Add a way to strip top-level folder

Closes #238

cmd/arc/main.go

@@ -21,6 +21,7 @@ var (
 	mkdirAll               bool
 	selectiveCompression   bool
 	implicitTopLevelFolder bool
+	stripComponents        int
 	continueOnError        bool
 	specifyFileType        string
 )
@@ -37,6 +38,7 @@ func init() {
 	flag.BoolVar(&mkdirAll, "mkdirs", false, "Make all necessary directories")
 	flag.BoolVar(&selectiveCompression, "smart", true, "Only compress files which are not already compressed (zip only)")
 	flag.BoolVar(&implicitTopLevelFolder, "folder-safe", true, "If an archive does not have a single top-level folder, create one implicitly")
+	flag.IntVar(&stripComponents, "strip-components", 0, "Strip number of leading paths")
 	flag.BoolVar(&continueOnError, "allow-errors", true, "Log errors and continue processing")
 	flag.StringVar(&specifyFileType, "ext", "", "specify file type")
 }
@@ -223,6 +225,7 @@ func getFormat(subcommand string) (interface{}, error) {
 		OverwriteExisting:      overwriteExisting,
 		MkdirAll:               mkdirAll,
 		ImplicitTopLevelFolder: implicitTopLevelFolder,
+		StripComponents:        stripComponents,
 		ContinueOnError:        continueOnError,
 	}
 

tar.go

@@ -40,6 +40,10 @@ type Tar struct {
 	// especially on extraction.
 	ImplicitTopLevelFolder bool
 
+	// Strip number of leading paths. This feature is available
+	// only during unpacking of the entire archive.
+	StripComponents int
+
 	// If true, errors encountered during reading
 	// or writing a single file will be logged and
 	// the operation will continue on remaining files.
@@ -233,6 +237,17 @@ func (t *Tar) untarNext(destination string) error {
 	if errPath != nil {
 		return fmt.Errorf("checking path traversal attempt: %v", errPath)
 	}
+
+	if t.StripComponents > 0 {
+		if strings.Count(header.Name, "/") < t.StripComponents {
+			return nil // skip path with fewer components
+		}
+
+		for i := 0; i < t.StripComponents; i++ {
+			slash := strings.Index(header.Name, "/")
+			header.Name = header.Name[slash+1:]
+		}
+	}
 	return t.untarFile(f, destination, header)
 }