Command line argument parser package in Swift.

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Swift 5.7


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Command line argument parser package in Swift.

The basic idea is to define a tree structure of parsers which then parses all command line arguments. This approach is very flexible and allows for quick and easy flag parsing for a simple script, as well as complicated parse trees for big command line programs.

Table of Contents


Swift Package Manager

let package = Package(
        dependencies: [
          .package(url: "", from: "1.5.4"),


At least clang-3.6 is required. On linux one might need to install it explicitly. There are no dependencies on macOS.

Getting started

The following example shows a hello world script with one flag (no option or command) and generated help.

// global modal for the application
var verbose = false

try! ArgTree(description:
usage: \(CommandLine.arguments[0])) [flags...]

hello world demo

    parsers: [
        Flag(longName: "verbose", shortName: "v", description: "print verbose output") { _ in
            verbose = true
// here comes the real program code after parsing the command line arguments
if verbose {
    print("hello world")
} else {

Help Text generation

Help texts can be generated automatically (partially), detailed in Automatic Help Flag. This is only true for global help. Help on individual commands is not generated, can however, easily be implemented by adding a Help flag to the command.


There are a variaty of parsers implemented to compose the parser tree.

If those are not sufficient, it is easy to implement a custom parser. Therefore, the Parser interface must be implemented, see Architecture for details.


A flag is a boolean property like -v or --verbose. A flag has a long and a short name, both are optional (however, not setting any of them does not make sense). Handling a flag can either be done via the parsed closure

let verbose = false
try! ArgTree(parsers: [
    Flag(longName: "verbose", shortName: "v") { value, path in verbose = true }

or by accessing the parsed values later.

let verboseFlag = Flag(longName: "verbose", shortName: "v")
try! ArgTree(parsers: [verboseFlag]).parse()
let verbose = verboseFlag.value != nil

Flag Prefixes

By default long names get the prefix "--" and short names get the prefix "-". Other prefixes, to handle e.g. +a, can be specified.

let a = Flag(shortName: "a", shortPrefix: "+")

The same can be done for long prefixes.

Passing a Flag Multiple Times

By default passing the same flag multiple times is reported as error (FlagParseError.flagAllowedOnlyOnce). Sometimes it is, however, useful to be able to pass the same flag multiple times, e.g. if -v should print a verbose output and -v -v should print a very verbose output. In this case the flag should have set the property multiAllowed to true.

let verboseFlag = Flag(longName: "verbose", shortName: "v", multiAllowed: true)
try! ArgTree(parsers: [verboseFlag]).parse()
let verbosity = verboseFlag.values.count

The number of times the value was parsed can be accassed via the values property. It is up to the implementation to decide if passing the same flag multiple times is simply ignored or meaning something useful.

Handling Unexpected Flags

By default, nothing happens if a flag was set at the command line, that has no meaning, i.e. that is not parsed. To report all flag like arguments that have not meaning as errors, simply add the UnexpectedFlagHandler to the parsers. The handler must be added after the flag parsers, to report errors correctly.

try! ArgTree(parsers: [
        Flag(longName: "verbose", shortName: "v")

In this case, all arguments starting with either the long or short prefix will be reported as errors. The UnexpectedFlagHandler supports a Stop Token, to allow for flag like var args. Also, the longPrefix and shortPrefix can be customized. If flags with different prefixes are used, e.g. -a and +a, two seperate UnexpectedFlagHandler can be added, one for the standard prefix and one for the + prefix.

Multi Flags

Multi flags are combined flags (for short names). For example if there are flags -a and -b one could also pass the combined flag -ab or -ba, which is equivalent to -a -b.

To achieve this kind of parsing use the MultiFlag.

try! ArgTree(parsers: [
    MultiFlag(parsers: [
        Flag(shortName: "a")
        Flag(shortName: "b")

Note that the mulit flag and all the added flags must have the same shortPrefix to get the expected result.

Automatic Help Flag

If initializing ArgTree with a description or helpText a help flag is automatically added, which will show a help text. A minimal example would be:

try! ArgTree(helpText: "usage...").parse()

The call to the script with -h or --help passed as flag will print


and exit afterwards.

If a helpText is passed it will simply be printed. Alternatively, a description can be passed, which will generate a help text from the description and all descriptions of the passed flags and options.

try! ArgTree(description: 
usage: \(CommandLine.arguments[0]) [flags...]

parsers: [
    Flag(longName: "foo", shortName: "f", description: "a foo flag"),

This example will print

usage: my_script [flags...]

