diff --git a/cat/cat.go b/cat/cat.go
index eca92e5..2b8dfd8 100644
--- a/cat/cat.go
+++ b/cat/cat.go
@@ -30,7 +30,7 @@ const FILE_LINK_REGEXP = `(?m)` + HR_BLOCK_STRIP + `([0-9]+\.[^\.]*\.*md\n)+\n`
 var fileLinkRegexp *regexp.Regexp = regexp.MustCompile(FILE_LINK_REGEXP)
 
 type options struct {
-	Filelist       flags.Arg
+	Filelist       flags.Glob
 	Base           flags.String
 	KeepHeader     flags.Bool
 	KeepTitle      flags.Bool
@@ -40,7 +40,7 @@ type options struct {
 
 func initOpts() options {
 	return options{
-		flags.Arg{"", "filelist. accepts from stdin if not provided"},
+		flags.Glob{nil, ".um filelist. accepts multiple. reads from stdin if not provided"},
 		flags.String{"--base", "-b", "", "base directory prepended to files in filelist"},
 		flags.Bool{"--keep-header", "-d", false, "preserve um headers in concatenated file. overrides --keep-title"},
 		flags.Bool{"--keep-title", "-t", false, "preserve um titles in concatenated file"},
@@ -120,7 +120,8 @@ func Cat(args []string) {
 		}
 		log.Fatalf("um %s: %s", CMD, err)
 	}
-	files, err := pipe.FileListSplitMaybeStdin(opts.Filelist.Val)
+	// NOTE: um cat expects .um files, the content of which is assembled:
+	files, err := pipe.FileListFromGlobOrStdin(opts.Filelist.Val)
 	if err != nil {
 		log.Fatalf("um %s: %s", CMD, err)
 	}
diff --git a/flags/flags.go b/flags/flags.go
index 84880e4..54ec9bd 100644
--- a/flags/flags.go
+++ b/flags/flags.go
@@ -31,6 +31,11 @@ type Arg struct {
 	Help string
 }
 
+type Glob struct {
+	Val  []string
+	Help string
+}
+
 type String struct {
 	Long  string
 	Short string
@@ -69,6 +74,33 @@ func (f *Arg) MaybeIncrement(i int) int {
 	return i
 }
 
+func (f *Glob) Set(arg string) {
+	f.Val = append(f.Val, arg)
+}
+
+func (f *Glob) Valid(args []string, i int) bool {
+	return true
+}
+
+func (f *Glob) Match(arg string, i, j int) bool {
+	// NOTE: this is the key difference from Arg: by not requiring that the arg and flag positions
+	// match, a shell-expanded glob of files of any length will populate the value.
+	return i >= j && !hasDashPrefix(arg)
+}
+
+func (f *Glob) IsSet() bool {
+	return f.Val != nil && len(f.Val) != 0
+}
+
+func (f *Glob) IsHelp() bool {
+	return false
+}
+
+func (f *Glob) MaybeIncrement(i int) int {
+	// NOTE: because this is only needed where a flag --prefix exists:
+	return i
+}
+
 func (f *String) Set(arg string) {
 	f.Val = arg
 }
@@ -145,16 +177,16 @@ func expandOpts(opts any) ([]Flag, error) {
 	for j := 0; j < v.NumField(); j++ {
 		field := v.Field(j)
 		if field.Kind() != reflect.Struct {
-			return nil, ParseError{fmt.Sprintf("field must be a struct: %#v", field.Kind())}
+			return nil, ParseError{fmt.Sprintf("field must be a struct: %v", field.Kind())}
 		}
 		if !field.CanAddr() {
-			return nil, ParseError{fmt.Sprintf("field must be addressable type: %#v", field.Type())}
+			return nil, ParseError{fmt.Sprintf("field must be addressable type: %v", field.Type())}
 		}
 		// NOTE: Addr() gets the underlying address:
 		if f, ok := field.Addr().Interface().(Flag); ok {
 			flags = append(flags, f)
 		} else {
-			return nil, ParseError{fmt.Sprintf("field must be a []Flag interface: %#v", field.Type())}
+			return nil, ParseError{fmt.Sprintf("field must be a []Flag interface: %v", field.Type())}
 		}
 	}
 	return flags, nil
diff --git a/flags/help.go b/flags/help.go
index 6408f11..5ec106c 100644
--- a/flags/help.go
+++ b/flags/help.go
@@ -61,9 +61,12 @@ func (h HelpError) Help(opts any) error {
 		tField := t.Field(i)
 		vField := v.Field(i)
 		switch vField.Interface().(type) {
-		case Arg:
+		case Arg, Glob:
 			positional += fmt.Sprintf(" [%s]", strings.ToLower(tField.Name))
-			fmt.Fprintf(w, "[%v]\tstring\t%v\n", strings.ToLower(tField.Name), vField.FieldByName("Help"))
+			fmt.Fprintf(w, "[%v]\t%v\t%v\n",
+				strings.ToLower(tField.Name),
+				vField.FieldByName("Val").Type(),
+				vField.FieldByName("Help"))
 		case String, Bool:
 			if tField.Name != "Help" {
 				positional += fmt.Sprintf(" [%s]", vField.FieldByName("Long"))
@@ -75,7 +78,7 @@ func (h HelpError) Help(opts any) error {
 				vField.FieldByName("Help"),
 			)
 		default:
-			return ParseError{fmt.Sprintf("needs a []Flag interface: %#v", vField.Type())}
+			return ParseError{fmt.Sprintf("Help did not recognize the Flag: %v", vField.Type())}
 
