First coreutil started, cat.

cat/go.mod

@@ -0,0 +1,3 @@
+module git.learnjsthehardway.com/learn-code-the-hard-way/go-coreutils/cat
+
+go 1.24.2

cat/main.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+  "fmt"
+  "flag"
+  "os"
+  "log"
+  "strings"
+)
+
+func Fail(err error, format string, v ...any) {
+  err_format := fmt.Sprintf("ERROR: %v; %s", err, format)
+  log.Printf(err_format, v...)
+  os.Exit(1)
+}
+
+func main() {
+  var number bool
+
+  flag.BoolVar(&number, "n", false, "Number all nonempty output lines, starting with 1")
+  flag.BoolVar(&number, "number", false, "Number all nonempty output lines, starting with 1")
+  flag.Parse()
+
+  if flag.NArg() < 1 {
+    log.Fatal("USAGE: cat [-n/--number] file0 [fileN]")
+    os.Exit(1)
+  }
+
+  filename := flag.Arg(0)
+  in_file, err := os.ReadFile(filename)
+
+  if err != nil { Fail(err, "cannot open %s:", filename) }
+
+  if(number) {
+    count := 1
+    for line := range strings.Lines(string(in_file)) {
+      fmt.Printf("%0.4d: %s", count, line)
+      count++
+    }
+  } else {
+    fmt.Print(string(in_file))
+  }
+}