--foo, -f a foo flag
Parse Order

The generated help flag is always added as first parser to make sure it plays together with Var Args nicely. The order of the parsers can be changed after creation of the ArgTree object by manipulating its elements via the MutableCollection protocol (like an array). For example, to move the auto generated help flag parser to the end, do:

var argTree = ArgTree(description: "foo")
Default Action

Generated help is set as default action automatically. If this is not intended, the default action can be unset or set to something else.

let argTree = ArgTree(description: "usage...")
argTree.defaultAction = nil
try! argtree.parse()
Exit After Help Printed

The generated help flag parser always exits with code 0 after printing the help text. If this is not the intended behaviour one can pass a closure, which is called after the help text is printed. The following example shows how to continue without any specific action after printing help.

let argTree = ArgTree(description: "usage...") { /* do nothing after help was printed */ }
try! argtree.parse()
Output Stream

Help text is printed to stdout by default. This can be customized, by setting the writeToOutStream delegate. For example one could redirect the output to a string.

let argTree = ArgTree(description: "usage...")
var out = ""
argTree.writeToOutStream = { s in
    print(s, to: &out)


An option is a key value property like --foo=bar or --foo bar. An option has a long and a short name, both are optional. Two syntaxes are supported, i.e. a key value pair can be passed separated by = or by passing the value as subsequent argument to the key. If the value to a key cannot be parsed, it will be reported as error (OptionParseError.missingValueForOption). The following example shows the basic usage of options.

var foo: String = "default"
try! ArgTree(parsers: [
    Option(longName: "foo") {value, _ in foo = value}

Option Prefixes

Prefixes can be changed as for flags, see Flag Prefixes.

Passing an Option Multiple Times

Options can be passed multiples times, specifying different values. By default, passing an option multiple times is reported as error (OptionParseError.optionAllowedOnlyOnce), as for flags, see Passing a Flag Multiple Times. The following example shows how to implement an options, for which different values can be passed.

let fooOption = Option(longName: "foo", multiAllowed: true)
try! ArgTree(parsers: [fooOption]).parse()
fooOption.values.forEach{ value in /* ... */ }

In this case, it makes sense to handle the parsed options via values after parsing instead of handling them via a closure. Although this is possible as well.

var foo: [String] = []
try! ArgTree(parsers: [
    Option(longName: "foo", multiAllowed: true) { value, _ in 

Int Option and Double Option

If only integers or floating point values are allowed for an option, the convenience parsers IntOption and DoubleOption can be employed. They work exactly like Option except that all values not parsable to an Int or Double will be reported as OptionParseError.valueNotIntConvertible or OptionParseError.valueNotDoubleConvertible, respectively.

Handling Unexpected Options

The mechanism is the same as for flags, see Handling Unexpected Flags. Simply add a UnexpectedOptionHandler to the parsers.


A command is a special argument to change control flow of the program. Simple scripts or programs like e.g. rm often do not have commands, more advanced command line programs, like. e.g. git support a variety of commands and even sub-commands. The following example shows an implementation of a program, handling the command foo.

try! ArgTree(parsers: [
    Command(name: "foo") { path in 
        /* handle foo command */ }

When it comes to handling commands, quickly things get complicated. For example the following questions arise:

  • Should a flag be supported only on the global level, or also on the level of each command?
    • For example, the --verbose flag might be supported on any level and have the same effect on any level, i.e. setting the output to verbose for the whole program.
    • The --help flag, on the other hand, might also be supported on every command, should however print a different help for each command.
    • Additional flags should only be supported for certain commands and for others not.
  • The same questions arises for options.
  • How are var args handled?
    • Some commands may take var args, others don't.
  • Must var args be handled on the global level and also on the command level?
  • Can commands be nested?
    • If there exist commands, there should be also sub-commands and sub-sub-commands.

The good thing is, everything can be done with ArgTree. However, it requires a bit of an understanding, how the parsing works, since the order in which parses are added to the parse tree matters. To support finding out the correct order, consider to switch on logging while parsing, see Logging.

Some of the cases listed above are detailed in examples in the following sections.

Global Flags (or Options)

A global flag, that does the same for every command is easy to implement. Just add it before all commands.

var verbose = false;
try! ArgTree(parsers: [
    Flag(longName: "verbose", shortName: "v") { _ in verbose = true }
    Command(name: "foo") 

The same can be done for options.

Semi-Global Flags (or Options)

A so called semi-global flag is one that can be set on any level, but has different effects. The following example shows how to implement a custom help flag, if the generated help should not be used.

try! ArgTree(parsers: [
        Command(name: "foo", parsers: [
            Flag(longName: "help", shortName: "h") { _ in print("help for foo") }
        Flag(longName: "help", shortName: "h") { _ in print("global help") }

The corresponding parse tree is

  +-- foo
  |   +-- help(1)
  +-- help(2)

Here two differnt Flag instances (help(1) and help(2)) are employed to parse the help flags.

