dump go.mod and go.sum

Merge remote-tracking branch 'flint/master' into jcarr
Signed-off-by: Jeff Carr <jcarr@wit.com>
2024-11-07 16:55:05 -06:00 · 2024-11-07 12:48:14 -06:00 · 2024-11-04 12:58:37 -05:00 · 2024-11-04 12:56:12 -05:00 · 2024-10-21 17:08:37 -04:00 · 2024-09-07 13:07:21 +01:00
20 changed files with 1672 additions and 427 deletions
--- a/.github/FUNDING.yml
+++ b/.github/FUNDING.yml
@ -0,0 +1 @@
+github: [alexflint]
--- a/.github/workflows/go.yml
+++ b/.github/workflows/go.yml
@ -15,17 +15,17 @@ jobs:
    strategy:
      fail-fast: false
      matrix:
-        go: ['1.17', '1.18', '1.19']
+        go: ['1.20', '1.21', '1.22']

    steps:
    - id: go
      name: Set up Go
-      uses: actions/setup-go@v3
+      uses: actions/setup-go@v5
      with:
        go-version: ${{ matrix.go }}

    - name: Checkout
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4

    - name: Build
      run: go build -v .
--- a/11
+++ b/11
@ -0,0 +1,11 @@
+all:
+	@echo
+	@echo
+
+clean:
+	rm -f go.*
+
+redomod:
+	rm -f go.*
+	GO111MODULE= go mod init
+	GO111MODULE= go mod tidy
--- a/README.md
+++ b/README.md
@ -64,7 +64,7 @@ fmt.Println("Input:", args.Input)
 fmt.Println("Output:", args.Output)
 ```

-```
+```shell
 $ ./example src.txt x.out y.out z.out
 Input: src.txt
 Output: [x.out y.out z.out]
@ -80,12 +80,12 @@ arg.MustParse(&args)
 fmt.Println("Workers:", args.Workers)
 ```

-```
+```shell
 $ WORKERS=4 ./example
 Workers: 4
 ```

-```
+```shell
 $ WORKERS=4 ./example --workers=6
 Workers: 6
 ```
@ -100,12 +100,12 @@ arg.MustParse(&args)
 fmt.Println("Workers:", args.Workers)
 ```

-```
+```shell
 $ NUM_WORKERS=4 ./example
 Workers: 4
 ```

-You can provide multiple values using the CSV (RFC 4180) format:
+You can provide multiple values in environment variables using commas:

 ```go
 var args struct {
@ -115,12 +115,50 @@ arg.MustParse(&args)
 fmt.Println("Workers:", args.Workers)
 ```

-```
+```shell
 $ WORKERS='1,99' ./example
 Workers: [1 99]
 ```

+Command line arguments take precedence over environment variables:
+
+```go
+var args struct {
+	Workers int `arg:"--count,env:NUM_WORKERS"`
+}
+arg.MustParse(&args)
+fmt.Println("Workers:", args.Workers)
+```
+
+```shell
+$ NUM_WORKERS=6 ./example
+Workers: 6
+$ NUM_WORKERS=6 ./example --count 4
+Workers: 4
+```
+
+Configuring a global environment variable name prefix is also possible:
+
+```go
+var args struct {
+	Workers int `arg:"--count,env:NUM_WORKERS"`
+}
+
+p, err := arg.NewParser(arg.Config{
+    EnvPrefix: "MYAPP_",
+}, &args)
+
+p.MustParse(os.Args[1:])
+fmt.Println("Workers:", args.Workers)
+```
+
+```shell
+$ MYAPP_NUM_WORKERS=6 ./example
+Workers: 6
+```
+
 ### Usage strings
+
 ```go
 var args struct {
 	Input    string   `arg:"positional"`
@ -158,20 +196,7 @@ var args struct {
 arg.MustParse(&args)
 ```

-### Default values (before v1.2)
-
-```go
-var args struct {
-	Foo string
-	Bar bool
-}
-arg.Foo = "abc"
-arg.MustParse(&args)
-```
-
-### Combining command line options, environment variables, and default values
-
-You can combine command line arguments, environment variables, and default values. Command line arguments take precedence over environment variables, which take precedence over default values. This means that we check whether a certain option was provided on the command line, then if not, we check for an environment variable (only if an `env` tag was provided), then if none is found, we check for a `default` tag containing a default value.
+Command line arguments take precedence over environment variables, which take precedence over default values. This means that we check whether a certain option was provided on the command line, then if not, we check for an environment variable (only if an `env` tag was provided), then if none is found, we check for a `default` tag containing a default value.

 ```go
 var args struct {
@ -182,9 +207,6 @@ arg.MustParse(&args)

 #### Ignoring environment variables and/or default values

-The values in an existing structure can be kept in-tact by ignoring environment
-variables and/or default values.
-
 ```go
 var args struct {
    Test  string `arg:"-t,env:TEST" default:"something"`
@ -195,10 +217,11 @@ p, err := arg.NewParser(arg.Config{
    IgnoreDefault: true,
 }, &args)

-err = p.Parse(os.Args)
+err = p.Parse(os.Args[1:])
 ```

 ### Arguments with multiple values
+
 ```go
 var args struct {
 	Database string
@ -214,6 +237,7 @@ Fetching the following IDs from foo: [1 2 3]
 ```

 ### Arguments that can be specified multiple times, mixed with positionals
+
 ```go
 var args struct {
    Commands  []string `arg:"-c,separate"`
@ -231,6 +255,7 @@ Databases [db1 db2 db3]
 ```

 ### Arguments with keys and values
+
 ```go
 var args struct {
 	UserIDs map[string]int
@ -244,24 +269,6 @@ fmt.Println(args.UserIDs)
 map[john:123 mary:456]
 ```

-### Custom validation
-```go
-var args struct {
-	Foo string
-	Bar string
-}
-p := arg.MustParse(&args)
-if args.Foo == "" && args.Bar == "" {
-	p.Fail("you must provide either --foo or --bar")
-}
-```
-
-```shell
-./example
-Usage: samples [--foo FOO] [--bar BAR]
-error: you must provide either --foo or --bar
-```
-
 ### Version strings

 ```go
@ -284,6 +291,28 @@ $ ./example --version
 someprogram 4.3.0
 ```

+> **Note**
+> If a `--version` flag is defined in `args` or any subcommand, it overrides the built-in versioning.
+
+### Custom validation
+
+```go
+var args struct {
+	Foo string
+	Bar string
+}
+p := arg.MustParse(&args)
+if args.Foo == "" && args.Bar == "" {
+	p.Fail("you must provide either --foo or --bar")
+}
+```
+
+```shell
+./example
+Usage: samples [--foo FOO] [--bar BAR]
+error: you must provide either --foo or --bar
+```
+
 ### Overriding option names

 ```go
@ -308,13 +337,11 @@ Options:
  --help, -h             display this help and exit
 ```

-
 ### Embedded structs

 The fields of embedded structs are treated just like regular fields:

 ```go
-
 type DatabaseOptions struct {
 	Host     string
 	Username string
@ -382,6 +409,7 @@ func main() {
 	fmt.Printf("%#v\n", args.Name)
 }
 ```
+
 ```shell
 $ ./example --name=foo.bar
 main.NameDotName{Head:"foo", Tail:"bar"}
@ -418,6 +446,7 @@ func main() {
 	fmt.Printf("%#v\n", args.Name)
 }
 ```
+
 ```shell
 $ ./example --help
 Usage: test [--name NAME]
@ -432,8 +461,6 @@ main.NameDotName{Head:"file", Tail:"txt"}

 ### Custom placeholders

-*Introduced in version 1.3.0*
-
 Use the `placeholder` tag to control which placeholder text is used in the usage text.

 ```go
@ -445,6 +472,7 @@ var args struct {
 }
 arg.MustParse(&args)
 ```
+
 ```shell
 $ ./example -h
 Usage: example [--optimize LEVEL] [--maxjobs N] SRC [DST [DST ...]]
@ -521,8 +549,6 @@ For more information visit github.com/alexflint/go-arg

 ### Subcommands

-*Introduced in version 1.1.0*
-
 Subcommands are commonly used in tools that wish to group multiple functions into a single program. An example is the `git` tool:
 ```shell
 $ git checkout [arguments specific to checking out code]
@ -583,15 +609,187 @@ if p.Subcommand() == nil {
 }
 ```

+### Custom handling of --help and --version
+
+The following reproduces the internal logic of `MustParse` for the simple case where
+you are not using subcommands or --version. This allows you to respond
+programatically to --help, and to any errors that come up.
+
+```go
+var args struct {
+	Something string
+}
+
+p, err := arg.NewParser(arg.Config{}, &args)
+if err != nil {
+	log.Fatalf("there was an error in the definition of the Go struct: %v", err)
+}
+
+err = p.Parse(os.Args[1:])
+switch {
+case err == arg.ErrHelp:  // indicates that user wrote "--help" on command line
+	p.WriteHelp(os.Stdout)
+	os.Exit(0)
+case err != nil:
+	fmt.Printf("error: %v\n", err)
+	p.WriteUsage(os.Stdout)
+	os.Exit(1)
+}
+```
+
+```shell
+$ go run ./example --help
+Usage: ./example --something SOMETHING
+
+Options:
+  --something SOMETHING
+  --help, -h             display this help and exit
+
+$ ./example --wrong
+error: unknown argument --wrong
+Usage: ./example --something SOMETHING
+
+$ ./example
+error: --something is required
+Usage: ./example --something SOMETHING
+```
+
+To also handle --version programatically, use the following:
+
+```go
+type args struct {
+	Something string
+}
+
+func (args) Version() string {
+	return "1.2.3"
+}
+
+func main() {
+	var args args
+	p, err := arg.NewParser(arg.Config{}, &args)
+	if err != nil {
+		log.Fatalf("there was an error in the definition of the Go struct: %v", err)
+	}
+
+	err = p.Parse(os.Args[1:])
+	switch {
+	case err == arg.ErrHelp: // found "--help" on command line
+		p.WriteHelp(os.Stdout)
+		os.Exit(0)
+	case err == arg.ErrVersion: // found "--version" on command line
+		fmt.Println(args.Version())
+		os.Exit(0)
+	case err != nil:
+		fmt.Printf("error: %v\n", err)
+		p.WriteUsage(os.Stdout)
+		os.Exit(1)
+	}
+
+	fmt.Printf("got %q\n", args.Something)
+}
+```
+
+```shell
+$ ./example --version
+1.2.3
+
+$ go run ./example --help
+1.2.3
+Usage: example --something SOMETHING
+
+Options:
+  --something SOMETHING
+  --help, -h             display this help and exit
+
+$ ./example --wrong
+1.2.3
+error: unknown argument --wrong
+Usage: example --something SOMETHING
+
+$ ./example
+error: --something is required
+Usage: example --something SOMETHING
+```
+
+To generate subcommand-specific help messages, use the following most general version
+(this also works in absence of subcommands but is a bit more complex):
+
+```go
+type fetchCmd struct {
+	Count int
+}
+
+type args struct {
+	Something string
+	Fetch     *fetchCmd `arg:"subcommand"`
+}
+
+func (args) Version() string {
+	return "1.2.3"
+}
+
+func main() {
+	var args args
+	p, err := arg.NewParser(arg.Config{}, &args)
+	if err != nil {
+		log.Fatalf("there was an error in the definition of the Go struct: %v", err)
+	}
+
+	err = p.Parse(os.Args[1:])
+	switch {
+	case err == arg.ErrHelp: // found "--help" on command line
+		p.WriteHelpForSubcommand(os.Stdout, p.SubcommandNames()...)
+		os.Exit(0)
+	case err == arg.ErrVersion: // found "--version" on command line
+		fmt.Println(args.Version())
+		os.Exit(0)
+	case err != nil:
+		fmt.Printf("error: %v\n", err)
+		p.WriteUsageForSubcommand(os.Stdout, p.SubcommandNames()...)
+		os.Exit(1)
+	}
+}
+```
+
+```shell
+$ ./example --version
+1.2.3
+
+$ ./example --help
+1.2.3
+Usage: example [--something SOMETHING] <command> [<args>]
+
+Options:
+  --something SOMETHING
+  --help, -h             display this help and exit
+  --version              display version and exit
+
+Commands:
+  fetch
+
+$ ./example fetch --help
+1.2.3
+Usage: example fetch [--count COUNT]
+
+Options:
+  --count COUNT
+
+Global options:
+  --something SOMETHING
+  --help, -h             display this help and exit
+  --version              display version and exit
+```
+
 ### API Documentation

-https://godoc.org/github.com/alexflint/go-arg
+https://pkg.go.dev/github.com/alexflint/go-arg

 ### Rationale

 There are many command line argument parsing libraries for Go, including one in the standard library, so why build another?

-The `flag` library that ships in the standard library seems awkward to me. Positional arguments must preceed options, so `./prog x --foo=1` does what you expect but `./prog --foo=1 x` does not. It also does not allow arguments to have both long (`--foo`) and short (`-f`) forms.
+The `flag` library that ships in the standard library seems awkward to me. Positional arguments must precede options, so `./prog x --foo=1` does what you expect but `./prog --foo=1 x` does not. It also does not allow arguments to have both long (`--foo`) and short (`-f`) forms.

 Many third-party argument parsing libraries are great for writing sophisticated command line interfaces, but feel to me like overkill for a simple script with a few flags.

