argparse.3erl - Man Page

Command line arguments parser.


This module implements command line parser. Parser operates with commands and arguments represented as a tree. Commands are branches, and arguments are leaves of the tree. Parser always starts with the root command, named after progname (the name of the program which started Erlang).

A command specification may contain handler definition for each command, and a number argument specifications. When parser is successful, argparse calls the matching handler, passing arguments extracted from the command line. Arguments can be positional (occupying specific position in the command line), and optional, residing anywhere but prefixed with a specified character.

argparse automatically generates help and usage messages. It will also issue errors when users give the program invalid arguments.

Quick Start

argparse is designed to work with escript. The example below is a fully functioning Erlang program accepting two command line arguments and printing their product.

#!/usr/bin/env escript

main(Args) ->
    argparse:run(Args, cli(), #{progname => mul}).

cli() ->
        arguments => [
            #{name => left, type => integer},
            #{name => right, type => integer}
        handler =>
            fun (#{left := Left, right := Right}) ->
                io:format("~b~n", [Left * Right])

Running this script with no arguments results in an error, accompanied by the usage information.

The cli function defines a single command with embedded handler accepting a map. Keys of the map are argument names as defined by the argument field of the command, left and right in the example. Values are taken from the command line, and converted into integers, as requested by the type specification. Both arguments in the example above are required (and therefore defined as positional).

Command Hierarchy

A command may contain nested commands, forming a hierarchy. Arguments defined at the upper level command are automatically added to all nested commands. Nested commands example (assuming progname is nested):

cli() ->
    %% top level argument applicable to all commands
    arguments => [#{name => top}],
      commands => #{
        "first" => #{
          %% argument applicable to "first" command and
          %%  all commands nested into "first"
          arguments => [#{name => mid}],
          commands => #{
            "second" => #{
              %% argument only applicable for "second" command
              arguments => [#{name => bottom}],
              handler => fun (A) -> io:format("~p~n", [A]) end

In the example above, a 3-level hierarchy is defined. First is the script itself (nested), accepting the only argument top. Since it has no associated handler, run/3 will not accept user input omitting nested command selection. For this example, user has to supply 5 arguments in the command line, two being command names, and another 3 - required positional arguments:

#{top => "one",mid => "two",bottom => "three"}

Commands have preference over positional argument values. In the example above, commands and positional arguments are interleaving, and argparse matches command name first.


argparse supports positional and optional arguments. Optional arguments, or options for short, must be prefixed with a special character (- is the default on all operating systems). Both options and positional arguments have 1 or more associated values. See argument specification to find more details about supported combinations.

In the user input, short options may be concatenated with their values. Long options support values separated by =. Consider this definition:

cli() ->
    arguments => [
      #{name => long, long => "-long"},
      #{name => short, short => $s}
    handler => fun (Args) -> io:format("~p~n", [Args]) end

Running ./args --long=VALUE prints #{long => "VALUE"}, running ./args -sVALUE prints #{short => "VALUE"}

argparse supports boolean flags concatenation: it is possible to shorten -r -f -v to -rfv.

Shortened option names are not supported: it is not possible to use --my-argum instead of --my-argument-name even when such option can be unambiguously found.

Data Types

arg_type() = 
    boolean | float |
    {float, Choice :: [float()]} |
    {float, [{min, float()} | {max, float()}]} |
    integer |
    {integer, Choices :: [integer()]} |
    {integer, [{min, integer()} | {max, integer()}]} |
    string |
    {string, Choices :: [string()]} |
    {string, Re :: string()} |
    {string, Re :: string(), ReOptions :: [term()]} |
    binary |
    {binary, Choices :: [binary()]} |
    {binary, Re :: binary()} |
    {binary, Re :: binary(), ReOptions :: [term()]} |
    atom |
    {atom, Choices :: [atom()]} |
    {atom, unsafe} |
    {custom, fun((string()) -> term())}

Defines type conversion applied to the string retrieved from the user input. If the conversion is successful, resulting value is validated using optional Choices, or minimums and maximums (for integer and floating point values only). Strings and binary values may be validated using regular expressions. It's possible to define custom type conversion function, accepting a string and returning Erlang term. If this function raises error with badarg reason, argument is treated as invalid.

argument_help() = 
     [unicode:chardata() | type | default] |
     fun(() -> unicode:chardata())}

