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@page
@node Command Line Handling
@chapter Handling Command Line Options and Arguments

@c This chapter was written and contributed by Martin Grabmueller.

The ability to accept and handle command line arguments is very
important when writing Guile scripts to solve particular problems, such
as extracting information from text files or interfacing with existing
command line applications.  This chapter describes how Guile makes
command line arguments available to a Guile script, and the utilities
that Guile provides to help with the processing of command line
arguments.

@menu
* Command Line Args::           Using command line arguments.
* getopt-long::                 The (ice-9 getopt-long) module.
@end menu


@node Command Line Args
@section Using Command Line Arguments

When a Guile script is invoked, Guile makes the command line arguments
accessible via the procedure @code{command-line}, which returns the
arguments as a list of strings.

For example, if the script

@example
#! /usr/local/bin/guile -s
!#
(write (command-line))
(newline)
@end example

@noindent
is saved in a file @file{cmdline-test.scm} and invoked using the command
line @code{./cmdline-test.scm bar.txt -o foo -frumple grob}, the output
is

@example
("./cmdline-test.scm" "bar.txt" "-o" "foo" "-frumple" "grob")
@end example

If the script invocation includes a @code{-e} option, specifying a
procedure to call after loading the script, Guile will call that
procedure with @code{(command-line)} as its argument.  So a script that
uses @code{-e} doesn't need to refer explicitly to @code{command-line}
in its code.  For example, the script above would have identical
behaviour if it was written instead like this:

@example
#! /usr/local/bin/guile \
-e main -s
!#
(define (main args)
  (write args)
  (newline))
@end example

(Note the use of the meta switch @code{\} so that the script invocation
can include more than one Guile option: @xref{The Meta Switch}.)

These scripts use the @code{#!} POSIX convention so that they can be
executed using their own file names directly, as in the example command
line @code{./cmdline-test.scm bar.txt -o foo -frumple grob}.  But they
can also be executed by typing out the implied Guile command line in
full, as in:

@example
$ guile -s ./cmdline-test.scm bar.txt -o foo -frumple grob
@end example

@noindent
or

@example
$ guile -e main -s ./cmdline-test2.scm bar.txt -o foo -frumple grob
@end example

Even when a script is invoked using this longer form, the arguments that
the script receives are the same as if it had been invoked using the
short form.  Guile ensures that the @code{(command-line)} or @code{-e}
arguments are independent of how the script is invoked, by stripping off
the arguments that Guile itself processes.


@node getopt-long
@section The (ice-9 getopt-long) Module

A script is free to parse and handle its command line arguments in any
way that it chooses.  Where the set of possible options and arguments is
complex, however, it can get tricky to extract all the options, check
the validity of given arguments, and so on.  This task can be greatly
simplified by taking advantage of the module @code{(ice-9 getopt-long)},
which is distributed with Guile.

The @code{(ice-9 getopt-long)} module exports two procedures:
@code{getopt-long} and @code{option-ref}.

@itemize @bullet
@item
@code{getopt-long} takes a list of strings --- the command line
arguments --- and an @dfn{option specification}.  It parses the command
line arguments according to the option specification and returns a data
structure that encapsulates the results of the parsing.

@item
@code{option-ref} then takes the parsed data structure and a specific
option's name, and returns information about that option in particular.
@end itemize

To make these procedures available to your Guile script, include the
expression @code{(use-modules (ice-9 getopt-long))} somewhere near the
top, before the first usage of @code{getopt-long} or @code{option-ref}.

@menu
* getopt-long Example::         A short getopt-long example.
* Option Specification::        How to write an option specification.
* Command Line Format::         The expected command line format.
* getopt-long Reference::       Full documentation for @code{getopt-long}.
* option-ref Reference::        Full documentation for @code{option-ref}.
@end menu


@node getopt-long Example
@subsection A Short getopt-long Example

This subsection illustrates how @code{getopt-long} is used by presenting
and dissecting a simple example.  The first thing that we need is an
@dfn{option specification} that tells @code{getopt-long} how to parse
the command line.  This specification is an association list with the
long option name as the key.  Here is how such a specification might
look:

@lisp
(define option-spec
  '((version (single-char #\v) (value #f))
    (help    (single-char #\h) (value #f))))
@end lisp

This alist tells @code{getopt-long} that it should accept two long
options, called @emph{version} and @emph{help}, and that these options
can also be selected by the single-letter abbreviations @emph{v} and
@emph{h}, respectively.  The @code{(value #f)} clauses indicate that
neither of the options accepts a value.

With this specification we can use @code{getopt-long} to parse a given
command line:

@lisp
(define options (getopt-long (command-line) option-spec))
@end lisp

After this call, @code{options} contains the parsed command line and is
ready to be examined by @code{option-ref}.  @code{option-ref} is called
like this:

@lisp
(option-ref options 'help #f)
@end lisp

@noindent
It expects the parsed command line, a symbol indicating the option to
examine, and a default value.  The default value is returned if the
option was not present in the command line, or if the option was present
but without a value; otherwise the value from the command line is
returned.  Usually @code{option-ref} is called once for each possible
option that a script supports.