@ -599,4 +797,4 @@ The idea behind `go-arg` is that Go already has an excellent way to describe dat

 ### Backward compatibility notes

-Earlier versions of this library required the help text to be part of the `arg` tag. This is still supported but is now deprecated. Instead, you should use a separate `help` tag, described above, which removes most of the limits on the text you can write. In particular, you will need to use the new `help` tag if your help text includes any commas.
+Earlier versions of this library required the help text to be part of the `arg` tag. This is still supported but is now deprecated. Instead, you should use a separate `help` tag, described above, which makes it possible to include commas inside help text.
--- a/go.mod
+++ b/go.mod
@ -1,8 +0,0 @@
-module github.com/alexflint/go-arg
-
-require (
-	github.com/alexflint/go-scalar v1.2.0
-	github.com/stretchr/testify v1.7.0
-)
-
-go 1.13
--- a/go.sum
+++ b/go.sum
@ -1,17 +0,0 @@
-github.com/alexflint/go-scalar v1.1.0 h1:aaAouLLzI9TChcPXotr6gUhq+Scr8rl0P9P4PnltbhM=
-github.com/alexflint/go-scalar v1.1.0/go.mod h1:LoFvNMqS1CPrMVltza4LvnGKhaSpc3oyLEBUZVhhS2o=
-github.com/alexflint/go-scalar v1.2.0 h1:WR7JPKkeNpnYIOfHRa7ivM21aWAdHD0gEWHCx+WQBRw=
-github.com/alexflint/go-scalar v1.2.0/go.mod h1:LoFvNMqS1CPrMVltza4LvnGKhaSpc3oyLEBUZVhhS2o=
-github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
-github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
-github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
-github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
-github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
-github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=
-github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
-gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
--- a/parse.go
+++ b/parse.go
@ -5,12 +5,13 @@ import (
 	"encoding/csv"
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 	"reflect"
 	"strings"

-	scalar "github.com/alexflint/go-scalar"
+	"go.wit.com/dev/alexflint/scalar"
 )

 // path represents a sequence of steps to find the output location for an
@ -43,23 +44,25 @@ func (p path) Child(f reflect.StructField) path {

 // spec represents a command line option
 type spec struct {
-	dest        path
-	field       reflect.StructField // the struct field from which this option was created
-	long        string              // the --long form for this option, or empty if none
-	short       string              // the -s short form for this option, or empty if none
-	cardinality cardinality         // determines how many tokens will be present (possible values: zero, one, multiple)
-	required    bool                // if true, this option must be present on the command line
-	positional  bool                // if true, this option will be looked for in the positional flags
-	separate    bool                // if true, each slice and map entry will have its own --flag
-	help        string              // the help text for this option
-	env         string              // the name of the environment variable for this option, or empty for none
-	defaultVal  string              // default value for this option
-	placeholder string              // name of the data in help
+	dest          path
+	field         reflect.StructField // the struct field from which this option was created
+	long          string              // the --long form for this option, or empty if none
+	short         string              // the -s short form for this option, or empty if none
+	cardinality   cardinality         // determines how many tokens will be present (possible values: zero, one, multiple)
+	required      bool                // if true, this option must be present on the command line
+	positional    bool                // if true, this option will be looked for in the positional flags
+	separate      bool                // if true, each slice and map entry will have its own --flag
+	help          string              // the help text for this option
+	env           string              // the name of the environment variable for this option, or empty for none
+	defaultValue  reflect.Value       // default value for this option
+	defaultString string              // default value for this option, in string form to be displayed in help text
+	placeholder   string              // placeholder string in help
 }

 // command represents a named subcommand, or the top-level command
 type command struct {
 	name        string
+	aliases     []string
 	help        string
 	dest        path
 	specs       []*spec
@ -67,33 +70,49 @@ type command struct {
 	parent      *command
 }

-// ErrHelp indicates that -h or --help were provided
+// ErrHelp indicates that the builtin -h or --help were provided
 var ErrHelp = errors.New("help requested by user")

-// ErrVersion indicates that --version was provided
+// ErrVersion indicates that the builtin --version was provided
 var ErrVersion = errors.New("version requested by user")

+// for monkey patching in example and test code
+var mustParseExit = os.Exit
+var mustParseOut io.Writer = os.Stdout
+
+// This stores the args sent from modules
+var register []interface{}
+
+/*
+	Use this in your packages to register
+	variables with go-arg. Then add this to your init()
+
+	package 'foo'
+	function init() {
+		args.Register(&argsFoo)
+	}
+*/
+func Register(dest ...interface{}) {
+	register = append(register, dest...)
+}
+
 // MustParse processes command line arguments and exits upon failure
 func MustParse(dest ...interface{}) *Parser {
-	p, err := NewParser(Config{}, dest...)
+	register = append(register, dest...)
+	return mustParse(Config{Exit: mustParseExit, Out: mustParseOut}, register...)
+}
+
+
+// mustParse is a helper that facilitates testing
+func mustParse(config Config, dest ...interface{}) *Parser {
+	p, err := NewParser(config, dest...)
 	if err != nil {
-		fmt.Fprintln(stdout, err)
-		osExit(-1)
-		return nil // just in case osExit was monkey-patched
-	}
-
-	err = p.Parse(flags())
-	switch {
-	case err == ErrHelp:
-		p.writeHelpForSubcommand(stdout, p.lastCmd)
-		osExit(0)
-	case err == ErrVersion:
-		fmt.Fprintln(stdout, p.version)
-		osExit(0)
-	case err != nil:
-		p.failWithSubcommand(err.Error(), p.lastCmd)
+		fmt.Fprintln(config.Out, err)
+		config.Exit(2)
+		return nil
 	}

+	p.MustParse(flags())
 	return p
 }

@ -125,6 +144,19 @@ type Config struct {
 	// IgnoreDefault instructs the library not to reset the variables to the
 	// default values, including pointers to sub commands
 	IgnoreDefault bool
+
+	// StrictSubcommands intructs the library not to allow global commands after
+	// subcommand
+	StrictSubcommands bool
+
+	// EnvPrefix instructs the library to use a name prefix when reading environment variables.
+	EnvPrefix string
+
+	// Exit is called to terminate the process with an error code (defaults to os.Exit)
+	Exit func(int)
+
+	// Out is where help text, usage text, and failure messages are printed (defaults to os.Stdout)
+	Out io.Writer
 }

 // Parser represents a set of command line options with destination values
@ -137,7 +169,7 @@ type Parser struct {
 	epilogue    string

 	// the following field changes during processing of command line arguments
-	lastCmd *command
+	subcommand []string
 }

 // Versioned is the interface that the destination struct should implement to
@ -187,6 +219,14 @@ func walkFieldsImpl(t reflect.Type, visit func(field reflect.StructField, owner

 // NewParser constructs a parser from a list of destination structs
 func NewParser(config Config, dests ...interface{}) (*Parser, error) {
+	// fill in defaults
+	if config.Exit == nil {
+		config.Exit = os.Exit
+	}
+	if config.Out == nil {
+		config.Out = os.Stdout
+	}
+
 	// first pick a name for the command for use in the usage text
 	var name string
 	switch {
@ -216,23 +256,36 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 			panic(fmt.Sprintf("%s is not a pointer (did you forget an ampersand?)", t))
 		}

-		cmd, err := cmdFromStruct(name, path{root: i}, t)
+		cmd, err := cmdFromStruct(name, path{root: i}, t, config.EnvPrefix)
 		if err != nil {
 			return nil, err
 		}

-		// add nonzero field values as defaults
+		// for backwards compatibility, add nonzero field values as defaults
+		// this applies only to the top-level command, not to subcommands (this inconsistency
+		// is the reason that this method for setting default values was deprecated)
 		for _, spec := range cmd.specs {
-			if v := p.val(spec.dest); v.IsValid() && !isZero(v) {
-				if defaultVal, ok := v.Interface().(encoding.TextMarshaler); ok {
-					str, err := defaultVal.MarshalText()
-					if err != nil {
-						return nil, fmt.Errorf("%v: error marshaling default value to string: %v", spec.dest, err)
-					}
-					spec.defaultVal = string(str)
-				} else {
-					spec.defaultVal = fmt.Sprintf("%v", v)
+			// get the value
+			v := p.val(spec.dest)
+
+			// if the value is the "zero value" (e.g. nil pointer, empty struct) then ignore
+			if isZero(v) {
+				continue
+			}
+
+			// store as a default
+			spec.defaultValue = v
+
+			// we need a string to display in help text
+			// if MarshalText is implemented then use that
+			if m, ok := v.Interface().(encoding.TextMarshaler); ok {
+				s, err := m.MarshalText()
+				if err != nil {
+					return nil, fmt.Errorf("%v: error marshaling default value to string: %v", spec.dest, err)
 				}
+				spec.defaultString = string(s)
+			} else {
+				spec.defaultString = fmt.Sprintf("%v", v)
 			}
 		}

@ -253,7 +306,7 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 	return &p, nil
 }

-func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
+func cmdFromStruct(name string, dest path, t reflect.Type, envPrefix string) (*command, error) {
 	// commands can only be created from pointers to structs
 	if t.Kind() != reflect.Ptr {
 		return nil, fmt.Errorf("subcommands must be pointers to structs but %s is a %s",
@ -304,13 +357,8 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 			spec.help = help
 		}

-		defaultVal, hasDefault := field.Tag.Lookup("default")
-		if hasDefault {
-			spec.defaultVal = defaultVal
-		}
-
-		// Look at the tag
-		var isSubcommand bool // tracks whether this field is a subcommand
+		// process each comma-separated part of the tag
+		var isSubcommand bool
 		for _, key := range strings.Split(tag, ",") {
 			if key == "" {
 				continue
@ -328,18 +376,13 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 			case strings.HasPrefix(key, "--"):
 				spec.long = key[2:]
 			case strings.HasPrefix(key, "-"):
-				if len(key) != 2 {
+				if len(key) > 2 {
 					errs = append(errs, fmt.Sprintf("%s.%s: short arguments must be one character only",
 						t.Name(), field.Name))
 					return false
 				}
 				spec.short = key[1:]
 			case key == "required":
-				if hasDefault {
-					errs = append(errs, fmt.Sprintf("%s.%s: 'required' cannot be used when a default value is specified",
-						t.Name(), field.Name))
-					return false
-				}
 				spec.required = true
 			case key == "positional":
 				spec.positional = true
@ -350,24 +393,30 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 			case key == "env":
 				// Use override name if provided
 				if value != "" {
-					spec.env = value
+					spec.env = envPrefix + value
 				} else {
-					spec.env = strings.ToUpper(field.Name)
+					spec.env = envPrefix + strings.ToUpper(field.Name)
 				}
 			case key == "subcommand":
 				// decide on a name for the subcommand
-				cmdname := value
-				if cmdname == "" {
-					cmdname = strings.ToLower(field.Name)
+				var cmdnames []string
+				if value == "" {
+					cmdnames = []string{strings.ToLower(field.Name)}
+				} else {
+					cmdnames = strings.Split(value, "|")
+				}
+				for i := range cmdnames {
+					cmdnames[i] = strings.TrimSpace(cmdnames[i])
 				}

 				// parse the subcommand recursively
-				subcmd, err := cmdFromStruct(cmdname, subdest, field.Type)
+				subcmd, err := cmdFromStruct(cmdnames[0], subdest, field.Type, envPrefix)
 				if err != nil {
 					errs = append(errs, err.Error())
 					return false
 				}

+				subcmd.aliases = cmdnames[1:]
 				subcmd.parent = &cmd
 				subcmd.help = field.Tag.Get("help")

@ -379,6 +428,7 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 			}
 		}

+		// placeholder is the string used in the help text like this: "--somearg PLACEHOLDER"
 		placeholder, hasPlaceholder := field.Tag.Lookup("placeholder")
 		if hasPlaceholder {
 			spec.placeholder = placeholder
@ -388,27 +438,60 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 			spec.placeholder = strings.ToUpper(spec.field.Name)
 		}

-		// Check whether this field is supported. It's good to do this here rather than
+		// if this is a subcommand then we've done everything we need to do
+		if isSubcommand {
+			return false
+		}
+
+		// check whether this field is supported. It's good to do this here rather than
 		// wait until ParseValue because it means that a program with invalid argument
 		// fields will always fail regardless of whether the arguments it received
 		// exercised those fields.
-		if !isSubcommand {
-			cmd.specs = append(cmd.specs, &spec)
+		var err error
+		spec.cardinality, err = cardinalityOf(field.Type)
+		if err != nil {
+			errs = append(errs, fmt.Sprintf("%s.%s: %s fields are not supported",
+				t.Name(), field.Name, field.Type.String()))
+			return false
+		}

-			var err error
-			spec.cardinality, err = cardinalityOf(field.Type)
-			if err != nil {
-				errs = append(errs, fmt.Sprintf("%s.%s: %s fields are not supported",
-					t.Name(), field.Name, field.Type.String()))
-				return false
-			}
-			if spec.cardinality == multiple && hasDefault {
+		defaultString, hasDefault := field.Tag.Lookup("default")
+		if hasDefault {
+			// we do not support default values for maps and slices
+			if spec.cardinality == multiple {
 				errs = append(errs, fmt.Sprintf("%s.%s: default values are not supported for slice or map fields",
 					t.Name(), field.Name))
 				return false
 			}
+
+			// a required field cannot also have a default value
+			if spec.required {
+				errs = append(errs, fmt.Sprintf("%s.%s: 'required' cannot be used when a default value is specified",
+					t.Name(), field.Name))
+				return false
+			}
+
+			// parse the default value
+			spec.defaultString = defaultString
+			if field.Type.Kind() == reflect.Ptr {
+				// here we have a field of type *T and we create a new T, no need to dereference
+				// in order for the value to be settable
+				spec.defaultValue = reflect.New(field.Type.Elem())
+			} else {
+				// here we have a field of type T and we create a new T and then dereference it
+				// so that the resulting value is settable
+				spec.defaultValue = reflect.New(field.Type).Elem()
+			}
+			err := scalar.ParseValue(spec.defaultValue, defaultString)
+			if err != nil {
+				errs = append(errs, fmt.Sprintf("%s.%s: error processing default value: %v", t.Name(), field.Name, err))
+				return false
+			}
 		}

+		// add the spec to the list of specs
+		cmd.specs = append(cmd.specs, &spec)
+
 		// if this was an embedded field then we already returned true up above
 		return false
 	})
@ -431,8 +514,15 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 	return &cmd, nil
 }

