refactor: use 'log' instead of 'fmt'

cli.go

@@ -3,6 +3,7 @@ package main
 import (
 	"flag"
 	"fmt"
+	"log"
 	"os"
 )
 
@@ -53,8 +54,7 @@ func initFlags() {
 	)
 
 	flag.Usage = func() {
-		fmt.Fprintf(
-			os.Stderr,
+		log.Printf(
 			`USAGE: %s [OPTIONS] INPUT
 
 INPUT is a template file or '-' for stdin

context.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
+	"log"
 	"os"
 	"strings"
 
@@ -43,7 +44,7 @@ func getFileVariables(file string) map[string]interface{} {
 
 	bytes, err := ioutil.ReadFile(file)
 	if err != nil {
-		fmt.Fprint(os.Stderr, fmt.Errorf("unable to read file\n%v\n", err))
+		log.Printf("unable to read file\n%v\n", err)
 		return vars
 	}
 
@@ -57,7 +58,7 @@ func getFileVariables(file string) map[string]interface{} {
 		err = fmt.Errorf("bad file type: %s", file)
 	}
 	if err != nil {
-		fmt.Fprint(os.Stderr, fmt.Errorf("unable to load data\n%v\n", err))
+		log.Printf("unable to load data\n%v\n", err)
 	}
 	return vars
 }

main.go

@@ -3,14 +3,18 @@ package main
 import (
 	"flag"
 	"fmt"
+	"log"
 	"os"
 	"path/filepath"
 )
 
 func main() {
+	log.SetFlags(0)
+	log.SetPrefix(fmt.Sprintf("%s: ", os.Args[0]))
 
 	initFlags()
 	if flag.NArg() == 0 {
+		log.SetPrefix("")
 		flag.Usage()
 		os.Exit(2)
 	}
@@ -26,8 +30,7 @@ func main() {
 
 	output, err := executeTemplateFile(input)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "%v", err)
-		return
+		log.Fatal(err)
 	}
 	fmt.Print(output)
 }