 		}
 	}
diff --git a/pipe/pipe.go b/pipe/pipe.go
index 47a6af1..7c78345 100644
--- a/pipe/pipe.go
+++ b/pipe/pipe.go
@@ -29,15 +29,30 @@ func GetStdin() ([]string, error) {
 	return strings.Split(s, Newline), nil
 }
 
-// reads files from stdin if present, otherwise from the filename:
-func FileListSplitMaybeStdin(f string) ([]string, error) {
+// reads files from stdin if present, otherwise just returns the glob:
+func GlobOrStdin(glob []string) ([]string, error) {
 	filelist, err := GetStdin()
 	if err != nil {
-		// BORK: don't see a neater way of wrapping these two when we depend on the non-nil in this block:
-		// if i just return FileListSplit(), we lose the first err
-		filelist, err2 := FileListSplit(f)
-		if err2 != nil {
-			return nil, fmt.Errorf("%w: %w", err, err2)
+		if glob == nil || len(glob) == 0 {
+			return nil, fmt.Errorf("glob is empty")
+		}
+		return glob, nil
+	}
+	return filelist, nil
+}
+
+// reads files from stdin if present, otherwise reads content from files in glob and assembles into a list.
+func FileListFromGlobOrStdin(glob []string) ([]string, error) {
+	filelist, err := GetStdin()
+	if err != nil {
+		for _, f := range glob {
+			// TODO: don't see a neater way of wrapping these two when we depend on the non-nil in this block:
+			// if i just return FileListSplit(), we lose the first err
+			fl, err2 := FileListSplit(f)
+			if err2 != nil {
+				return nil, fmt.Errorf("%w: %w", err, err2)
+			}
+			filelist = append(filelist, fl...)
 		}
 		return filelist, nil
 	}
diff --git a/sort/sort.go b/sort/sort.go
index 5bdda0a..7f67c1f 100644
--- a/sort/sort.go
+++ b/sort/sort.go
@@ -19,15 +19,15 @@ const (
 )
 
 type options struct {
-	Source flags.String
-	Key    flags.String
-	Write  flags.Bool
-	Help   flags.Bool
+	Filelist flags.Glob
+	Key      flags.String
+	Write    flags.Bool
+	Help     flags.Bool
 }
 
 func initOpts() options {
 	return options{
-		flags.String{"--source", "-s", "", "path to source list. reads from stdin if omitted."},
+		flags.Glob{nil, ".md filelist. accepts multiple. reads from stdin if not provided"},
 		flags.String{"--key", "-k", "", "path to sort key"},
 		flags.Bool{"--write", "-w", false, "write sorted list back to --key file"},
 		flags.Bool{"--help", "-h", false, "show help"},
@@ -84,7 +84,8 @@ func Sort(args []string) {
 		return
 	}
 
-	sslice, err := pipe.FileListSplitMaybeStdin(opts.Source.Val)
+	// NOTE: um sort expects a list of .md files, in contrast to um cat.
+	sslice, err := pipe.GlobOrStdin(opts.Filelist.Val)
 	if err != nil {
 		log.Fatalf("um %s: %s", CMD, err)
 	}