-// Parse processes the given command line option, storing the results in the field
-// of the structs from which NewParser was constructed
+// Parse processes the given command line option, storing the results in the fields
+// of the structs from which NewParser was constructed.
+//
+// It returns ErrHelp if "--help" is one of the command line args and ErrVersion if
+// "--version" is one of the command line args (the latter only applies if the
+// destination struct passed to NewParser implements Versioned.)
+//
+// To respond to --help and --version in the way that MustParse does, see examples
+// in the README under "Custom handling of --help and --version".
 func (p *Parser) Parse(args []string) error {
 	err := p.process(args)
 	if err != nil {
@ -449,6 +539,20 @@ func (p *Parser) Parse(args []string) error {
 	return err
 }

+func (p *Parser) MustParse(args []string) {
+	err := p.Parse(args)
+	switch {
+	case err == ErrHelp:
+		p.WriteHelpForSubcommand(p.config.Out, p.subcommand...)
+		p.config.Exit(0)
+	case err == ErrVersion:
+		fmt.Fprintln(p.config.Out, p.version)
+		p.config.Exit(0)
+	case err != nil:
+		p.FailSubcommand(err.Error(), p.subcommand...)
+	}
+}
+
 // process environment vars for the given arguments
 func (p *Parser) captureEnvVars(specs []*spec, wasPresent map[*spec]bool) error {
 	for _, spec := range specs {
@ -502,7 +606,7 @@ func (p *Parser) process(args []string) error {

 	// union of specs for the chain of subcommands encountered so far
 	curCmd := p.cmd
-	p.lastCmd = curCmd
+	p.subcommand = nil

 	// make a copy of the specs because we will add to this list each time we expand a subcommand
 	specs := make([]*spec, len(curCmd.specs))
@ -516,6 +620,15 @@ func (p *Parser) process(args []string) error {
 		}
 	}

+	// determine if the current command has a version option spec
+	var hasVersionOption bool
+	for _, spec := range curCmd.specs {
+		if spec.long == "version" {
+			hasVersionOption = true
+			break
+		}
+	}
+
 	// process each string from the command line
 	var allpositional bool
 	var positionals []string
@ -523,7 +636,7 @@ func (p *Parser) process(args []string) error {
 	// must use explicit for loop, not range, because we manipulate i inside the loop
 	for i := 0; i < len(args); i++ {
 		arg := args[i]
-		if arg == "--" {
+		if arg == "--" && !allpositional {
 			allpositional = true
 			continue
 		}
@ -548,7 +661,12 @@ func (p *Parser) process(args []string) error {
 			}

 			// add the new options to the set of allowed options
-			specs = append(specs, subcmd.specs...)
+			if p.config.StrictSubcommands {
+				specs = make([]*spec, len(subcmd.specs))
+				copy(specs, subcmd.specs)
+			} else {
+				specs = append(specs, subcmd.specs...)
+			}

 			// capture environment vars for these new options
 			if !p.config.IgnoreEnv {
@ -559,7 +677,7 @@ func (p *Parser) process(args []string) error {
 			}

 			curCmd = subcmd
-			p.lastCmd = curCmd
+			p.subcommand = append(p.subcommand, arg)
 			continue
 		}

@ -568,7 +686,9 @@ func (p *Parser) process(args []string) error {
 		case "-h", "--help":
 			return ErrHelp
 		case "--version":
-			return ErrVersion
+			if !hasVersionOption && p.version != "" {
+				return ErrVersion
+			}
 		}

 		// check for an equals sign, as in "--foo=bar"
@ -582,7 +702,7 @@ func (p *Parser) process(args []string) error {
 		// lookup the spec for this option (note that the "specs" slice changes as
 		// we expand subcommands so it is better not to use a map)
 		spec := findOption(specs, opt)
-		if spec == nil {
+		if spec == nil || opt == "" {
 			return fmt.Errorf("unknown argument %s", arg)
 		}
 		wasPresent[spec] = true
@ -671,17 +791,26 @@ func (p *Parser) process(args []string) error {
 		}

 		if spec.required {
+			if spec.short == "" && spec.long == "" {
+				msg := fmt.Sprintf("environment variable %s is required", spec.env)
+				return errors.New(msg)
+			}
+
 			msg := fmt.Sprintf("%s is required", name)
 			if spec.env != "" {
 				msg += " (or environment variable " + spec.env + ")"
 			}
+
 			return errors.New(msg)
 		}
-		if !p.config.IgnoreDefault && spec.defaultVal != "" {
-			err := scalar.ParseValue(p.val(spec.dest), spec.defaultVal)
-			if err != nil {
-				return fmt.Errorf("error processing default value for %s: %v", name, err)
-			}
+
+		if spec.defaultValue.IsValid() && !p.config.IgnoreDefault {
+			// One issue here is that if the user now modifies the value then
+			// the default value stored in the spec will be corrupted. There
+			// is no general way to "deep-copy" values in Go, and we still
+			// support the old-style method for specifying defaults as
+			// Go values assigned directly to the struct field, so we are stuck.
+			p.val(spec.dest).Set(spec.defaultValue)
 		}
 	}

@ -742,6 +871,11 @@ func findSubcommand(cmds []*command, name string) *command {
 		if cmd.name == name {
 			return cmd
 		}
+		for _, alias := range cmd.aliases {
+			if alias == name {
+				return cmd
+			}
+		}
 	}
 	return nil
 }
--- a/reflect.go
+++ b/reflect.go
@ -7,15 +7,15 @@ import (
 	"unicode"
 	"unicode/utf8"

-	scalar "github.com/alexflint/go-scalar"
+	"go.wit.com/dev/alexflint/scalar"
 )

 var textUnmarshalerType = reflect.TypeOf([]encoding.TextUnmarshaler{}).Elem()

 // cardinality tracks how many tokens are expected for a given spec
-//  - zero is a boolean, which does to expect any value
-//  - one is an ordinary option that will be parsed from a single token
-//  - multiple is a slice or map that can accept zero or more tokens
+//   - zero is a boolean, which does to expect any value
+//   - one is an ordinary option that will be parsed from a single token
+//   - multiple is a slice or map that can accept zero or more tokens
 type cardinality int

 const (
@ -74,10 +74,10 @@ func cardinalityOf(t reflect.Type) (cardinality, error) {
 	}
 }

-// isBoolean returns true if the type can be parsed from a single string
+// isBoolean returns true if the type is a boolean or a pointer to a boolean
 func isBoolean(t reflect.Type) bool {
 	switch {
-	case t.Implements(textUnmarshalerType):
+	case isTextUnmarshaler(t):
 		return false
 	case t.Kind() == reflect.Bool:
 		return true
@ -88,6 +88,11 @@ func isBoolean(t reflect.Type) bool {
 	}
 }

+// isTextUnmarshaler returns true if the type or its pointer implements encoding.TextUnmarshaler
+func isTextUnmarshaler(t reflect.Type) bool {
+	return t.Implements(textUnmarshalerType) || reflect.PtrTo(t).Implements(textUnmarshalerType)
+}
+
 // isExported returns true if the struct field name is exported
 func isExported(field string) bool {
 	r, _ := utf8.DecodeRuneInString(field) // returns RuneError for empty string or invalid UTF8
@ -97,7 +102,7 @@ func isExported(field string) bool {
 // isZero returns true if v contains the zero value for its type
 func isZero(v reflect.Value) bool {
 	t := v.Type()
-	if t.Kind() == reflect.Slice || t.Kind() == reflect.Map {
+	if t.Kind() == reflect.Ptr || t.Kind() == reflect.Slice || t.Kind() == reflect.Map || t.Kind() == reflect.Chan || t.Kind() == reflect.Interface {
 		return v.IsNil()
 	}
 	if !t.Comparable() {
--- a/sequence.go
+++ b/sequence.go
@ -5,7 +5,7 @@ import (
 	"reflect"
 	"strings"

-	scalar "github.com/alexflint/go-scalar"
+	"go.wit.com/dev/alexflint/scalar"
 )

 // setSliceOrMap parses a sequence of strings into a slice or map. If clear is
--- a/subcommand.go
+++ b/subcommand.go
@ -1,5 +1,7 @@
 package arg

+import "fmt"
+
 // Subcommand returns the user struct for the subcommand selected by
 // the command line arguments most recently processed by the parser.
 // The return value is always a pointer to a struct. If no subcommand
@ -7,31 +9,35 @@ package arg
 // no command line arguments have been processed by this parser then it
 // returns nil.
 func (p *Parser) Subcommand() interface{} {
-	if p.lastCmd == nil || p.lastCmd.parent == nil {
+	if len(p.subcommand) == 0 {
 		return nil
 	}
-	return p.val(p.lastCmd.dest).Interface()
+	cmd, err := p.lookupCommand(p.subcommand...)
+	if err != nil {
+		return nil
+	}
+	return p.val(cmd.dest).Interface()
 }

 // SubcommandNames returns the sequence of subcommands specified by the
 // user. If no subcommands were given then it returns an empty slice.
 func (p *Parser) SubcommandNames() []string {
-	if p.lastCmd == nil {
-		return nil
-	}
-
-	// make a list of ancestor commands
-	var ancestors []string
-	cur := p.lastCmd
-	for cur.parent != nil { // we want to exclude the root
-		ancestors = append(ancestors, cur.name)
-		cur = cur.parent
-	}
-
-	// reverse the list
-	out := make([]string, len(ancestors))
-	for i := 0; i < len(ancestors); i++ {
-		out[i] = ancestors[len(ancestors)-i-1]
-	}
-	return out
+	return p.subcommand
+}
+
+// lookupCommand finds a subcommand based on a sequence of subcommand names. The
+// first string should be a top-level subcommand, the next should be a child
+// subcommand of that subcommand, and so on. If no strings are given then the
+// root command is returned. If no such subcommand exists then an error is
+// returned.
+func (p *Parser) lookupCommand(path ...string) (*command, error) {
+	cmd := p.cmd
+	for _, name := range path {
+		found := findSubcommand(cmd.subcommands, name)
+		if found == nil {
+			return nil, fmt.Errorf("%q is not a subcommand of %s", name, cmd.name)
+		}
+		cmd = found
+	}
+	return cmd, nil
 }
--- a/test/Makefile
+++ b/test/Makefile
@ -0,0 +1,10 @@
+all:
+	@echo
+	@echo
+
+test:
+
+redomod:
+	rm -f go.*
+	GO111MODULE= go mod init
+	GO111MODULE= go mod tidy
--- a/test/example_test.go
+++ b/test/example_test.go
@ -162,8 +162,8 @@ func Example_helpText() {
 	}

 	// This is only necessary when running inside golang's runnable example harness
-	osExit = func(int) {}
-	stdout = os.Stdout
+	mustParseExit = func(int) {}
+	mustParseOut = os.Stdout

 	MustParse(&args)

@ -195,19 +195,18 @@ func Example_helpPlaceholder() {
 	}

 	// This is only necessary when running inside golang's runnable example harness
-	osExit = func(int) {}
-	stdout = os.Stdout
+	mustParseExit = func(int) {}
+	mustParseOut = os.Stdout

 	MustParse(&args)

 	// output:
-
 	// Usage: example [--optimize LEVEL] [--maxjobs N] SRC [DST [DST ...]]
-
+	//
 	// Positional arguments:
 	//   SRC
 	//   DST
-
+	//
 	// Options:
 	//   --optimize LEVEL, -O LEVEL
 	//                          optimization level
@ -236,8 +235,8 @@ func Example_helpTextWithSubcommand() {
 	}

 	// This is only necessary when running inside golang's runnable example harness
-	osExit = func(int) {}
-	stdout = os.Stdout
+	mustParseExit = func(int) {}
+	mustParseOut = os.Stdout

 	MustParse(&args)

@ -274,8 +273,8 @@ func Example_helpTextWhenUsingSubcommand() {
 	}

 	// This is only necessary when running inside golang's runnable example harness
-	osExit = func(int) {}
-	stdout = os.Stdout
+	mustParseExit = func(int) {}
+	mustParseOut = os.Stdout

 	MustParse(&args)

@ -311,10 +310,9 @@ func Example_writeHelpForSubcommand() {
 	}

 	// This is only necessary when running inside golang's runnable example harness
-	osExit = func(int) {}
-	stdout = os.Stdout
+	exit := func(int) {}

-	p, err := NewParser(Config{}, &args)
+	p, err := NewParser(Config{Exit: exit}, &args)
 	if err != nil {
 		fmt.Println(err)
 		os.Exit(1)
@ -360,10 +358,9 @@ func Example_writeHelpForSubcommandNested() {
 	}

 	// This is only necessary when running inside golang's runnable example harness
-	osExit = func(int) {}
-	stdout = os.Stdout
+	exit := func(int) {}

-	p, err := NewParser(Config{}, &args)
+	p, err := NewParser(Config{Exit: exit}, &args)
 	if err != nil {
 		fmt.Println(err)
 		os.Exit(1)
@ -397,8 +394,8 @@ func Example_errorText() {
 	}

 	// This is only necessary when running inside golang's runnable example harness
-	osExit = func(int) {}
-	stderr = os.Stdout
+	mustParseExit = func(int) {}
+	mustParseOut = os.Stdout

 	MustParse(&args)

@ -421,8 +418,8 @@ func Example_errorTextForSubcommand() {
 	}

 	// This is only necessary when running inside golang's runnable example harness
-	osExit = func(int) {}
-	stderr = os.Stdout
+	mustParseExit = func(int) {}
+	mustParseOut = os.Stdout

 	MustParse(&args)

@ -457,8 +454,8 @@ func Example_subcommand() {
 	}

 	// This is only necessary when running inside golang's runnable example harness
-	osExit = func(int) {}
-	stderr = os.Stdout
+	mustParseExit = func(int) {}
+	mustParseOut = os.Stdout

 	MustParse(&args)

