docopt helps you:
docopt is based on conventions that have been used for decades in help messages and
man pages for describing a program's interface. An interface description
in docopt is such a help message, but formalized. Here is an example:
Naval Fate.
Usage:
naval_fate ship new <name>...
naval_fate ship <name> move <x> <y> [--speed=<kn>]
naval_fate ship shoot <x> <y>
naval_fate mine (set|remove) <x> <y> [--moored|--drifting]
naval_fate -h | --help
naval_fate --version
Options:
-h --help Show this screen.
--version Show version.
--speed=<kn> Speed in knots [default: 10].
--moored Moored (anchored) mine.
--drifting Drifting mine.
The example describes the interface of executable naval_fate, which can be
invoked with different combinations of commands (ship, new, move,
etc.), options (-h, --help, --speed=<kn>, etc.) and positional
arguments (<name>, <x>, <y>).
The example uses brackets "[ ]", parens "( )", pipes "|" and ellipsis
"..." to
describe optional, required, mutually exclusive, and repeating
elements. Together, these elements form valid usage patterns, each starting
with program's name naval_fate.
Below the usage patterns, there is a list of options with descriptions.
They describe whether an option has short/long forms (-h, --help), whether
an option has an argument (--speed=<kn>), and whether that argument has a
default value ([default: 10]).
A docopt implementation will extract all that information and generate a
command-line arguments parser, with the text of the interface description as the
help message shown when the program is invoked with
the -h or --help options.
Text occuring between keyword usage: (case-insensitive) and a visibly
empty line is interpreted as list of usage patterns. The first word after
usage: is interpreted as the program's name. Here is a minimal example for
program that takes no command-line arguments:
Usage: my_program
Program can have several patterns listed with various elements used to describe the pattern:
Usage:
my_program command --option <argument>
my_program [<optional-argument>]
my_program --another-option=<with-argument>
my_program (--either-that-option | <or-this-argument>)
my_program <repeating-argument> <repeating-argument>...
Each of the elements and constructs is described below.
We will use the word "word" to describe a sequence of characters delimited
by either whitespace, one of "[]()|" characters, or "...".
Words starting with "<", ending with ">" and upper-case words are
interpreted as positional arguments.
Usage: my_program <host> <port>
Words starting with one or two dashes (with exception of "-", "--"
by themselves) are interpreted as short (one-letter) or long options,
respectively.
-abc is equivalent to
-a -b -c.=" sign:--input=ARG is equivalent to --input ARG.-f FILE is equivalent to -fFILE.Note, writing --input ARG (as opposed to --input=ARG) is ambiguous, meaning
it is not possible to tell whether ARG is option's argument or a positional
argument. In usage patterns this will be interpreted as an option with argument
only if a description (covered below) for that option is
provided. Otherwise it will be interpreted as an option and separate
positional argument.
There is the same ambiguity with the -f FILE and -fFILE notation. In the latter
case it is not possible to tell whether it is a number of stacked short
options, or an option with an argument. These notations will be interpreted as an
option with argument only if a description for the option is provided.
All other words that do not follow the above conventions of --options or
<arguments> are interpreted as (sub)commands.
Elements (options, arguments, commands) enclosed with square brackets "[ ]"
are marked to be optional. It does not matter if elements are enclosed
in the same or different pairs of brackets, for example:
Usage: my_program [command --option <argument>]
is equivalent to:
Usage: my_program [command] [--option] [<argument>]
All elements are required by default, if not included in brackets "[ ]".
However, sometimes it is necessary to mark elements as required explicitly
with parens "( )".
For example, when you need to group mutually-exclusive elements (see next
section):
Usage: my_program (--either-this <and-that> | <or-this>)
Another use case is when you need to specify that if one element is present, then another one is required, which you can achieve as:
Usage: my_program [(<one-argument> <another-argument>)]
In this case, a valid program invocation could be with either no arguments, or with 2 arguments.
Mutually-exclusive elements can be separated with a pipe "|" as follows:
Usage: my_program go (--up | --down | --left | --right)
Use parens "( )" to group elements when one of the mutually exclusive
cases is required. Use brackets "[ ]" to group elements when none of the
mutually exclusive cases is required:
Usage: my_program go [--up | --down | --left | --right]
Note, that specifying several patterns works exactly like pipe "|", that is:
Usage: my_program run [--fast]
my_program jump [--high]
is equivalent to:
Usage: my_program (run [--fast] | jump [--high])
Use ellipsis "..." to specify that the argument (or group of arguments)
to the left could be repeated one or more times:
Usage: my_program open <file>...
my_program move (<from> <to>)...
You can flexibly specify the number of arguments that are required. Here are 3 (redundant) ways of requiring zero or more arguments:
Usage: my_program [<file>...]
my_program [<file>]...
my_program [<file> [<file> ...]]
One or more arguments:
Usage: my_program <file>...
Two or more arguments (and so on):
Usage: my_program <file> <file>...
"[options]" is a shortcut that allows to avoid listing all options
(from list of options with descriptions) in a pattern. For example:
Usage: my_program [options] <path>
--all List everything.
--long Long output.
--human-readable Display in human-readable format.
is equivalent to:
Usage: my_program [--all --long --human-readable] <path>
--all List everything.
--long Long output.
--human-readable Display in human-readable format.
This can be useful if you have many options and all of them are applicable to one of patterns. Alternatively, if you have both short and long versions of options (specified in option description part), you can list either of them in a pattern:
Usage: my_program [-alh] <path>
-a, --all List everything.
-l, --long Long output.
-h, --human-readable Display in human-readable format.
More details on how to write options' descriptions will follow below.
A double dash "--", when not part of an option, is often used as a convention
to separate options and positional arguments, in order to handle cases when,
e.g., file names could be mistaken for options. In order to support this
convention, add "[--]" into your patterns before positional arguments.
Usage: my_program [options] [--] <file>...
Apart from this special meaning, "--" is just a normal command, so you can
apply any previously-described operations, for example, make it required
(by dropping brackets "[ ]")
A single dash "-", when not part of an option, is often used by convention
to signify that a program should process stdin, as opposed to a file.
If you want to follow this convention add "[-]" to your pattern.
"-" by itself is just a normal command, which you can use with any meaning.
Option descriptions consist of a list of options that you put below your
usage patterns. It is optional to specify them if there is no ambiguity
in usage patterns (described in the --option section above).
An option's description allows to specify:
The rules are as follows:
Every line that starts with "-" or "--" (not counting spaces)
is treated as an option description, e.g.:
Options:
--verbose # GOOD
-o FILE # GOOD
Other: --bad # BAD, line does not start with dash "-"
To specify that an option has an argument, put a word describing that
argument after a space (or equals "=" sign) as shown below. Follow
either <angular-brackets> or UPPER-CASE convention for options' arguments.
You can use a comma if you want to separate options. In the example below, both
lines are valid, however it is recommended to stick to a single style.
-o FILE --output=FILE # without comma, with "=" sign
-i <file>, --input <file> # with comma, without "=" sign
Use two spaces to separate options with their informal description.
--verbose MORE text. # BAD, will be treated as if verbose
# option had an argument MORE, so use
# 2 spaces instead
-q Quit. # GOOD
-o FILE Output file. # GOOD
--stdout Use stdout. # GOOD, 2 spaces
If you want to set a default value for an option with an argument, put it
into the option's description, in the form [default: <the-default-value>].
--coefficient=K The K coefficient [default: 2.95]
--output=FILE Output file [default: test.txt]
--directory=DIR Some directory [default: ./]
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Official implementations are listed under the docopt organization on GitHub.