feat: add filecontent named mapper

README.md

@@ -485,6 +485,7 @@ function `NamedMapper(name, mapper)`.
 | `existingfile` | An existing file. ~ expansion is applied. `-` is accepted for stdin, and will be passed unaltered. |
 | `existingdir`  | An existing directory. ~ expansion is applied.                                                     |
 | `counter`      | Increment a numeric field. Useful for `-vvv`. Can accept `-s`, `--long` or `--long=N`.             |
+| `filecontent`  | Read the file at path into the field. ~ expansion is applied. `-` is accepted for stdin, and will be passed unaltered. |
 
 
 Slices and maps treat type tags specially. For slices, the `type:""` tag

mapper.go

@@ -283,6 +283,7 @@ func (r *Registry) RegisterDefaults() *Registry {
 		RegisterName("existingfile", existingFileMapper(r)).
 		RegisterName("existingdir", existingDirMapper(r)).
 		RegisterName("counter", counterMapper()).
+		RegisterName("filecontent", fileContentMapper(r)).
 		RegisterKind(reflect.Ptr, ptrMapper{r})
 }
 
@@ -684,6 +685,39 @@ func existingDirMapper(r *Registry) MapperFunc {
 	}
 }
 
+func fileContentMapper(r *Registry) MapperFunc {
+	return func(ctx *DecodeContext, target reflect.Value) error {
+		if target.Kind() != reflect.Slice && target.Elem().Kind() != reflect.Uint8 {
+			return fmt.Errorf("\"filecontent\" must be applied to []byte not %s", target.Type())
+		}
+		var path string
+		err := ctx.Scan.PopValueInto("file", &path)
+		if err != nil {
+			return err
+		}
+
+		if !ctx.Value.Active || ctx.Value.Set {
+			// early return to avoid checking extra dirs that may not exist;
+			// this hack only works because the value provided on the cli is
+			// checked before the default value is checked (if default is set).
+			return nil
+		}
+
+		var data []byte
+		if path != "-" {
+			path = ExpandPath(path)
+			data, err = ioutil.ReadFile(path) //nolint:gosec
+		} else {
+			data, err = ioutil.ReadAll(os.Stdin)
+		}
+		if err != nil {
+			return err
+		}
+		target.SetBytes(data)
+		return nil
+	}
+}
+
 type ptrMapper struct {
 	r *Registry
 }

mapper_test.go

@@ -469,6 +469,24 @@ func TestFileMapper(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, os.Stdin, cli.File)
 }
+func TestFileContentMapper(t *testing.T) {
+	type CLI struct {
+		File []byte `type:"filecontent"`
+	}
+	var cli CLI
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--file", "testdata/file.txt"})
+	assert.NoError(t, err)
+	assert.Equal(t, []byte(`Hello world.`), cli.File)
+	p = mustNew(t, &cli)
+	_, err = p.Parse([]string{"--file", "testdata/missing.txt"})
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "missing.txt: no such file or directory")
+	p = mustNew(t, &cli)
+	_, err = p.Parse([]string{"--file", "testdata/"})
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "is a directory")
+}
 
 //nolint:dupl
 func TestExistingFileMapper(t *testing.T) {