@ -505,3 +502,45 @@ func Example_allSupportedTypes() {

 	// output:
 }
+
+func Example_envVarOnly() {
+	os.Args = split("./example")
+	_ = os.Setenv("AUTH_KEY", "my_key")
+
+	defer os.Unsetenv("AUTH_KEY")
+
+	var args struct {
+		AuthKey string `arg:"--,env:AUTH_KEY"`
+	}
+
+	MustParse(&args)
+
+	fmt.Println(args.AuthKey)
+	// output: my_key
+}
+
+func Example_envVarOnlyShouldIgnoreFlag() {
+	os.Args = split("./example --=my_key")
+
+	var args struct {
+		AuthKey string `arg:"--,env:AUTH_KEY"`
+	}
+
+	err := Parse(&args)
+
+	fmt.Println(err)
+	// output: unknown argument --=my_key
+}
+
+func Example_envVarOnlyShouldIgnoreShortFlag() {
+	os.Args = split("./example -=my_key")
+
+	var args struct {
+		AuthKey string `arg:"--,env:AUTH_KEY"`
+	}
+
+	err := Parse(&args)
+
+	fmt.Println(err)
+	// output: unknown argument -=my_key
+}
--- a/test/go.mod
+++ b/test/go.mod
@ -0,0 +1,11 @@
+module go.wit.com/dev/alexflint/arg/test
+
+go 1.21.4
+
+require github.com/stretchr/testify v1.8.4
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
--- a/test/go.sum
+++ b/test/go.sum
@ -0,0 +1,10 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.8.4 h1:CcVxjf3Q8PM0mHUKJCdn+eZZtm5yQwehR5yeSVQQcUk=
+github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
--- a/test/parse_test.go
+++ b/test/parse_test.go
@ -2,6 +2,7 @@ package arg

 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"net"
 	"net/mail"
@ -27,11 +28,11 @@ func parse(cmdline string, dest interface{}) error {
 }

 func pparse(cmdline string, dest interface{}) (*Parser, error) {
-	return parseWithEnv(cmdline, nil, dest)
+	return parseWithEnv(Config{}, cmdline, nil, dest)
 }

-func parseWithEnv(cmdline string, env []string, dest interface{}) (*Parser, error) {
-	p, err := NewParser(Config{}, dest)
+func parseWithEnv(config Config, cmdline string, env []string, dest interface{}) (*Parser, error) {
+	p, err := NewParser(config, dest)
 	if err != nil {
 		return nil, err
 	}
@ -97,9 +98,9 @@ func TestInt(t *testing.T) {

 func TestHexOctBin(t *testing.T) {
 	var args struct {
-		Hex int
-		Oct int
-		Bin int
+		Hex         int
+		Oct         int
+		Bin         int
 		Underscored int
 	}
 	err := parse("--hex 0xA --oct 0o10 --bin 0b101 --underscored 123_456", &args)
@ -226,6 +227,14 @@ func TestRequiredWithEnv(t *testing.T) {
 	require.Error(t, err, "--foo is required (or environment variable FOO)")
 }

+func TestRequiredWithEnvOnly(t *testing.T) {
+	var args struct {
+		Foo string `arg:"required,--,-,env:FOO"`
+	}
+	_, err := parseWithEnv(Config{}, "", []string{}, &args)
+	require.Error(t, err, "environment variable FOO is required")
+}
+
 func TestShortFlag(t *testing.T) {
 	var args struct {
 		Foo string `arg:"-f"`
@ -600,6 +609,15 @@ func TestNoMoreOptionsBeforeHelp(t *testing.T) {
 	assert.NotEqual(t, ErrHelp, err)
 }

+func TestNoMoreOptionsTwice(t *testing.T) {
+	var args struct {
+		X []string `arg:"positional"`
+	}
+	err := parse("-- --", &args)
+	require.NoError(t, err)
+	assert.Equal(t, []string{"--"}, args.X)
+}
+
 func TestHelpFlag(t *testing.T) {
 	var args struct {
 		Foo string
@ -683,11 +701,26 @@ func TestMustParse(t *testing.T) {
 	assert.NotNil(t, parser)
 }

+func TestMustParseError(t *testing.T) {
+	var args struct {
+		Foo []string `default:""`
+	}
+	var exitCode int
+	var stdout bytes.Buffer
+	mustParseExit = func(code int) { exitCode = code }
+	mustParseOut = &stdout
+	os.Args = []string{"example"}
+	parser := MustParse(&args)
+	assert.Nil(t, parser)
+	assert.Equal(t, 2, exitCode)
+	assert.Contains(t, stdout.String(), "default values are not supported for slice or map fields")
+}
+
 func TestEnvironmentVariable(t *testing.T) {
 	var args struct {
 		Foo string `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{"FOO=bar"}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{"FOO=bar"}, &args)
 	require.NoError(t, err)
 	assert.Equal(t, "bar", args.Foo)
 }
@ -696,7 +729,7 @@ func TestEnvironmentVariableNotPresent(t *testing.T) {
 	var args struct {
 		NotPresent string `arg:"env"`
 	}
-	_, err := parseWithEnv("", nil, &args)
+	_, err := parseWithEnv(Config{}, "", nil, &args)
 	require.NoError(t, err)
 	assert.Equal(t, "", args.NotPresent)
 }
@ -705,7 +738,7 @@ func TestEnvironmentVariableOverrideName(t *testing.T) {
 	var args struct {
 		Foo string `arg:"env:BAZ"`
 	}
-	_, err := parseWithEnv("", []string{"BAZ=bar"}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{"BAZ=bar"}, &args)
 	require.NoError(t, err)
 	assert.Equal(t, "bar", args.Foo)
 }
@ -714,7 +747,7 @@ func TestEnvironmentVariableOverrideArgument(t *testing.T) {
 	var args struct {
 		Foo string `arg:"env"`
 	}
-	_, err := parseWithEnv("--foo zzz", []string{"FOO=bar"}, &args)
+	_, err := parseWithEnv(Config{}, "--foo zzz", []string{"FOO=bar"}, &args)
 	require.NoError(t, err)
 	assert.Equal(t, "zzz", args.Foo)
 }
@ -723,7 +756,7 @@ func TestEnvironmentVariableError(t *testing.T) {
 	var args struct {
 		Foo int `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{"FOO=bar"}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{"FOO=bar"}, &args)
 	assert.Error(t, err)
 }

@ -731,7 +764,7 @@ func TestEnvironmentVariableRequired(t *testing.T) {
 	var args struct {
 		Foo string `arg:"env,required"`
 	}
-	_, err := parseWithEnv("", []string{"FOO=bar"}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{"FOO=bar"}, &args)
 	require.NoError(t, err)
 	assert.Equal(t, "bar", args.Foo)
 }
@ -740,7 +773,7 @@ func TestEnvironmentVariableSliceArgumentString(t *testing.T) {
 	var args struct {
 		Foo []string `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{`FOO=bar,"baz, qux"`}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{`FOO=bar,"baz, qux"`}, &args)
 	require.NoError(t, err)
 	assert.Equal(t, []string{"bar", "baz, qux"}, args.Foo)
 }
@ -749,7 +782,7 @@ func TestEnvironmentVariableSliceEmpty(t *testing.T) {
 	var args struct {
 		Foo []string `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{`FOO=`}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{`FOO=`}, &args)
 	require.NoError(t, err)
 	assert.Len(t, args.Foo, 0)
 }
@ -758,7 +791,7 @@ func TestEnvironmentVariableSliceArgumentInteger(t *testing.T) {
 	var args struct {
 		Foo []int `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{`FOO=1,99`}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{`FOO=1,99`}, &args)
 	require.NoError(t, err)
 	assert.Equal(t, []int{1, 99}, args.Foo)
 }
@ -767,7 +800,7 @@ func TestEnvironmentVariableSliceArgumentFloat(t *testing.T) {
 	var args struct {
 		Foo []float32 `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{`FOO=1.1,99.9`}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{`FOO=1.1,99.9`}, &args)
 	require.NoError(t, err)
 	assert.Equal(t, []float32{1.1, 99.9}, args.Foo)
 }
@ -776,7 +809,7 @@ func TestEnvironmentVariableSliceArgumentBool(t *testing.T) {
 	var args struct {
 		Foo []bool `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{`FOO=true,false,0,1`}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{`FOO=true,false,0,1`}, &args)
 	require.NoError(t, err)
 	assert.Equal(t, []bool{true, false, false, true}, args.Foo)
 }
@ -785,7 +818,7 @@ func TestEnvironmentVariableSliceArgumentWrongCsv(t *testing.T) {
 	var args struct {
 		Foo []int `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{`FOO=1,99\"`}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{`FOO=1,99\"`}, &args)
 	assert.Error(t, err)
 }

@ -793,7 +826,7 @@ func TestEnvironmentVariableSliceArgumentWrongType(t *testing.T) {
 	var args struct {
 		Foo []bool `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{`FOO=one,two`}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{`FOO=one,two`}, &args)
 	assert.Error(t, err)
 }

@ -801,7 +834,7 @@ func TestEnvironmentVariableMap(t *testing.T) {
 	var args struct {
 		Foo map[int]string `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{`FOO=1=one,99=ninetynine`}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{`FOO=1=one,99=ninetynine`}, &args)
 	require.NoError(t, err)
 	assert.Len(t, args.Foo, 2)
 	assert.Equal(t, "one", args.Foo[1])
@ -812,11 +845,21 @@ func TestEnvironmentVariableEmptyMap(t *testing.T) {
 	var args struct {
 		Foo map[int]string `arg:"env"`
 	}
-	_, err := parseWithEnv("", []string{`FOO=`}, &args)
+	_, err := parseWithEnv(Config{}, "", []string{`FOO=`}, &args)
 	require.NoError(t, err)
 	assert.Len(t, args.Foo, 0)
 }

+func TestEnvironmentVariableWithPrefix(t *testing.T) {
+	var args struct {
+		Foo string `arg:"env"`
+	}
+
+	_, err := parseWithEnv(Config{EnvPrefix: "MYAPP_"}, "", []string{"MYAPP_FOO=bar"}, &args)
+	require.NoError(t, err)
+	assert.Equal(t, "bar", args.Foo)
+}
+
 func TestEnvironmentVariableIgnored(t *testing.T) {
 	var args struct {
 		Foo string `arg:"env"`
@ -844,6 +887,24 @@ func TestDefaultValuesIgnored(t *testing.T) {
 	assert.Equal(t, "", args.Foo)
 }

+func TestRequiredEnvironmentOnlyVariableIsMissing(t *testing.T) {
+	var args struct {
+		Foo string `arg:"required,--,env:FOO"`
+	}
+
+	_, err := parseWithEnv(Config{}, "", []string{""}, &args)
+	assert.Error(t, err)
+}
+
+func TestOptionalEnvironmentOnlyVariable(t *testing.T) {
+	var args struct {
+		Foo string `arg:"env:FOO"`
+	}
+
+	_, err := parseWithEnv(Config{}, "", []string{}, &args)
+	assert.NoError(t, err)
+}
+
 func TestEnvironmentVariableInSubcommandIgnored(t *testing.T) {
 	var args struct {
 		Sub *struct {
@ -856,10 +917,51 @@ func TestEnvironmentVariableInSubcommandIgnored(t *testing.T) {
 	require.NoError(t, err)

 	err = p.Parse([]string{"sub"})
-	assert.NoError(t, err)
+	require.NoError(t, err)
+	require.NotNil(t, args.Sub)
 	assert.Equal(t, "", args.Sub.Foo)
 }

+func TestParserMustParseEmptyArgs(t *testing.T) {
+	// this mirrors TestEmptyArgs
+	p, err := NewParser(Config{}, &struct{}{})
+	require.NoError(t, err)
+	assert.NotNil(t, p)
+	p.MustParse(nil)
+}
+
+func TestParserMustParse(t *testing.T) {
+	tests := []struct {
+		name    string
+		args    versioned
+		cmdLine []string
+		code    int
+		output  string
+	}{
+		{name: "help", args: struct{}{}, cmdLine: []string{"--help"}, code: 0, output: "display this help and exit"},
+		{name: "version", args: versioned{}, cmdLine: []string{"--version"}, code: 0, output: "example 3.2.1"},
+		{name: "invalid", args: struct{}{}, cmdLine: []string{"invalid"}, code: 2, output: ""},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			var exitCode int
+			var stdout bytes.Buffer
+			exit := func(code int) { exitCode = code }
+
+			p, err := NewParser(Config{Exit: exit, Out: &stdout}, &tt.args)
+			require.NoError(t, err)
+			assert.NotNil(t, p)
+
+			p.MustParse(tt.cmdLine)
+			assert.NotNil(t, exitCode)
+			assert.Equal(t, tt.code, exitCode)
+			assert.Contains(t, stdout.String(), tt.output)
+		})
+	}
+}
+
 type textUnmarshaler struct {
 	val int
 }
@ -1313,11 +1415,55 @@ func TestReuseParser(t *testing.T) {
 	assert.Error(t, err)
 }

-func TestVersion(t *testing.T) {
+func TestNoVersion(t *testing.T) {
 	var args struct{}
-	err := parse("--version", &args)
-	assert.Equal(t, ErrVersion, err)

