode(); mode&os.ModeSetuid != 0 {
  // ... oh no
- log.Fatalf("error: %q appears to be installed with the 'setuid' bit set, which is an *extremely* insecure and completely unsupported configuration! (what you want instead is likely 'sudo' or 'su')", os.Args[0])
- } else if fi.Mode()&os.ModeSetgid != 0 {
+ exit(1, os.Stderr, "error:", os.Args[0], "appears to be installed with the 'setuid' bit set, which is an *extremely* insecure and completely unsupported configuration! (what you want instead is likely 'sudo' or 'su')")
+ } else if mode&os.ModeSetgid != 0 {
  // ... oh no
- log.Fatalf("error: %q appears to be installed with the 'setgid' bit set, which is not quite *as* insecure as 'setuid', but still not great, and definitely a completely unsupported configuration! (what you want instead is likely 'sudo' or 'su')", os.Args[0])
+ exit(1, os.Stderr, "error:", os.Args[0], "appears to be installed with the 'setgid' bit set, which is not quite *as* insecure as 'setuid', but still not great, and definitely a completely unsupported configuration! (what you want instead is likely 'sudo' or 'su')")
  }
  }
 
  if len(os.Args) >= 2 {
  switch os.Args[1] {
  case "--help", "-h", "-?":
- fmt.Println(usage())
- os.Exit(0)
+ exit(0, os.Stdout, usage())
  case "--version", "-v":
- fmt.Println(version())
- os.Exit(0)
+ exit(0, os.Stdout, version())
  }
  }
  if len(os.Args) <= 2 {
- log.Println(usage())
- os.Exit(1)
+ exit(1, os.Stderr, usage())
  }
 
  // clear HOME so that SetupUser will set it
  os.Unsetenv("HOME")
 
  if err := SetupUser(os.Args[1]); err != nil {
- log.Fatalf("error: failed switching to %q: %v", os.Args[1], err)
+ exit(1, os.Stderr, "error: failed switching to '"+os.Args[1]+"':", err.Error())
  }
 
  name, err := exec.LookPath(os.Args[2])
  if err != nil {
- log.Fatalf("error: %v", err)
+ exit(1, os.Stderr, "error:", err.Error())
  }
 
  if err = syscall.Exec(name, os.Args[2:], os.Environ()); err != nil {
- log.Fatalf("error: exec failed: %v", err)
+ exit(1, os.Stderr, "error: exec failed:", err.Error())
  }
 }