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) {
@@ -32,28 +32,31 @@ 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(opts.Number) {
-    count := 1
-    for line := range strings.Lines(string(in_file)) {
-      if opts.Squeeze && len(line) <= 1 {
-        continue
-      }
+    if err != nil { Fail(err, "cannot open %s:", filename) }
+
+    if(opts.Number) {
+      count := 1
+      for line := range strings.Lines(string(in_file)) {
+        if opts.Squeeze && len(line) <= 1 {
+          continue
+        }
 
-      fmt.Printf("%0.4d: %s", count, line)
-      count++
+        fmt.Printf("%0.4d: %s", count, line)
+        count++
+      }
+    } else {
+      fmt.Print(string(in_file))
     }
-  } else {
-    fmt.Print(string(in_file))
   }
 }