+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	err = p.Parse([]string{"--version"})
+	assert.Error(t, err)
+	assert.NotEqual(t, ErrVersion, err)
+}
+
+func TestBuiltinVersion(t *testing.T) {
+	var args struct{}
+
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	p.version = "example 3.2.1"
+
+	err = p.Parse([]string{"--version"})
+	assert.Equal(t, ErrVersion, err)
+}
+
+func TestArgsVersion(t *testing.T) {
+	var args struct {
+		Version bool `arg:"--version"`
+	}
+
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	err = p.Parse([]string{"--version"})
+	require.NoError(t, err)
+	require.Equal(t, args.Version, true)
+}
+
+func TestArgsAndBuiltinVersion(t *testing.T) {
+	var args struct {
+		Version bool `arg:"--version"`
+	}
+
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	p.version = "example 3.2.1"
+
+	err = p.Parse([]string{"--version"})
+	require.NoError(t, err)
+	require.Equal(t, args.Version, true)
 }

 func TestMultipleTerminates(t *testing.T) {
@ -1348,13 +1494,21 @@ func TestDefaultOptionValues(t *testing.T) {
 	require.NoError(t, err)

 	assert.Equal(t, 123, args.A)
-	assert.Equal(t, 123, *args.B)
+	if assert.NotNil(t, args.B) {
+		assert.Equal(t, 123, *args.B)
+	}
 	assert.Equal(t, "xyz", args.C)
-	assert.Equal(t, "abc", *args.D)
+	if assert.NotNil(t, args.D) {
+		assert.Equal(t, "abc", *args.D)
+	}
 	assert.Equal(t, 4.56, args.E)
-	assert.Equal(t, 1.23, *args.F)
-	assert.True(t, args.G)
+	if assert.NotNil(t, args.F) {
+		assert.Equal(t, 1.23, *args.F)
+	}
 	assert.True(t, args.G)
+	if assert.NotNil(t, args.H) {
+		assert.True(t, *args.H)
+	}
 }

 func TestDefaultUnparseable(t *testing.T) {
@ -1363,7 +1517,7 @@ func TestDefaultUnparseable(t *testing.T) {
 	}

 	err := parse("", &args)
-	assert.EqualError(t, err, `error processing default value for --a: strconv.ParseInt: parsing "x": invalid syntax`)
+	assert.EqualError(t, err, `.A: error processing default value: strconv.ParseInt: parsing "x": invalid syntax`)
 }

 func TestDefaultPositionalValues(t *testing.T) {
@ -1382,13 +1536,21 @@ func TestDefaultPositionalValues(t *testing.T) {
 	require.NoError(t, err)

 	assert.Equal(t, 456, args.A)
-	assert.Equal(t, 789, *args.B)
+	if assert.NotNil(t, args.B) {
+		assert.Equal(t, 789, *args.B)
+	}
 	assert.Equal(t, "abc", args.C)
-	assert.Equal(t, "abc", *args.D)
+	if assert.NotNil(t, args.D) {
+		assert.Equal(t, "abc", *args.D)
+	}
 	assert.Equal(t, 1.23, args.E)
-	assert.Equal(t, 1.23, *args.F)
-	assert.True(t, args.G)
+	if assert.NotNil(t, args.F) {
+		assert.Equal(t, 1.23, *args.F)
+	}
 	assert.True(t, args.G)
+	if assert.NotNil(t, args.H) {
+		assert.True(t, *args.H)
+	}
 }

 func TestDefaultValuesNotAllowedWithRequired(t *testing.T) {
@ -1402,7 +1564,7 @@ func TestDefaultValuesNotAllowedWithRequired(t *testing.T) {

 func TestDefaultValuesNotAllowedWithSlice(t *testing.T) {
 	var args struct {
-		A []int `default:"123"` // required not allowed with default!
+		A []int `default:"invalid"` // default values not allowed with slices
 	}

 	err := parse("", &args)
@ -1419,68 +1581,201 @@ func TestUnexportedFieldsSkipped(t *testing.T) {
 }

 func TestMustParseInvalidParser(t *testing.T) {
-	originalExit := osExit
-	originalStdout := stdout
-	defer func() {
-		osExit = originalExit
-		stdout = originalStdout
-	}()
-
 	var exitCode int
-	osExit = func(code int) { exitCode = code }
-	stdout = &bytes.Buffer{}
+	var stdout bytes.Buffer
+	exit := func(code int) { exitCode = code }

 	var args struct {
 		CannotParse struct{}
 	}
-	parser := MustParse(&args)
+	parser := mustParse(Config{Out: &stdout, Exit: exit}, &args)
 	assert.Nil(t, parser)
-	assert.Equal(t, -1, exitCode)
+	assert.Equal(t, 2, exitCode)
 }

 func TestMustParsePrintsHelp(t *testing.T) {
-	originalExit := osExit
-	originalStdout := stdout
 	originalArgs := os.Args
 	defer func() {
-		osExit = originalExit
-		stdout = originalStdout
 		os.Args = originalArgs
 	}()

-	var exitCode *int
-	osExit = func(code int) { exitCode = &code }
 	os.Args = []string{"someprogram", "--help"}
-	stdout = &bytes.Buffer{}
+
+	var exitCode int
+	var stdout bytes.Buffer
+	exit := func(code int) { exitCode = code }

 	var args struct{}
-	parser := MustParse(&args)
+	parser := mustParse(Config{Out: &stdout, Exit: exit}, &args)
 	assert.NotNil(t, parser)
-	require.NotNil(t, exitCode)
-	assert.Equal(t, 0, *exitCode)
+	assert.Equal(t, 0, exitCode)
 }

 func TestMustParsePrintsVersion(t *testing.T) {
-	originalExit := osExit
-	originalStdout := stdout
 	originalArgs := os.Args
 	defer func() {
-		osExit = originalExit
-		stdout = originalStdout
 		os.Args = originalArgs
 	}()

-	var exitCode *int
-	osExit = func(code int) { exitCode = &code }
+	var exitCode int
+	var stdout bytes.Buffer
+	exit := func(code int) { exitCode = code }
+
 	os.Args = []string{"someprogram", "--version"}

-	var b bytes.Buffer
-	stdout = &b
-
 	var args versioned
-	parser := MustParse(&args)
+	parser := mustParse(Config{Out: &stdout, Exit: exit}, &args)
 	require.NotNil(t, parser)
-	require.NotNil(t, exitCode)
-	assert.Equal(t, 0, *exitCode)
-	assert.Equal(t, "example 3.2.1\n", b.String())
+	assert.Equal(t, 0, exitCode)
+	assert.Equal(t, "example 3.2.1\n", stdout.String())
+}
+
+type mapWithUnmarshalText struct {
+	val map[string]string
+}
+
+func (v *mapWithUnmarshalText) UnmarshalText(data []byte) error {
+	return json.Unmarshal(data, &v.val)
+}
+
+func TestTextUnmarshalerEmpty(t *testing.T) {
+	// based on https://github.com/alexflint/go-arg/issues/184
+	var args struct {
+		Config mapWithUnmarshalText `arg:"--config"`
+	}
+
+	err := parse("", &args)
+	require.NoError(t, err)
+	assert.Empty(t, args.Config)
+}
+
+func TestTextUnmarshalerEmptyPointer(t *testing.T) {
+	// a slight variant on https://github.com/alexflint/go-arg/issues/184
+	var args struct {
+		Config *mapWithUnmarshalText `arg:"--config"`
+	}
+
+	err := parse("", &args)
+	require.NoError(t, err)
+	assert.Nil(t, args.Config)
+}
+
+// similar to the above but also implements MarshalText
+type mapWithMarshalText struct {
+	val map[string]string
+}
+
+func (v *mapWithMarshalText) MarshalText(data []byte) error {
+	return json.Unmarshal(data, &v.val)
+}
+
+func (v *mapWithMarshalText) UnmarshalText(data []byte) error {
+	return json.Unmarshal(data, &v.val)
+}
+
+func TestTextMarshalerUnmarshalerEmpty(t *testing.T) {
+	// based on https://github.com/alexflint/go-arg/issues/184
+	var args struct {
+		Config mapWithMarshalText `arg:"--config"`
+	}
+
+	err := parse("", &args)
+	require.NoError(t, err)
+	assert.Empty(t, args.Config)
+}
+
+func TestTextMarshalerUnmarshalerEmptyPointer(t *testing.T) {
+	// a slight variant on https://github.com/alexflint/go-arg/issues/184
+	var args struct {
+		Config *mapWithMarshalText `arg:"--config"`
+	}
+
+	err := parse("", &args)
+	require.NoError(t, err)
+	assert.Nil(t, args.Config)
+}
+
+func TestSubcommandGlobalFlag_Before(t *testing.T) {
+	var args struct {
+		Global bool `arg:"-g"`
+		Sub    *struct {
+		} `arg:"subcommand"`
+	}
+
+	p, err := NewParser(Config{StrictSubcommands: false}, &args)
+	require.NoError(t, err)
+
+	err = p.Parse([]string{"-g", "sub"})
+	assert.NoError(t, err)
+	assert.True(t, args.Global)
+}
+
+func TestSubcommandGlobalFlag_InCommand(t *testing.T) {
+	var args struct {
+		Global bool `arg:"-g"`
+		Sub    *struct {
+		} `arg:"subcommand"`
+	}
+
+	p, err := NewParser(Config{StrictSubcommands: false}, &args)
+	require.NoError(t, err)
+
+	err = p.Parse([]string{"sub", "-g"})
+	assert.NoError(t, err)
+	assert.True(t, args.Global)
+}
+
+func TestSubcommandGlobalFlag_Before_Strict(t *testing.T) {
+	var args struct {
+		Global bool `arg:"-g"`
+		Sub    *struct {
+		} `arg:"subcommand"`
+	}
+
+	p, err := NewParser(Config{StrictSubcommands: true}, &args)
+	require.NoError(t, err)
+
+	err = p.Parse([]string{"-g", "sub"})
+	assert.NoError(t, err)
+	assert.True(t, args.Global)
+}
+
+func TestSubcommandGlobalFlag_InCommand_Strict(t *testing.T) {
+	var args struct {
+		Global bool `arg:"-g"`
+		Sub    *struct {
+		} `arg:"subcommand"`
+	}
+
+	p, err := NewParser(Config{StrictSubcommands: true}, &args)
+	require.NoError(t, err)
+
+	err = p.Parse([]string{"sub", "-g"})
+	assert.Error(t, err)
+}
+
+func TestSubcommandGlobalFlag_InCommand_Strict_Inner(t *testing.T) {
+	var args struct {
+		Global bool `arg:"-g"`
+		Sub    *struct {
+			Guard bool `arg:"-g"`
+		} `arg:"subcommand"`
+	}
+
+	p, err := NewParser(Config{StrictSubcommands: true}, &args)
+	require.NoError(t, err)
+
+	err = p.Parse([]string{"sub", "-g"})
+	require.NoError(t, err)
+	assert.False(t, args.Global)
+	require.NotNil(t, args.Sub)
+	assert.True(t, args.Sub.Guard)
+}
+
+func TestExitFunctionAndOutStreamGetFilledIn(t *testing.T) {
+	var args struct{}
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+	assert.NotNil(t, p.config.Exit) // go prohibits function pointer comparison
+	assert.Equal(t, p.config.Out, os.Stdout)
 }
--- a/test/reflect_test.go
+++ b/test/reflect_test.go
--- a/test/sequence_test.go
+++ b/test/sequence_test.go
--- a/test/subcommand_test.go
+++ b/test/subcommand_test.go
@ -83,6 +83,19 @@ func TestNamedSubcommand(t *testing.T) {
 	assert.Equal(t, []string{"ls"}, p.SubcommandNames())
 }

+func TestSubcommandAliases(t *testing.T) {
+	type listCmd struct {
+	}
+	var args struct {
+		List *listCmd `arg:"subcommand:list|ls"`
+	}
+	p, err := pparse("ls", &args)
+	require.NoError(t, err)
+	assert.NotNil(t, args.List)
+	assert.Equal(t, args.List, p.Subcommand())
+	assert.Equal(t, []string{"ls"}, p.SubcommandNames())
+}
+
 func TestEmptySubcommand(t *testing.T) {
 	type listCmd struct {
 	}
@ -113,6 +126,23 @@ func TestTwoSubcommands(t *testing.T) {
 	assert.Equal(t, []string{"list"}, p.SubcommandNames())
 }

+func TestTwoSubcommandsWithAliases(t *testing.T) {
+	type getCmd struct {
+	}
+	type listCmd struct {
+	}
+	var args struct {
+		Get  *getCmd  `arg:"subcommand:get|g"`
+		List *listCmd `arg:"subcommand:list|ls"`
+	}
+	p, err := pparse("ls", &args)
+	require.NoError(t, err)
+	assert.Nil(t, args.Get)
+	assert.NotNil(t, args.List)
+	assert.Equal(t, args.List, p.Subcommand())
+	assert.Equal(t, []string{"ls"}, p.SubcommandNames())
+}
+
 func TestSubcommandsWithOptions(t *testing.T) {
 	type getCmd struct {
 		Name string
@ -275,6 +305,60 @@ func TestNestedSubcommands(t *testing.T) {
 	}
 }

+func TestNestedSubcommandsWithAliases(t *testing.T) {
+	type child struct{}
+	type parent struct {
+		Child *child `arg:"subcommand:child|ch"`
+	}
+	type grandparent struct {
+		Parent *parent `arg:"subcommand:parent|pa"`
+	}
+	type root struct {
+		Grandparent *grandparent `arg:"subcommand:grandparent|gp"`
+	}
+
+	{
+		var args root
+		p, err := pparse("gp parent child", &args)
+		require.NoError(t, err)
+		require.NotNil(t, args.Grandparent)
+		require.NotNil(t, args.Grandparent.Parent)
+		require.NotNil(t, args.Grandparent.Parent.Child)
+		assert.Equal(t, args.Grandparent.Parent.Child, p.Subcommand())
+		assert.Equal(t, []string{"gp", "parent", "child"}, p.SubcommandNames())
+	}
+
+	{
+		var args root
+		p, err := pparse("grandparent pa", &args)
+		require.NoError(t, err)
+		require.NotNil(t, args.Grandparent)
+		require.NotNil(t, args.Grandparent.Parent)
+		require.Nil(t, args.Grandparent.Parent.Child)
+		assert.Equal(t, args.Grandparent.Parent, p.Subcommand())
+		assert.Equal(t, []string{"grandparent", "pa"}, p.SubcommandNames())
+	}
+
+	{
+		var args root
+		p, err := pparse("grandparent", &args)
+		require.NoError(t, err)
+		require.NotNil(t, args.Grandparent)
+		require.Nil(t, args.Grandparent.Parent)
+		assert.Equal(t, args.Grandparent, p.Subcommand())
+		assert.Equal(t, []string{"grandparent"}, p.SubcommandNames())
+	}
+
+	{
+		var args root
+		p, err := pparse("", &args)
+		require.NoError(t, err)
+		require.Nil(t, args.Grandparent)
+		assert.Nil(t, p.Subcommand())
+		assert.Empty(t, p.SubcommandNames())
+	}
+}
+
 func TestSubcommandsWithPositionals(t *testing.T) {
 	type listCmd struct {
 		Pattern string `arg:"positional"`
@ -411,3 +495,14 @@ func TestValForNilStruct(t *testing.T) {
 	v := p.val(path{fields: []reflect.StructField{subField, subField}})
 	assert.False(t, v.IsValid())
 }
+
+func TestSubcommandInvalidInternal(t *testing.T) {
+	// this situation should never arise in practice but still good to test for it
+	var cmd struct{}
+	p, err := NewParser(Config{}, &cmd)
+	require.NoError(t, err)
+
+	p.subcommand = []string{"should", "never", "happen"}
+	sub := p.Subcommand()
+	assert.Nil(t, sub)
+}
--- a/test/usage_test.go
+++ b/test/usage_test.go
@ -50,12 +50,16 @@ Options:
  --optimize OPTIMIZE, -O OPTIMIZE
                         optimization level
  --ids IDS              Ids
-  --values VALUES        Values [default: [3.14 42 256]]
+  --values VALUES        Values
  --workers WORKERS, -w WORKERS
                         number of workers to start [default: 10, env: WORKERS]
  --testenv TESTENV, -a TESTENV [env: TEST_ENV]
  --file FILE, -f FILE   File with mandatory extension [default: scratch.txt]
  --help, -h             display this help and exit
+
+Environment variables:
+  API_KEY                Required. Only via env-var for security reasons
+  TRACE                  Optional. Record low-level trace
 `