Instead of defining separate Flag instances, different actions can also be performed, based on the parse path.

let help = Flag(longName: "help", shortName: "h")
let foo = Command(name: "foo", parsers: [help])
help.parsed = { path in
    switch path.last {
        case let cmd as Command where cmd === foo:
            print("foo help")
        // case let cmd as Command where ...  (other commands)
            print("global help")
try! ArgTree(parsers: [help foo]).parse()

By using the path, to determine the context of a flag, very generic implementations are possible.

The corresponding parse tree is

  +-- foo
  |   +-- help
  +-- help

Which strategy is better depends on the use case. If the flag should have the same description on evey command, it might be better to use the same instance everywhere and implement the logic based on the path segment. On the other hand, the description is different for all commands, it might be simpler to use different instances.

Var Args on Commands

For var args in general see Var Args. If var args should be passed to a specific command, it can be done simply by adding the var args on the command.

let fooVarArgs = VarArgs()
try! ArgTree(parsers: [
    Command(name: "foo", parsers: [fooVarArgs]) 

To add support for var args also on the global level, simply another var args object can be added at this level. It must be added after the command, otherwise the command will be parsed as var arg.

let globalVarArgs = VarArgs()
let fooVarArgs = VarArgs()
try! ArgTree(parsers: [
    Command(name: "foo", parsers: [fooVarArgs]),

Look at some examples, how var args will be parsed in this case.

my_script a b         # a and b parsed by globalVarArgs
my_script foo a b     # a and b parsed by fooVarArgs
my_script a foo b     # a parsed by globalVarArgs and b parsed by fooVarArgs

This is straightforward, when looking a the parse tree.

  +-- foo
  |   +-- globalVarArgs
  +-- fooVarArgs

Command Default Action

As on the root node ArgTree of the parse tree, there is an optional default action on each command. The default action is called, if no child parser consumed any further argument. The default action can be set on the command directly.

let foo = Command(name: "foo" parsers: [
        Flag(longName: "bar") { _ in print("--bar parsed") }
    ]) { _ in 
    print("foo (maybe also --bar parsed)") 
foo.defaultAction = { () in print("foo (--bar not parsed)") }
try! ArgTree(parsers: [foo, baz]).parse()

parsed and afterChildrenParsed

The command has two optional delegates: parsed and afterChildrenParsed. The first, parsed is called, directly after the command was parsed, as for flags and options. At this time, however, no further arguments are parsed by parsers in the parsers property. The second delegate afterChildrenParsed is called, when the command was parsed and also all subsequent arguments are parsed by parsers from the parsers property. So when a command is defined like in the following example

let bar = Flag(longName: "bar")
let foo = Command(name: "foo", parsers: [bar]) { _ in print("bar?: \(bar.value)") }

the trailing closure refers to afterChildrenParsed and all parsed values from any child parser can be accessed.

Nested Commands

Commands, Flags, Options and so on can be nested to arbitrary depth. This is why the package is called ArgTree. Here is a simple example.

let bar = Command(name: "bar") { _ in print("foo bar") }
let foo = Command(name: "foo", parsers: [foo])
let baz = Command(name: "baz") { _ in print("baz") }
foo.defaultAction = { () in print("foo (no sub command)") }
try! ArgTree(parsers: [foo, baz]).parse()

Here is the parse tree.

  +-- foo
  |   +-- bar
  +-- baz

Var Args

Var args are all arguments, that are not specifically parsed by any other parser. Quite often, a script takes an arbitrary number of files as var args. Consider the following example

my_script -v file1 file2
my_script file1 file2 --verbose

Both scripts should process file1 and file2 as var args and handle -v or --verbose as flag, regardless of its position (to handle -v or --verbose as file, see Stop Token). This can be achieved, by defining the following parse tree.

let varArgs = VarArgs()
let argTree = ArgTree(parsers: [
    Flag(longName: "verbose", shortName: "-v", description: "verbose output") { _ _ in }

As can be seen in the example, defining VarArgs inline, as for Flag is possible but does not make sense. Notice that it is important to place varArgs after the Flag, otherwise every argument would be parsed as var arg instead of parsing it as flag. So usually VarArgs is added last in the parsers array. Parsed var args are usually handled, after the parsing was completed via the RandomAccessCollection protocol (like an array).

try! argTree.parse()
varArgs.values.forEach{ value in /* ... */ }

Handling Unexpected Arguments

If no var args are used, it might by helpful to report all errors as unexpected arg. In this case the UnexpectedArgHandler can be added as last parser. This will report any arg, not parsed by another parser before as ArgParseError.unexpectedArg.

See also Handling Unexpected Flags and Handling Unexpected Options.

Stop Token

A stop token stops parsing subsequent arguments as they would normally be parsed. By default -- is used as stop token. This means, all arguments passed after -- will be parsed as var args. This is helpful, if e.g. files with names that clash with a command or flag name should be passed as arguments. An example would be handling a file with name -h

my_script -- -h

which would normally print the help text. In this case -h is treated as var arg, e.g. a file name.