User-defined help template to print in the command usage. First element of a tuple must be a string. It is printed as a part of the usage header. Second element of the tuple can be either a list containing strings, type and default atoms, or a user-defined function that must return a string. A plain string should be wrapped as a list such as ["string is nested"].

argument_name() = atom() | string() | binary()

Argument name is used to populate argument map.

argument() = 
    #{name := argument_name(),
      short => char(),
      long => string(),
      required => boolean(),
      default => term(),
      type => arg_type(),
      action =>
          store |
          {store, term()} |
          append |
          {append, term()} |
          count | extend,
      nargs =>
          integer() >= 1 |
          maybe |
          {maybe, term()} |
          list | nonempty_list | all,
      help => hidden | unicode:chardata() | argument_help()}

Argument specification. Defines a single named argument that is returned in the argument map. The only required field is name, all other fields have defaults.

If either of the short or long fields is specified, the argument is treated as optional. Optional arguments do not have specific order and may appear anywhere in the command line. Positional arguments are ordered the same way as they appear in the arguments list of the command specification.

By default, all positional arguments must be present in the command line. The parser will return an error otherwise. Options, however, may be omitted, in which case resulting argument map will either contain the default value, or not have the key at all.


Sets the argument name in the parsed argument map. If help is not defined, name is also used to generate the default usage message.


Defines a short (single character) form of an optional argument.

%% Define a command accepting argument named myarg, with short form $a:
1> Cmd = #{arguments => [#{name => myarg, short => $a}]}.
%% Parse command line "-a str":
2> {ok, ArgMap, _, _} = argparse:parse(["-a", "str"], Cmd), ArgMap.

#{myarg => "str"}

%% Option value can be concatenated with the switch: "-astr"
3> {ok, ArgMap, _, _} = argparse:parse(["-astr"], Cmd), ArgMap.

#{myarg => "str"}

By default all options expect a single value following the option switch. The only exception is an option of a boolean type.


Defines a long form of an optional argument.

1> Cmd = #{arguments => [#{name => myarg, long => "name"}]}.
%% Parse command line "-name Erlang":
2> {ok, ArgMap, _, _} = argparse:parse(["-name", "Erlang"], Cmd), ArgMap.

#{myarg => "Erlang"}
%% Or use "=" to separate the switch and the value:
3> {ok, ArgMap, _, _} = argparse:parse(["-name=Erlang"], Cmd), ArgMap.

#{myarg => "Erlang"}

If neither short not long is defined, the argument is treated as positional.


Forces the parser to expect the argument to be present in the command line. By default, all positional argument are required, and all options are not.


Specifies the default value to put in the parsed argument map if the value is not supplied in the command line.

1> argparse:parse([], #{arguments => [#{name => myarg, short => $m}]}).

{ok,#{}, ...
2> argparse:parse([], #{arguments => [#{name => myarg, short => $m, default => "def"}]}).

{ok,#{myarg => "def"}, ...

Defines type conversion and validation routine. The default is string, assuming no conversion.


Defines the number of following arguments to consume from the command line. By default, the parser consumes the next argument and converts it into an Erlang term according to the specified type.


Consume exactly this number of positional arguments, fail if there is not enough. Value in the argument map contains a list of exactly this length. Example, defining a positional argument expecting 3 integer values:

1> Cmd = #{arguments => [#{name => ints, type => integer, nargs => 3}]},
argparse:parse(["1", "2", "3"], Cmd).

{ok, #{ints => [1, 2, 3]}, ...

Another example defining an option accepted as -env and expecting two string arguments:

1> Cmd = #{arguments => [#{name => env, long => "env", nargs => 2}]},
argparse:parse(["-env", "key", "value"], Cmd).

{ok, #{env => ["key", "value"]}, ...

Consume all following arguments until hitting the next option (starting with an option prefix). May result in an empty list added to the arguments map.

1> Cmd = #{arguments => [
  #{name => nodes, long => "nodes", nargs => list},
  #{name => verbose, short => $v, type => boolean}
argparse:parse(["-nodes", "one", "two", "-v"], Cmd).

{ok, #{nodes => ["one", "two"], verbose => true}, ...

Same as list, but expects at least one argument. Returns an error if the following command line argument is an option switch (starting with the prefix).


Consumes the next argument from the command line, if it does not start with an option prefix. Otherwise, adds a default value to the arguments map.