 	var args struct {
@ -70,11 +74,12 @@ Options:
 		Values   []float64    `help:"Values"`
 		Workers  int          `arg:"-w,env:WORKERS" help:"number of workers to start" default:"10"`
 		TestEnv  string       `arg:"-a,env:TEST_ENV"`
+		ApiKey   string       `arg:"required,-,--,env:API_KEY" help:"Only via env-var for security reasons"`
+		Trace    bool         `arg:"-,--,env" help:"Record low-level trace"`
 		File     *NameDotName `arg:"-f" help:"File with mandatory extension"`
 	}
 	args.Name = "Foo Bar"
 	args.Value = 42
-	args.Values = []float64{3.14, 42, 256}
 	args.File = &NameDotName{"scratch", "txt"}
 	p, err := NewParser(Config{Program: "example"}, &args)
 	require.NoError(t, err)
@ -232,7 +237,7 @@ func (versioned) Version() string {
 }

 func TestUsageWithVersion(t *testing.T) {
-	expectedUsage := "example 3.2.1\nUsage: example"
+	expectedUsage := "Usage: example"

 	expectedHelp := `
 example 3.2.1
@ -255,6 +260,233 @@ Options:
 	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
 }

+func TestUsageWithUserDefinedVersionFlag(t *testing.T) {
+	expectedUsage := "Usage: example [--version]"
+
+	expectedHelp := `
+Usage: example [--version]
+
+Options:
+  --version              this is a user-defined version flag
+  --help, -h             display this help and exit
+`
+
+	var args struct {
+		ShowVersion bool `arg:"--version" help:"this is a user-defined version flag"`
+	}
+
+	os.Args[0] = "example"
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	var usage bytes.Buffer
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+}
+
+func TestUsageWithVersionAndUserDefinedVersionFlag(t *testing.T) {
+	expectedUsage := "Usage: example [--version]"
+
+	expectedHelp := `
+Usage: example [--version]
+
+Options:
+  --version              this is a user-defined version flag
+  --help, -h             display this help and exit
+`
+
+	var args struct {
+		versioned
+		ShowVersion bool `arg:"--version" help:"this is a user-defined version flag"`
+	}
+
+	os.Args[0] = "example"
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	var usage bytes.Buffer
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+}
+
+type subcommand struct {
+	Number int `arg:"-n,--number" help:"compute something on the given number"`
+}
+
+func TestUsageWithVersionAndSubcommand(t *testing.T) {
+	expectedUsage := "Usage: example <command> [<args>]"
+
+	expectedHelp := `
+example 3.2.1
+Usage: example <command> [<args>]
+
+Options:
+  --help, -h             display this help and exit
+  --version              display version and exit
+
+Commands:
+  cmd
+`
+
+	var args struct {
+		versioned
+		Cmd *subcommand `arg:"subcommand"`
+	}
+
+	os.Args[0] = "example"
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	var usage bytes.Buffer
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+
+	expectedUsage = "Usage: example cmd [--number NUMBER]"
+
+	expectedHelp = `
+example 3.2.1
+Usage: example cmd [--number NUMBER]
+
+Options:
+  --number NUMBER, -n NUMBER
+                         compute something on the given number
+  --help, -h             display this help and exit
+  --version              display version and exit
+`
+	_ = p.Parse([]string{"cmd"})
+
+	help = bytes.Buffer{}
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	usage = bytes.Buffer{}
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+}
+
+func TestUsageWithUserDefinedVersionFlagAndSubcommand(t *testing.T) {
+	expectedUsage := "Usage: example [--version] <command> [<args>]"
+
+	expectedHelp := `
+Usage: example [--version] <command> [<args>]
+
+Options:
+  --version              this is a user-defined version flag
+  --help, -h             display this help and exit
+
+Commands:
+  cmd
+`
+
+	var args struct {
+		Cmd         *subcommand `arg:"subcommand"`
+		ShowVersion bool        `arg:"--version" help:"this is a user-defined version flag"`
+	}
+
+	os.Args[0] = "example"
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	var usage bytes.Buffer
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+
+	expectedUsage = "Usage: example cmd [--number NUMBER]"
+
+	expectedHelp = `
+Usage: example cmd [--number NUMBER]
+
+Options:
+  --number NUMBER, -n NUMBER
+                         compute something on the given number
+
+Global options:
+  --version              this is a user-defined version flag
+  --help, -h             display this help and exit
+`
+	_ = p.Parse([]string{"cmd"})
+
+	help = bytes.Buffer{}
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	usage = bytes.Buffer{}
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+}
+
+func TestUsageWithVersionAndUserDefinedVersionFlagAndSubcommand(t *testing.T) {
+	expectedUsage := "Usage: example [--version] <command> [<args>]"
+
+	expectedHelp := `
+Usage: example [--version] <command> [<args>]
+
+Options:
+  --version              this is a user-defined version flag
+  --help, -h             display this help and exit
+
+Commands:
+  cmd
+`
+
+	var args struct {
+		versioned
+		Cmd         *subcommand `arg:"subcommand"`
+		ShowVersion bool        `arg:"--version" help:"this is a user-defined version flag"`
+	}
+
+	os.Args[0] = "example"
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	var usage bytes.Buffer
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+
+	expectedUsage = "Usage: example cmd [--number NUMBER]"
+
+	expectedHelp = `
+Usage: example cmd [--number NUMBER]
+
+Options:
+  --number NUMBER, -n NUMBER
+                         compute something on the given number
+
+Global options:
+  --version              this is a user-defined version flag
+  --help, -h             display this help and exit
+`
+	_ = p.Parse([]string{"cmd"})
+
+	help = bytes.Buffer{}
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	usage = bytes.Buffer{}
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+}
+
 type described struct{}

 // Described returns the description for this program
@ -410,6 +642,50 @@ Options:
 	assert.Equal(t, expectedUsage, usage.String())
 }

+func TestUsageWithSubcommands(t *testing.T) {
+	expectedUsage := "Usage: example child [--values VALUES]"
+
+	expectedHelp := `
+Usage: example child [--values VALUES]
+
+Options:
+  --values VALUES        Values
+
+Global options:
+  --verbose, -v          verbosity level
+  --help, -h             display this help and exit
+`
+
+	var args struct {
+		Verbose bool `arg:"-v" help:"verbosity level"`
+		Child   *struct {
+			Values []float64 `help:"Values"`
+		} `arg:"subcommand:child"`
+	}
+
+	os.Args[0] = "example"
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	_ = p.Parse([]string{"child"})
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	var help2 bytes.Buffer
+	p.WriteHelpForSubcommand(&help2, "child")
+	assert.Equal(t, expectedHelp[1:], help2.String())
+
+	var usage bytes.Buffer
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+
+	var usage2 bytes.Buffer
+	p.WriteUsageForSubcommand(&usage2, "child")
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage2.String()))
+}
+
 func TestUsageWithNestedSubcommands(t *testing.T) {
 	expectedUsage := "Usage: example child nested [--enable] OUTPUT"

@ -445,6 +721,8 @@ Global options:

 	_ = p.Parse([]string{"child", "nested", "value"})

+	assert.Equal(t, []string{"child", "nested"}, p.SubcommandNames())
+
 	var help bytes.Buffer
 	p.WriteHelp(&help)
 	assert.Equal(t, expectedHelp[1:], help.String())
@ -466,7 +744,7 @@ func TestNonexistentSubcommand(t *testing.T) {
 	var args struct {
 		sub *struct{} `arg:"subcommand"`
 	}
-	p, err := NewParser(Config{}, &args)
+	p, err := NewParser(Config{Exit: func(int) {}}, &args)
 	require.NoError(t, err)

 	var b bytes.Buffer
@ -506,7 +784,36 @@ Options:
 		ShortOnly2 string `arg:"-b,--,required" help:"some help2"`
 	}
 	p, err := NewParser(Config{Program: "example"}, &args)
-	assert.NoError(t, err)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	var usage bytes.Buffer
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+}
+
+func TestUsageWithEmptyPlaceholder(t *testing.T) {
+	expectedUsage := "Usage: example [-a] [--b] [--c]"
+
+	expectedHelp := `
+Usage: example [-a] [--b] [--c]
+
+Options:
+  -a                     some help for a
+  --b                    some help for b
+  --c, -c                some help for c
+  --help, -h             display this help and exit
+`
+	var args struct {
+		ShortOnly string `arg:"-a,--" placeholder:"" help:"some help for a"`
+		LongOnly  string `arg:"--b" placeholder:"" help:"some help for b"`
+		Both      string `arg:"-c,--c" placeholder:"" help:"some help for c"`
+	}
+	p, err := NewParser(Config{Program: "example"}, &args)
+	require.NoError(t, err)

 	var help bytes.Buffer
 	p.WriteHelp(&help)
@ -553,10 +860,16 @@ Usage: example [-s SHORT]
 Options:
  -s SHORT [env: SHORT]
  --help, -h             display this help and exit
+
+Environment variables:
+  ENVONLY                Optional.
+  ENVONLY2               Optional.
+  CUSTOM                 Optional.
 `
 	var args struct {
 		Short            string `arg:"--,-s,env"`
 		EnvOnly          string `arg:"--,env"`
+		EnvOnly2         string `arg:"--,-,env"`
 		EnvOnlyOverriden string `arg:"--,env:CUSTOM"`
 	}

@ -572,19 +885,39 @@ Options:
 	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
 }

+func TestEnvOnlyArgs(t *testing.T) {
+	expectedUsage := "Usage: example [--arg ARG]"
+
+	expectedHelp := `
+Usage: example [--arg ARG]
+
+Options:
+  --arg ARG, -a ARG [env: MY_ARG]
+  --help, -h             display this help and exit
+
+Environment variables:
+  AUTH_KEY               Required.
+`
+	var args struct {
+		ArgParam string `arg:"-a,--arg,env:MY_ARG"`
+		AuthKey  string `arg:"required,--,env:AUTH_KEY"`
+	}
+	p, err := NewParser(Config{Program: "example"}, &args)
+	assert.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	var usage bytes.Buffer
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+}
+
 func TestFail(t *testing.T) {
-	originalStderr := stderr
-	originalExit := osExit
-	defer func() {
-		stderr = originalStderr
-		osExit = originalExit
-	}()
-
-	var b bytes.Buffer
-	stderr = &b
-
+	var stdout bytes.Buffer
 	var exitCode int
-	osExit = func(code int) { exitCode = code }
+	exit := func(code int) { exitCode = code }

 	expectedStdout := `
 Usage: example [--foo FOO]
@ -594,27 +927,18 @@ error: something went wrong
 	var args struct {
 		Foo int
 	}
-	p, err := NewParser(Config{Program: "example"}, &args)
+	p, err := NewParser(Config{Program: "example", Exit: exit, Out: &stdout}, &args)
 	require.NoError(t, err)
 	p.Fail("something went wrong")

-	assert.Equal(t, expectedStdout[1:], b.String())
-	assert.Equal(t, -1, exitCode)
+	assert.Equal(t, expectedStdout[1:], stdout.String())
+	assert.Equal(t, 2, exitCode)
 }

 func TestFailSubcommand(t *testing.T) {
-	originalStderr := stderr
-	originalExit := osExit
-	defer func() {
-		stderr = originalStderr
-		osExit = originalExit
-	}()
-
-	var b bytes.Buffer
-	stderr = &b
-
+	var stdout bytes.Buffer
 	var exitCode int
-	osExit = func(code int) { exitCode = code }
+	exit := func(code int) { exitCode = code }

 	expectedStdout := `
 Usage: example sub
@ -624,12 +948,139 @@ error: something went wrong
 	var args struct {
 		Sub *struct{} `arg:"subcommand"`
 	}
-	p, err := NewParser(Config{Program: "example"}, &args)
+	p, err := NewParser(Config{Program: "example", Exit: exit, Out: &stdout}, &args)
 	require.NoError(t, err)

 	err = p.FailSubcommand("something went wrong", "sub")
 	require.NoError(t, err)