Default Action

If no argument could be parsed, a default action can be performed. If there is a generated help flag, the default action will be set to printing the help text and exit. This can be customized by setting (or unsetting) a default action.

let argTree = ArgTree()
argTree.defaultAction = { () in print("this is the default") }

See also Command Default Action.

Error Handling

The parse is supposed to throw errors on parsing the arguments. A variety of errors can be thrown and in simple scripts, it may be sufficient to force try the parse call (as in all examples in this document). Any parse error will be reported to stderr and the program will exit. While this default behaviour is sufficient for simple scripts and programs, more suffisticated programs, might print nice error messages. This can be done by catching errors and doing some nice error handling.

do {
    try ArgTree(parsers: [UnexpectedArgHandler()]).parse()
} catch ArgParseError.unexpectedArg(argument:let arg, atIndex:_) {
    print("got an unexpected argument: \(arg)")
} catch let error {
    print("unknown error \(error)")


To get a deeper understanding of why a certain argument is parsed or not parsed it can be very helpful to switch on logging.

Logging is done via the swift-log API. So by default nothing is logged. To activate logging, one must configure a logger such as (HeliumLogger) or the StreamLogHandler which can be employed in the following way.

import Logging
LoggingSystem.bootstrap({ label in
    var logHandler = StreamLogHandler.standardError(label: label)
    logHandler.logLevel = .trace/
    logHandler = logHandler
    return logHandler

let argTree = ArgTree()
try! argtree.parse()

Note that most of the logging is done on debug level, so this level should be activated to see any log output.


The basic idead it so define a tree of parsers, which then consume argument after argument. This package helps to define the parser tree and invoke it. Each node in the parser tree usually parses specific types of arguments, e.g. flags or options. To understand, how the parser tree must be set up, it is important to know how the tree is traversed. Consider the following example

arguments = ['arg_0', 'arg_1', 'arg_2']

The first argument is always ignored, since it refers to the script name. Parsing is started at arg_1. Now, if there is the following tree

  +-- parser_0
  |   +-- parser_0_1
  +-- parser_1

first, arg_1 will be parsed. Thereby argTree calls each child parser with the arguments array and the index i, where parsing should be done. Each child parser, parser_0 and parser_1 in this case can decidide to consume any number of arguments starting from i and returns how many arguments it consumed. Hence, if parser_0 decides to consume all arguments, parser_1 will never get called, since there are no arguments left. If parser_0 consumes no argument, parser_1 get called on the same index i and may also consume an arbitrary amount of arguments. For any index i calling subsequent child parsers is stopped, as soon as one child parser consumes a non-zero amount of argumts. After that, the index i is incremented by the number of consumed arguments and the list of child parsers ist iterated again from the beginning. So if parser_0 consumes arg_1, parser_1 is not called. Instead i is incremented by one and parser_0 is called again for arg_2. So it is important to understand that parsers with low indices always have higher precedence than the following parsers. One corner case is, if an argument is not consumed by any parser. In this case i is incremented by one and the next argument is parsed. This means that the argument not parsed is simply ignored. If ignoring arguments is not the expected behaviour, an UnexpectedArgHandler can be added to throw an error. It should be clear now that the UnexpectedArgHandler must be added as last parser, since it simply consumes any argument and converts it into an error.

Having understood the parsing process for one node, it is straightforward to understand the whole tree. Since every parser node can consume any number of arguments, it is not important how the node parses arguments. So each node may can itself delegate to child parses in the described way. This makes it very easy to reuse simple parsers for flags and options. Here is a short example for a command line program, which takes a global flag -v and two commands foo and bar which themselves take flags -f and -b respectively.

  +-- -v
  +-- foo
  |   +-- -f
  +-- bar
      +-- -b

Parse Path

To define the context of a parsed argument, a parse path is always specified. This is simply an array of parsers in the call chain. Note, that the root parser is not added to the path. So for the following example

  +-- -v
  +-- foo
  |   +-- -f
  +-- bar
      +-- -b

there would be the folowing paths when parsing

[] : -v
[foo] : -f
[bar] : -b

This path can be used, if a parser should be used multiple times, but should act context aware. If e.g. -v should do something different for the foo and for bar the following tree should be defined

  +-- foo
  |   +-- -v
  |   +-- -f
  +-- bar
      +-- -v
      +-- -b

Or, alternatively

  +-- -v
  +-- foo
  |   +-- -v
  |   +-- -f
  +-- bar
      +-- -v
      +-- -b

if -v should also be supported on the global level. For an example on path specific actions see Semi Global Flags (or Options).


Read the generated docs.


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Last updated: Mon Jun 17 2024 20:50:00 GMT-0900 (Hawaii-Aleutian Daylight Time)