1> Cmd = #{arguments => [
  #{name => level, short => $l, nargs => 'maybe', default => "error"},
  #{name => verbose, short => $v, type => boolean}
argparse:parse(["-l", "info", "-v"], Cmd).

{ok,#{level => "info",verbose => true}, ...

%% When "info" is omitted, argument maps receives the default "error"
2> argparse:parse(["-l", "-v"], Cmd).

{ok,#{level => "error",verbose => true}, ...
{'maybe', term()}:

Consumes the next argument from the command line, if it does not start with an option prefix. Otherwise, adds a specified Erlang term to the arguments map.


Fold all remaining command line arguments into a list, ignoring any option prefixes or switches. Useful for proxying arguments into another command line utility.

1> Cmd = #{arguments => [
    #{name => verbose, short => $v, type => boolean},
    #{name => raw, long => "-", nargs => all}
argparse:parse(["-v", "--", "-kernel", "arg", "opt"], Cmd).

{ok,#{raw => ["-kernel","arg","opt"],verbose => true}, ...

Defines an action to take when the argument is found in the command line. The default action is store.


Store the value in the arguments map. Overwrites the value previously written.

1> Cmd = #{arguments => [#{name => str, short => $s}]},
argparse:parse(["-s", "one", "-s", "two"], Cmd).

{ok, #{str => "two"}, ...
{store, term()}:

Stores the specified term instead of reading the value from the command line.

1> Cmd = #{arguments => [#{name => str, short => $s, action => {store, "two"}}]},
argparse:parse(["-s"], Cmd).

{ok, #{str => "two"}, ...

Appends the repeating occurrences of the argument instead of overwriting.

1> Cmd = #{arguments => [#{name => node, short => $n, action => append}]},
argparse:parse(["-n", "one", "-n", "two", "-n", "three"], Cmd).

{ok, #{node => ["one", "two", "three"]}, ...

%% Always produces a list - even if there is one occurrence
2> argparse:parse(["-n", "one"], Cmd).

{ok, #{node => ["one"]}, ...
{append, term()}:

Same as append, but instead of consuming the argument from the command line, appends a provided term().


Puts a counter as a value in the arguments map. Useful for implementing verbosity option:

1> Cmd = #{arguments => [#{name => verbose, short => $v, action => count}]},
argparse:parse(["-v"], Cmd).

{ok, #{verbose => 1}, ...

2> argparse:parse(["-vvvv"], Cmd).

{ok, #{verbose => 4}, ...

Works as append, but flattens the resulting list. Valid only for nargs set to list, nonempty_list, all or pos_integer().

1> Cmd = #{arguments => [#{name => duet, short => $d, nargs => 2, action => extend}]},
argparse:parse(["-d", "a", "b", "-d", "c", "d"], Cmd).

{ok, #{duet => ["a", "b", "c", "d"]}, ...

%% 'append' would result in {ok, #{duet => [["a", "b"],["c", "d"]]},

Specifies help/usage text for the argument. argparse provides automatic generation based on the argument name, type and default value, but for better usability it is recommended to have a proper description. Setting this field to hidden suppresses usage output for this argument.

arg_map() = #{argument_name() => term()}

Arguments map is the map of argument names to the values extracted from the command line. It is passed to the matching command handler. If an argument is omitted, but has the default value is specified, it is added to the map. When no default value specified, and argument is not present in the command line, corresponding key is not present in the resulting map.

handler() = 
    optional |
    fun((arg_map()) -> term()) |
    {module(), Fn :: atom()} |
    {fun(() -> term()), term()} |
    {module(), atom(), term()}

Command handler specification. Called by run/3 upon successful parser return.

fun((arg_map()) -> term()):

Function accepting argument map. See the basic example in the Quick Start section.

{Module :: module(), Function :: atom()}:

Function named Function, exported from Module, accepting argument map.

{fun(() -> term()), Default :: term()}:

Function accepting as many arguments as there are in the arguments list for this command. Arguments missing from the parsed map are replaced with the Default. Convenient way to expose existing functions.

1> Cmd = #{arguments => [
        #{name => x, type => float},
        #{name => y, type => float, short => $p}],
    handler => {fun math:pow/2, 1}},
argparse:run(["2", "-p", "3"], Cmd, #{}).


%% default term 1 is passed to math:pow/2
2> argparse:run(["2"], Cmd, #{}).