-	assert.Equal(t, expectedStdout[1:], b.String())
-	assert.Equal(t, -1, exitCode)
+	assert.Equal(t, expectedStdout[1:], stdout.String())
+	assert.Equal(t, 2, exitCode)
+}
+
+type lengthOf struct {
+	Length int
+}
+
+func (p *lengthOf) UnmarshalText(b []byte) error {
+	p.Length = len(b)
+	return nil
+}
+
+func TestHelpShowsDefaultValueFromOriginalTag(t *testing.T) {
+	// check that the usage text prints the original string from the default tag, not
+	// the serialization of the parsed value
+
+	expectedHelp := `
+Usage: example [--test TEST]
+
+Options:
+  --test TEST [default: some_default_value]
+  --help, -h             display this help and exit
+`
+
+	var args struct {
+		Test *lengthOf `default:"some_default_value"`
+	}
+	p, err := NewParser(Config{Program: "example"}, &args)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+}
+
+func TestHelpShowsSubcommandAliases(t *testing.T) {
+	expectedHelp := `
+Usage: example <command> [<args>]
+
+Options:
+  --help, -h             display this help and exit
+
+Commands:
+  remove, rm, r          remove something from somewhere
+  simple                 do something simple
+  halt, stop             stop now
+`
+
+	var args struct {
+		Remove *struct{} `arg:"subcommand:remove|rm|r" help:"remove something from somewhere"`
+		Simple *struct{} `arg:"subcommand" help:"do something simple"`
+		Stop   *struct{} `arg:"subcommand:halt|stop" help:"stop now"`
+	}
+	p, err := NewParser(Config{Program: "example"}, &args)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+}
+
+func TestHelpShowsPositionalWithDefault(t *testing.T) {
+	expectedHelp := `
+Usage: example [FOO]
+
+Positional arguments:
+  FOO                    this is a positional with a default [default: bar]
+
+Options:
+  --help, -h             display this help and exit
+`
+
+	var args struct {
+		Foo string `arg:"positional" default:"bar" help:"this is a positional with a default"`
+	}
+
+	p, err := NewParser(Config{Program: "example"}, &args)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+}
+
+func TestHelpShowsPositionalWithEnv(t *testing.T) {
+	expectedHelp := `
+Usage: example [FOO]
+
+Positional arguments:
+  FOO                    this is a positional with an env variable [env: FOO]
+
+Options:
+  --help, -h             display this help and exit
+`
+
+	var args struct {
+		Foo string `arg:"positional,env:FOO" help:"this is a positional with an env variable"`
+	}
+
+	p, err := NewParser(Config{Program: "example"}, &args)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+}
+
+func TestHelpShowsPositionalWithDefaultAndEnv(t *testing.T) {
+	expectedHelp := `
+Usage: example [FOO]
+
+Positional arguments:
+  FOO                    this is a positional with a default and an env variable [default: bar, env: FOO]
+
+Options:
+  --help, -h             display this help and exit
+`
+
+	var args struct {
+		Foo string `arg:"positional,env:FOO" default:"bar" help:"this is a positional with a default and an env variable"`
+	}
+
+	p, err := NewParser(Config{Program: "example"}, &args)
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
 }
--- a/usage.go
+++ b/usage.go
@ -3,54 +3,37 @@ package arg
 import (
 	"fmt"
 	"io"
-	"os"
 	"strings"
 )

 // the width of the left column
 const colWidth = 25

-// to allow monkey patching in tests
-var (
-	stdout io.Writer = os.Stdout
-	stderr io.Writer = os.Stderr
-	osExit           = os.Exit
-)
-
-// Fail prints usage information to stderr and exits with non-zero status
+// Fail prints usage information to p.Config.Out and exits with status code 2.
 func (p *Parser) Fail(msg string) {
-	p.failWithSubcommand(msg, p.cmd)
+	p.FailSubcommand(msg)
 }

-// FailSubcommand prints usage information for a specified subcommand to stderr,
-// then exits with non-zero status. To write usage information for a top-level
+// FailSubcommand prints usage information for a specified subcommand to p.Config.Out,
+// then exits with status code 2. To write usage information for a top-level
 // subcommand, provide just the name of that subcommand. To write usage
 // information for a subcommand that is nested under another subcommand, provide
 // a sequence of subcommand names starting with the top-level subcommand and so
 // on down the tree.
 func (p *Parser) FailSubcommand(msg string, subcommand ...string) error {
-	cmd, err := p.lookupCommand(subcommand...)
+	err := p.WriteUsageForSubcommand(p.config.Out, subcommand...)
 	if err != nil {
 		return err
 	}
-	p.failWithSubcommand(msg, cmd)
-	return nil
-}

-// failWithSubcommand prints usage information for the given subcommand to stderr and exits with non-zero status
-func (p *Parser) failWithSubcommand(msg string, cmd *command) {
-	p.writeUsageForSubcommand(stderr, cmd)
-	fmt.Fprintln(stderr, "error:", msg)
-	osExit(-1)
+	fmt.Fprintln(p.config.Out, "error:", msg)
+	p.config.Exit(2)
+	return nil
 }

 // WriteUsage writes usage information to the given writer
 func (p *Parser) WriteUsage(w io.Writer) {
-	cmd := p.cmd
-	if p.lastCmd != nil {
-		cmd = p.lastCmd
-	}
-	p.writeUsageForSubcommand(w, cmd)
+	p.WriteUsageForSubcommand(w, p.subcommand...)
 }

 // WriteUsageForSubcommand writes the usage information for a specified
@ -63,12 +46,7 @@ func (p *Parser) WriteUsageForSubcommand(w io.Writer, subcommand ...string) erro
 	if err != nil {
 		return err
 	}
-	p.writeUsageForSubcommand(w, cmd)
-	return nil
-}

-// writeUsageForSubcommand writes usage information for the given subcommand
-func (p *Parser) writeUsageForSubcommand(w io.Writer, cmd *command) {
 	var positionals, longOptions, shortOptions []*spec
 	for _, spec := range cmd.specs {
 		switch {
@ -81,22 +59,10 @@ func (p *Parser) writeUsageForSubcommand(w io.Writer, cmd *command) {
 		}
 	}

-	if p.version != "" {
-		fmt.Fprintln(w, p.version)
-	}
-
-	// make a list of ancestor commands so that we print with full context
-	var ancestors []string
-	ancestor := cmd
-	for ancestor != nil {
-		ancestors = append(ancestors, ancestor.name)
-		ancestor = ancestor.parent
-	}
-
 	// print the beginning of the usage string
-	fmt.Fprint(w, "Usage:")
-	for i := len(ancestors) - 1; i >= 0; i-- {
-		fmt.Fprint(w, " "+ancestors[i])
+	fmt.Fprintf(w, "Usage: %s", p.cmd.name)
+	for _, s := range subcommand {
+		fmt.Fprint(w, " "+s)
 	}

 	// write the option component of the usage message
@ -157,47 +123,66 @@ func (p *Parser) writeUsageForSubcommand(w io.Writer, cmd *command) {
 	}

 	fmt.Fprint(w, "\n")
+	return nil
 }

-func printTwoCols(w io.Writer, left, help string, defaultVal string, envVal string) {
-	lhs := "  " + left
+// print prints a line like this:
+//
+//	--option FOO            A description of the option [default: 123]
+//
+// If the text on the left is longer than a certain threshold, the description is moved to the next line:
+//
+//	--verylongoptionoption VERY_LONG_VARIABLE
+//	                        A description of the option [default: 123]
+//
+// If multiple "extras" are provided then they are put inside a single set of square brackets:
+//
+//	--option FOO            A description of the option [default: 123, env: FOO]
+func print(w io.Writer, item, description string, bracketed ...string) {
+	lhs := "  " + item
 	fmt.Fprint(w, lhs)
-	if help != "" {
+	if description != "" {
 		if len(lhs)+2 < colWidth {
 			fmt.Fprint(w, strings.Repeat(" ", colWidth-len(lhs)))
 		} else {
 			fmt.Fprint(w, "\n"+strings.Repeat(" ", colWidth))
 		}
-		fmt.Fprint(w, help)
+		fmt.Fprint(w, description)
 	}

-	bracketsContent := []string{}
-
-	if defaultVal != "" {
-		bracketsContent = append(bracketsContent,
-			fmt.Sprintf("default: %s", defaultVal),
-		)
+	var brack string
+	for _, s := range bracketed {
+		if s != "" {
+			if brack != "" {
+				brack += ", "
+			}
+			brack += s
+		}
 	}

-	if envVal != "" {
-		bracketsContent = append(bracketsContent,
-			fmt.Sprintf("env: %s", envVal),
-		)
-	}
-
-	if len(bracketsContent) > 0 {
-		fmt.Fprintf(w, " [%s]", strings.Join(bracketsContent, ", "))
+	if brack != "" {
+		fmt.Fprintf(w, " [%s]", brack)
 	}
 	fmt.Fprint(w, "\n")
 }

+func withDefault(s string) string {
+	if s == "" {
+		return ""
+	}
+	return "default: " + s
+}
+
+func withEnv(env string) string {
+	if env == "" {
+		return ""
+	}
+	return "env: " + env
+}
+
 // WriteHelp writes the usage string followed by the full help string for each option
 func (p *Parser) WriteHelp(w io.Writer) {
-	cmd := p.cmd
-	if p.lastCmd != nil {
-		cmd = p.lastCmd
-	}
-	p.writeHelpForSubcommand(w, cmd)
+	p.WriteHelpForSubcommand(w, p.subcommand...)
 }

 // WriteHelpForSubcommand writes the usage string followed by the full help
@ -210,34 +195,55 @@ func (p *Parser) WriteHelpForSubcommand(w io.Writer, subcommand ...string) error
 	if err != nil {
 		return err
 	}
-	p.writeHelpForSubcommand(w, cmd)
-	return nil
-}

-// writeHelp writes the usage string for the given subcommand
-func (p *Parser) writeHelpForSubcommand(w io.Writer, cmd *command) {
-	var positionals, longOptions, shortOptions []*spec
+	var positionals, longOptions, shortOptions, envOnlyOptions []*spec
+	var hasVersionOption bool
 	for _, spec := range cmd.specs {
 		switch {
 		case spec.positional:
 			positionals = append(positionals, spec)
 		case spec.long != "":
 			longOptions = append(longOptions, spec)
+			if spec.long == "version" {
+				hasVersionOption = true
+			}
 		case spec.short != "":
 			shortOptions = append(shortOptions, spec)
+		case spec.short == "" && spec.long == "":
+			envOnlyOptions = append(envOnlyOptions, spec)
 		}
 	}

+	// obtain a flattened list of options from all ancestors
+	// also determine if any ancestor has a version option spec
+	var globals []*spec
+	ancestor := cmd.parent
+	for ancestor != nil {
+		for _, spec := range ancestor.specs {
+			if spec.long == "version" {
+				hasVersionOption = true
+				break
+			}
+		}
+		globals = append(globals, ancestor.specs...)
+		ancestor = ancestor.parent
+	}
+
 	if p.description != "" {
 		fmt.Fprintln(w, p.description)
 	}
-	p.writeUsageForSubcommand(w, cmd)
+
+	if !hasVersionOption && p.version != "" {
+		fmt.Fprintln(w, p.version)
+	}
+
+	p.WriteUsageForSubcommand(w, subcommand...)

 	// write the list of positionals
 	if len(positionals) > 0 {
 		fmt.Fprint(w, "\nPositional arguments:\n")
 		for _, spec := range positionals {
-			printTwoCols(w, spec.placeholder, spec.help, "", "")
+			print(w, spec.placeholder, spec.help, withDefault(spec.defaultString), withEnv(spec.env))
 		}
 	}

@ -252,14 +258,6 @@ func (p *Parser) writeHelpForSubcommand(w io.Writer, cmd *command) {
 		}
 	}

-	// obtain a flattened list of options from all ancestors
-	var globals []*spec
-	ancestor := cmd.parent
-	for ancestor != nil {
-		globals = append(globals, ancestor.specs...)
-		ancestor = ancestor.parent
-	}
-
 	// write the list of global options
 	if len(globals) > 0 {
 		fmt.Fprint(w, "\nGlobal options:\n")
@ -275,7 +273,7 @@ func (p *Parser) writeHelpForSubcommand(w io.Writer, cmd *command) {
 		short:       "h",
 		help:        "display this help and exit",
 	})
-	if p.version != "" {
+	if !hasVersionOption && p.version != "" {
 		p.printOption(w, &spec{
 			cardinality: zero,
 			long:        "version",
@ -283,17 +281,27 @@ func (p *Parser) writeHelpForSubcommand(w io.Writer, cmd *command) {
 		})
 	}

+	// write the list of environment only variables
+	if len(envOnlyOptions) > 0 {
+		fmt.Fprint(w, "\nEnvironment variables:\n")
+		for _, spec := range envOnlyOptions {
+			p.printEnvOnlyVar(w, spec)
+		}
+	}
+
 	// write the list of subcommands
 	if len(cmd.subcommands) > 0 {
 		fmt.Fprint(w, "\nCommands:\n")
 		for _, subcmd := range cmd.subcommands {
-			printTwoCols(w, subcmd.name, subcmd.help, "", "")
+			names := append([]string{subcmd.name}, subcmd.aliases...)
+			print(w, strings.Join(names, ", "), subcmd.help)
 		}
 	}

 	if p.epilogue != "" {
 		fmt.Fprintln(w, "\n"+p.epilogue)
 	}
+	return nil
 }

 func (p *Parser) printOption(w io.Writer, spec *spec) {
@ -305,34 +313,30 @@ func (p *Parser) printOption(w io.Writer, spec *spec) {
 		ways = append(ways, synopsis(spec, "-"+spec.short))
 	}
 	if len(ways) > 0 {
-		printTwoCols(w, strings.Join(ways, ", "), spec.help, spec.defaultVal, spec.env)
+		print(w, strings.Join(ways, ", "), spec.help, withDefault(spec.defaultString), withEnv(spec.env))
 	}
 }

