add WithFilePrefix

This adds an option to prefix every file path with a given string.

main.go

@@ -2,6 +2,7 @@
 package main
 
 import (
+	"flag"
 	"fmt"
 	"io"
 	"os"
@@ -15,33 +16,56 @@ var Version = "unknown"
 var VersionHash = "unknown"
 
 const _usage = `Usage:
-  %s <infile> <outfile>
+  %s [OPTION]... <infile> <outfile>
 
 Flatten a Docker container image to a root file system.
 
 Arguments:
-  <infile>:  Input Docker container. Tarball.
-  <outfile>: Output tarball, the root file system. '-' is stdout.
+  <infile>   Input Docker container. Tarball.
+  <outfile>  Output tarball, the root file system. '-' is stdout.
+
+Options:
+  --prefix=<prefix>  prefix all destination files with a given string.
 
 undocker %s (%s)
 Built with %s
 `
 
+func usage(pre string, out io.Writer) {
+	fmt.Fprintf(out, pre+_usage,
+		filepath.Base(os.Args[0]),
+		Version,
+		VersionHash,
+		runtime.Version(),
+	)
+}
+
+func usageErr(pre string) {
+	usage(pre, os.Stderr)
+	os.Exit(2)
+}
+
 func main() {
 	runtime.GOMAXPROCS(1) // no need to create that many threads
 
-	if len(os.Args) != 3 {
-		fmt.Fprintf(os.Stderr, _usage,
-			filepath.Base(os.Args[0]),
-			Version,
-			VersionHash,
-			runtime.Version(),
-		)
-		os.Exit(1)
+	var filePrefix string
+	fs := flag.NewFlagSet("undocker", flag.ExitOnError)
+	fs.Usage = func() { usageErr("") }
+	fs.StringVar(&filePrefix, "prefix", "", "prefix files in the tarball")
+
+	if len(os.Args) == 1 {
+		usageErr("")
+	}
+
+	_ = fs.Parse(os.Args[1:]) // ExitOnError captures it
+
+	args := fs.Args()
+	if len(args) != 2 {
+		usageErr("invalid number of arguments\n")
 	}
 
 	c := &command{flattener: rootfs.Flatten, Stdout: os.Stdout}
-	if err := c.execute(os.Args[1], os.Args[2]); err != nil {
+	if err := c.execute(args[0], args[1], filePrefix); err != nil {
 		fmt.Printf("Error: %v\n", err)
 		os.Exit(1)
 	}
@@ -49,11 +73,11 @@ func main() {
 }
 
 type command struct {
-	flattener func(io.ReadSeeker, io.Writer) error
+	flattener func(io.ReadSeeker, io.Writer, ...rootfs.Option) error
 	Stdout    io.Writer
 }
 
-func (c *command) execute(infile string, outfile string) (_err error) {
+func (c *command) execute(infile, outfile, filePrefix string) (_err error) {
 	rd, err := os.Open(infile)
 	if err != nil {
 		return err
@@ -84,5 +108,5 @@ func (c *command) execute(infile string, outfile string) (_err error) {
 		out = outf
 	}
 
-	return c.flattener(rd, out)
+	return c.flattener(rd, out, rootfs.WithFilePrefix(filePrefix))
 }