{Module :: module(), Function :: atom(), Default :: term()}:

Function named Function, exported from Module, accepting as many arguments as defined for this command. Arguments missing from the parsed map are replaced with the Default. Effectively, just a different syntax to the same functionality as demonstrated in the code above.

command_help() = 
    [unicode:chardata() | usage | commands | arguments | options]

User-defined help template. Use this option to mix custom and predefined usage text. Help template may contain unicode strings, and following atoms:


Formatted command line usage text, e.g. rm [-rf] <directory>.


Expanded list of sub-commands.


Detailed description of positional arguments.


Detailed description of optional arguments.

command() = 
    #{commands => #{string() => command()},
      arguments => [argument()],
      help => hidden | unicode:chardata() | command_help(),
      handler => handler()}

Command specification. May contain nested commands, forming a hierarchy.


Maps of nested commands. Keys must be strings, matching command line input. Basic utilities do not need to specify any nested commands.


List of arguments accepted by this command, and all nested commands in the hierarchy.


Specifies help/usage text for this command. Pass hidden to remove this command from the usage output.


Specifies a callback function to call by run/3 when the parser is successful.

cmd_path() = [string()]

Path to the nested command. First element is always the progname, subsequent elements are nested command names.

parser_error() = 
    {Path :: cmd_path(),
     Expected :: argument() | undefined,
     Actual :: string() | undefined,
     Details :: unicode:chardata()}

Returned from parse/2,3 when the user input cannot be parsed according to the command specification.

First element is the path to the command that was considered when the parser detected an error. Second element, Expected, is the argument specification that caused an error. It could be undefined, meaning that Actual argument had no corresponding specification in the arguments list for the current command.

When Actual is set to undefined, it means that a required argument is missing from the command line. If both Expected and Actual have values, it means validation error.

Use format_error/1 to generate a human-readable error description, unless there is a need to provide localised error messages.

parser_options() = 
    #{prefixes => [char()],
      default => term(),
      progname => string() | atom(),
      command => cmd_path(),
      columns => integer() >= 1}

Options changing parser behaviour.


Changes the option prefix (the default is -).


Specifies the default value for all optional arguments. When this field is set, resulting argument map will contain all argument names. Useful for easy pattern matching on the argument map in the handler function.


Specifies the program (root command) name. Returned as the first element of the command path, and printed in help/usage text. It is recommended to have this value set, otherwise the default one is determined with init:get_argument(progname) and is often set to erl instead of the actual script name.


Specifies the path to the nested command for help/2. Useful to limit output for complex utilities with multiple commands, and used by the default error handling logic.


Specifies the help/usage text width (characters) for help/2. Default value is 80.

parse_result() = 
    {ok, arg_map(), Path :: cmd_path(), command()} |
    {error, parser_error()}

Returned from parse/2,3. Contains arguments extracted from the command line, path to the nested command (if any), and a (potentially nested) command specification that was considered when the parser finished successfully. It is expected that the command contains a handler definition, that will be called passing the argument map.


format_error(Reason :: parser_error()) -> unicode:chardata()

Generates human-readable text for parser error. Does not include help/usage information, and does not provide localisation.

help(Command :: command()) -> string()
help(Command :: command(), Options :: parser_options()) ->

Generates help/usage information text for the command supplied, or any nested command when command option is specified. Arguments are displayed in the same order as specified in Command. Does not provide localisation. Expects progname to be set, otherwise defaults to return value of init:get_argument(progname).

parse(Args :: [string()], Command :: command()) -> parse_result()
parse(Args :: [string()],
      Command :: command(),
      Options :: parser_options()) ->

Parses command line arguments according to the command specification. Raises an exception if the command specification is not valid. Use erl_error:format_exception/3,4 to see a friendlier message. Invalid command line input does not raise an exception, but makes parse/2,3 to return a tuple {error, parser_error()}.

This function does not call command handler.

run(Args :: [string()],
    Command :: command(),
    Options :: parser_options()) ->

Parses command line arguments and calls the matching command handler. Prints human-readable error, help/usage information for the discovered command, and halts the emulator with code 1 if there is any error in the command specification or user-provided command line input.


This function is designed to work as an entry point to a standalone escript. Therefore, it halts the emulator for any error detected. Do not use this function through remote procedure call, or it may result in an unexpected shutdown of a remote node.


stdlib 5.2.2 Maxim Fedorov Erlang Module Definition