And process any number of files.

cat/main.go

@@ -17,7 +17,7 @@ func Fail(err error, format string, v ...any) {
 type Opts struct {
   Number bool
   Squeeze bool
-  Filename string
+  Filenames []string
 }
 
 func parse_opts() (Opts) {
@@ -32,16 +32,18 @@ func parse_opts() (Opts) {
     os.Exit(1)
   }
 
-  opts.Filename = flag.Arg(0)
+  opts.Filenames = flag.Args()
 
   return opts
 }
 
 func main() {
   opts := parse_opts()
-  in_file, err := os.ReadFile(opts.Filename)
 
-  if err != nil { Fail(err, "cannot open %s:", opts.Filename) }
+  for _, filename := range opts.Filenames {
+    in_file, err := os.ReadFile(filename)
+
+    if err != nil { Fail(err, "cannot open %s:", filename) }
 
     if(opts.Number) {
       count := 1
@@ -57,3 +59,4 @@ func main() {
       fmt.Print(string(in_file))
     }
   }
+}