-// lookupCommand finds a subcommand based on a sequence of subcommand names. The
-// first string should be a top-level subcommand, the next should be a child
-// subcommand of that subcommand, and so on. If no strings are given then the
-// root command is returned. If no such subcommand exists then an error is
-// returned.
-func (p *Parser) lookupCommand(path ...string) (*command, error) {
-	cmd := p.cmd
-	for _, name := range path {
-		var found *command
-		for _, child := range cmd.subcommands {
-			if child.name == name {
-				found = child
-			}
-		}
-		if found == nil {
-			return nil, fmt.Errorf("%q is not a subcommand of %s", name, cmd.name)
-		}
-		cmd = found
+func (p *Parser) printEnvOnlyVar(w io.Writer, spec *spec) {
+	ways := make([]string, 0, 2)
+	if spec.required {
+		ways = append(ways, "Required.")
+	} else {
+		ways = append(ways, "Optional.")
 	}
-	return cmd, nil
+
+	if spec.help != "" {
+		ways = append(ways, spec.help)
+	}
+
+	print(w, spec.env, strings.Join(ways, " "), withDefault(spec.defaultString))
 }

 func synopsis(spec *spec, form string) string {
-	if spec.cardinality == zero {
+	// if the user omits the placeholder tag then we pick one automatically,
+	// but if the user explicitly specifies an empty placeholder then we
+	// leave out the placeholder in the help message
+	if spec.cardinality == zero || spec.placeholder == "" {
 		return form
 	}
 	return form + " " + spec.placeholder
Author	SHA1	Message	Date
Jeff Carr	3d847431ab	dump go.mod and go.sum	2024-11-07 16:55:05 -06:00
Jeff Carr	f92d210ca7	Merge remote-tracking branch 'flint/master' into jcarr Signed-off-by: Jeff Carr <jcarr@wit.com>	2024-11-07 12:48:14 -06:00
Alex Flint	438bbfff1e	Merge pull request #258 from hhromic/implement-204 Add support for setting a global env var prefix	2024-11-04 12:58:37 -05:00
Alex Flint	efb1be7122	Merge pull request #273 from alexflint/pass-dash-dash-through Passing the no-more-options string "--" twice or more	2024-11-04 12:56:12 -05:00
Alex Flint	51d9bef113	passing the no-more-options string "--" twice or more should pass the second and subsequent ones through as positionals	2024-10-21 17:08:37 -04:00
Hugo Hromic	cb7e5c1905	Add global env prefix example to README * Also made newline separations around sections consistent * Also fixed usage of `p.Parse()` in env variable ignore example	2024-09-07 13:07:21 +01:00
Hugo Hromic	9b5c76b1c4	Add support for setting a global env var prefix	2024-09-07 12:19:10 +01:00
Alex Flint	b218ad854d	Merge pull request #271 from alexflint/update-funding Update FUNDING.yml	2024-09-06 09:00:31 -04:00
Alex Flint	dcb5577c2b	Update FUNDING.yml	2024-09-06 09:00:13 -04:00
Alex Flint	d10a064207	Merge pull request #262 from hhromic/fix-261 Fix help text for positional args with default and env var	2024-09-05 17:20:09 -04:00
Alex Flint	a5045bbe85	Merge pull request #259 from hhromic/fix-193 Move writing program version from usage to help writer	2024-09-05 17:19:12 -04:00
Alex Flint	3925edf11a	Merge pull request #270 from alexflint/parse-docs Update API docs for Parser.Parse	2024-09-05 17:17:34 -04:00
Alex Flint	12fffac1d8	field -> fields	2024-09-05 17:16:23 -04:00
Alex Flint	b13a62172a	update api docs for Parser.Parse	2024-09-05 17:15:02 -04:00
Alex Flint	7cf32414af	Merge pull request #269 from alexflint/sponsorship Living in a monastery; looking for funding	2024-09-05 17:06:35 -04:00
Alex Flint	bdb7560b8d	Living in a monastery looking for funding The first commit to this project was on October 31, 2015, almost 9 years ago. It was [268 lines of code](`408290f7c2`) and it worked pretty well! That was just about three and a half years after Go 1.0 was released. What fun! At that time there was no Go module system, so there was no need for versioned releases. Later, I started releasing official versions from time to time. v1.0.0 was published in December 2018. Over the years I've resisted adding a lot of features, and as a result the library is in pretty good shape. I use it in almost every Go program I write, personally, both servers/daemons and command line tools. It's nice! I live in a Buddhist monastery in Vermont now, not as a monk but as a lay practitioner. I'm working on building a form of Buddhism fit for consumption by AI systems. I love maintaining this little piece of software and I'd love some financial support to do so. I don't have a day job, and I need money to buy firewood, pay for car insurance, and travel to see my folks back home in Australia from time to time. If you use go-arg please consider sponsoring me. It would make a huge difference to me, and it will create a connection between us. I look forward to many long relationships.	2024-09-05 17:05:51 -04:00
Alex Flint	50166cae2c	Merge pull request #268 from alexflint/readme-custom-error-handling Add info to README about programmatically reproducing behavior of MustParse	2024-09-04 10:31:40 -04:00
Alex Flint	7fd624cf1c	add info to README about programmatically reproducing behavior of MustParse	2024-09-04 10:27:34 -04:00
Hugo Hromic	bf156d17a3	Fix help text for positional args with default and env var	2024-07-22 19:25:51 +01:00
Hugo Hromic	3673177bf9	Move writing program version from usage to help writer * Writing the version on usage text is unexpected and confusing	2024-07-06 11:52:47 +01:00
Alex Flint	3de7278c4f	Merge pull request #257 from hhromic/fix-testable-example Fix testable example output comment formatting	2024-07-04 12:40:29 -04:00
Hugo Hromic	b8282df4c4	Fix testable example output comment formatting	2024-06-30 23:46:34 +01:00
Alex Flint	ec0ced7467	Merge pull request #232 from alexflint/bump-go-versions bump go versions used in CI	2024-06-30 16:42:12 -04:00
Alex Flint	0cc152dce5	Merge pull request #224 from hhromic/better-version-v2 Fix usage writing when using custom version flag	2024-06-30 12:27:39 -04:00
Alex Flint	67353a8bcf	Update version of github actions	2024-06-30 10:35:08 -04:00
Alex Flint	af368523db	Update go.yml	2024-06-30 10:33:03 -04:00
Alex Flint	b6422dcbc3	Merge pull request #233 from testwill/typo fix: typo	2024-06-30 10:32:09 -04:00
Alex Flint	56ee7c97ac	Merge pull request #237 from purpleidea/feat/env-docs add an example for environment vars with arg names	2024-06-30 10:31:46 -04:00
Alex Flint	177b84441e	Merge pull request #256 from hhromic/fix-246 Use standard exit status code for usage errors	2024-06-30 10:31:00 -04:00
Hugo Hromic	c087d71802	Add note for version flag overriding to README	2024-06-30 00:13:28 +01:00
Hugo Hromic	c992aa8627	Add more test cases for version help/usage writing	2024-06-30 00:12:34 +01:00
Hugo Hromic	bed89eb683	Implement scanning of version flag in specs for usage generation	2024-06-29 23:42:22 +01:00
Hugo Hromic	4ed4ce751f	Better scanning of version flag in specs for help generation	2024-06-29 23:42:22 +01:00
Hugo Hromic	a7c40c36a3	Use standard exit status code for usage errors * The stdlib `flags` package and most command line utilities use status code `2`.	2024-06-29 15:44:50 +01:00
Alex Flint	bee5cf5d7c	Merge pull request #255 from hhromic/fix-254 Fix crash on errors in package-level `MustParse`	2024-06-28 11:03:08 -04:00
Hugo Hromic	aa844c7de9	Fix crash on errors in package-level `MustParse`	2024-06-27 00:33:09 +01:00
Alex Flint	dfca71d159	Merge pull request #243 from alexflint/handle-empty-placeholder Handle explicit empty placeholders	2024-04-02 12:16:06 -04:00
Alex Flint	188bd31bf6	Merge pull request #244 from alexflint/restore-100pct-coverage Restore 100% test coverage	2024-04-02 12:14:49 -04:00
Alex Flint	8a917260c3	add a test case with single-level subcommands	2024-04-02 12:10:52 -04:00
Alex Flint	3ddfffdcd3	add test for help and usage when a --version flag is present	2024-04-02 12:05:00 -04:00
Alex Flint	68948b2ac1	restore 100% code coverage	2024-03-31 12:05:26 -04:00
Alex Flint	be792f1f8b	ping	2024-03-31 11:52:16 -04:00
Alex Flint	8e35a4f0d4	handle explicit empty placeholders	2024-03-31 10:30:12 -04:00
James Shubin	84ddf1d244	add an example for environment vars with arg names If you want to specify both of these, and if they should have different names, then this shows you how it can be done.	2024-02-28 22:29:16 -05:00
Jeff Carr	6b16520795	go mod update against scalar Signed-off-by: Jeff Carr <jcarr@wit.com>	2024-01-14 14:37:17 -06:00
Jeff Carr	0af6f25365	add register() Signed-off-by: Jeff Carr <jcarr@wit.com>	2024-01-14 14:26:47 -06:00
Jeff Carr	530fcb84d4	isolate tests Signed-off-by: Jeff Carr <jcarr@wit.com>	2024-01-14 14:25:54 -06:00
guoguangwu	582e6d537a	fix: typo Signed-off-by: guoguangwu <guoguangwu@magic-shield.com>	2023-11-15 17:58:55 +08:00
Alex Flint	bf629a16cb	Merge pull request #231 from alexflint/subcommand-aliases add subcommand aliases	2023-10-10 18:36:46 -04:00
Alex Flint	f02da4cd10	bump go versions used in CI	2023-10-08 20:39:23 -04:00
Alex Flint	e7a4f77ed0	add a unittest for an internally messed up subcommand path	2023-10-08 20:24:18 -04:00
Alex Flint	960d38c3ce	add some more tests for subcommand aliases	2023-10-08 20:14:34 -04:00
Alex Flint	0142b0b842	add subcommand aliases	2023-10-08 20:09:05 -04:00
Alex Flint	5ec29ce755	Merge pull request #229 from alexflint/dependabot/go_modules/gopkg.in/yaml.v3-3.0.0 Bump gopkg.in/yaml.v3 from 3.0.0-20200313102051-9f266ea9e77c to 3.0.0	2023-09-10 15:05:06 -07:00
dependabot[bot]	8e9f60aafc	Bump gopkg.in/yaml.v3 from 3.0.0-20200313102051-9f266ea9e77c to 3.0.0 Bumps gopkg.in/yaml.v3 from 3.0.0-20200313102051-9f266ea9e77c to 3.0.0. --- updated-dependencies: - dependency-name: gopkg.in/yaml.v3 dependency-type: indirect ... Signed-off-by: dependabot[bot] <support@github.com>	2023-08-30 13:58:31 +00:00
Alex Flint	660b9045e1	Merge pull request #223 from hhromic/fix-version-flag Improve handling of version flag	2023-07-14 15:52:33 -04:00
Hugo Hromic	c73f38cd54	Improve handling of version flag * Only use/show builtin `--version` flag if args are versioned with a non-empty `Version()` * If args define a `--version` flag, honor it and disable/hide the builtin version flag * Only return `ErrVersion` when using the builtin version flag	2023-07-14 20:12:52 +01:00
Alex Flint	463902ef7d	Merge pull request #222 from IljaN/env-only-args Support for parameters which can only be passed via env	2023-07-02 10:07:10 -04:00
Ilja Neumann	259c83fd5a	Remove usage of additional envOnly struct variable	2023-06-29 21:26:34 +02:00
Ilja Neumann	18623d869b	help,usage and error messages and tests	2023-06-03 12:47:47 +02:00
Ilja Neumann	b928a1839a	Parse env-only vars	2023-06-03 09:50:42 +02:00
Ilja Neumann	ccf62e0ffc	don't print env-vars in usage line	2023-06-03 03:33:10 +02:00
Pablo Diaz	5f10667949	fixed tests	2023-06-03 02:39:56 +02:00
Pablo Diaz	c3cac76438	added tests and fixed usage	2023-06-03 02:39:56 +02:00
Pablo Diaz	0280e6e591	ignores short and long parameters	2023-06-03 02:39:42 +02:00
Alex Flint	e25b4707a7	Merge pull request #211 from alexflint/clean-up-osexit-stderr-stdout clean up customizable stdout, stderr, and exit in parser config	2023-02-08 06:56:56 -08:00
Alex Flint	df28e7154b	clean up customizable stdout, stderr, and exit in parser config	2023-02-08 09:49:03 -05:00
Alex Flint	5dbdd5d0c5	Merge pull request #210 from cabuda/master feat: support more env than terminal	2023-02-08 06:13:50 -08:00
duxinlong	efae1938fd	feat: support more env than terminal Change-Id: I7f35e90b8f19f4ea781832885d35e2f1e275207a	2023-02-08 12:01:48 +00:00
Alex Flint	c0a8e20a0a	Merge pull request #205 from dmzkrsk/strict-subgroup-parsing add strict subcommand parsing	2023-01-27 08:35:12 -08:00
Alexey Trofimov	5036dce2d6	fix typo	2023-01-18 11:52:13 +03:00
Alexey Trofimov	cef66fd2f6	add strict subcommand parsing	2023-01-18 11:50:50 +03:00
Alex Flint	727f8533ac	Merge pull request #185 from alexflint/default-value-issue Do not turn values intro strings and then back into values when processing default values	2022-10-29 12:29:07 -07:00
Alex Flint	3489ea5b2e	in a second place: use reflect.Ptr not reflect.Pointer since the latter was added in Go 1.18	2022-10-29 15:23:56 -04:00
Alex Flint	763072452f	use reflect.Ptr not reflect.Pointer since the latter was added in Go 1.18	2022-10-29 15:21:21 -04:00
Alex Flint	3d95a706a6	Merge remote-tracking branch 'origin/master' into default-value-issue	2022-10-29 15:19:23 -04:00
Alex Flint	d949871b67	add further comment about backwards-compatible method for setting default values	2022-10-29 15:13:57 -04:00
Alex Flint	9d5e97ac8a	drop unnecessary test	2022-10-29 15:12:53 -04:00
Alex Flint	67f7183b85	remove unused textMarshalerType and isTextMarshaler	2022-10-29 15:10:11 -04:00
Alex Flint	522dbbcea8	add test for the new default value parsing logic as it shows up in help messages	2022-10-29 15:08:48 -04:00
Alex Flint	27c832b934	store both a default value and a string representation of that default value in the spec for each option	2022-10-29 14:47:13 -04:00
Alex Flint	197e226c77	drop unnecessary use of templates in this test	2022-10-29 12:28:06 -04:00
Alex Flint	dbc2ba5d0c	Merge pull request #198 from daenney/mustparse Implement MustParse on Parser	2022-10-10 08:41:57 -07:00
Daniele Sluijters	4fc9666f79	Implement MustParse on Parse This moves most of the body of the MustParse function into a MustParse method on a Parser. The MustParse function is now implemented by calling the MustParse function on the Parser it implicitly creates. Closes: #194	2022-10-10 17:25:14 +02:00
Alex Flint	23b2b67fe2	fix issue #184	2022-06-09 11:21:29 -04:00