The following example shows a main program which puts all this together
to parse its command line and figure out what the user wanted.

@lisp
(define (main args)
  (let* ((option-spec '((version (single-char #\v) (value #f))
                        (help    (single-char #\h) (value #f))))
         (options (getopt-long args option-spec))
         (help-wanted (option-ref options 'help #f))
         (version-wanted (option-ref options 'version #f)))
    (if (or version-wanted help-wanted)
        (begin
          (if version-wanted
              (display "getopt-long-example version 0.3\n"))
          (if help-wanted
              (display "\
getopt-long-example [options]
  -v, --version    Display version
  -h, --help       Display this help
")))
        (begin
          (display "Hello, World!") (newline)))))
@end lisp


@node Option Specification
@subsection How to Write an Option Specification

An option specification is an association list (@pxref{Association
Lists}) with one list element for each supported option. The key of each
list element is a symbol that names the option, while the value is a
list of option properties:

@lisp
OPTION-SPEC ::=  '( (OPT-NAME1 (PROP-NAME PROP-VALUE) @dots{})
                    (OPT-NAME2 (PROP-NAME PROP-VALUE) @dots{})
                    (OPT-NAME3 (PROP-NAME PROP-VALUE) @dots{})
                    @dots{}
                  )
@end lisp

Each @var{opt-name} specifies the long option name for that option.  For
example, a list element with @var{opt-name} @code{background} specifies
an option that can be specified on the command line using the long
option @code{--background}.  Further information about the option ---
whether it takes a value, whether it is required to be present in the
command line, and so on --- is specified by the option properties.

In the example of the preceding subsection, we already saw that a long
option name can have a equivalent @dfn{short option} character.  The
equivalent short option character can be set for an option by specifying
a @code{single-char} property in that option's property list.  For
example, a list element like @code{'(output (single-char #\o) @dots{})}
specifies an option with long name @code{--output} that can also be
specified by the equivalent short name @code{-o}.

The @code{value} property specifies whether an option requires or
accepts a value.  If the @code{value} property is set to @code{#t}, the
option requires a value: @code{getopt-long} will signal an error if the
option name is present without a corresponding value.  If set to
@code{#f}, the option does not take a value; in this case, a non-option
word that follows the option name in the command line will be treated as
a non-option argument.  If set to the symbol @code{optional}, the option
accepts a value but does not require one: a non-option word that follows
the option name in the command line will be interpreted as that option's
value.  If the option name for an option with @code{'(value optional)}
is immediately followed in the command line by @emph{another} option
name, the value for the first option is implicitly @code{#t}.

The @code{required?} property indicates whether an option is required to
be present in the command line.  If the @code{required?}  property is
set to @code{#t}, @code{getopt-long} will signal an error if the option
is not specified.

Finally, the @code{predicate} property can be used to constrain the
possible values of an option.  If used, the @code{predicate} property
should be set to a procedure that takes one argument --- the proposed
option value as a string --- and returns either @code{#t} or @code{#f}
according as the proposed value is or is not acceptable.  If the
predicate procedure returns @code{#f}, @code{getopt-long} will signal an
error.

By default, options do not have single-character equivalents, are not
required, and do not take values.  Where the list element for an option
includes a @code{value} property but no @code{predicate} property, the
option values are unconstrained.


@node Command Line Format
@subsection Expected Command Line Format

In order for @code{getopt-long} to correctly parse a command line, that
command line must conform to a standard set of rules for how command
line options are specified.  This subsection explains what those rules
are.

@code{getopt-long} splits a given command line into several pieces.  All
elements of the argument list are classified to be either options or
normal arguments.  Options consist of two dashes and an option name
(so-called @dfn{long} options), or of one dash followed by a single
letter (@dfn{short} options).

Options can behave as switches, when they are given without a value, or
they can be used to pass a value to the program.  The value for an
option may be specified using an equals sign, or else is simply the next
word in the command line, so the following two invocations are
equivalent:

@example
$ ./foo.scm --output=bar.txt
$ ./foo.scm --output bar.txt
@end example

Short options can be used instead of their long equivalents and can be
grouped together after a single dash.  For example, the following
commands are equivalent.

@example
$ ./foo.scm --version --help
$ ./foo.scm -v --help
$ ./foo.scm -vh
@end example

If an option requires a value, it can only be grouped together with other
short options if it is the last option in the group; the value is the
next argument.  So, for example, with the following option
specification ---

@lisp
((apples    (single-char #\a))
 (blimps    (single-char #\b) (value #t))
 (catalexis (single-char #\c) (value #t)))
@end lisp

@noindent
--- the following command lines would all be acceptable:

@example
$ ./foo.scm -a -b bang -c couth
$ ./foo.scm -ab bang -c couth
$ ./foo.scm -ac couth -b bang
@end example

But the next command line is an error, because @code{-b} is not the last
option in its combination, and because a group of short options cannot
include two options that both require values:

@example
$ ./foo.scm -abc couth bang
@end example

If an option's value is optional, @code{getopt-long} decides whether the
option has a value by looking at what follows it in the argument list.
If the next element is a string, and it does not appear to be an option
itself, then that string is the option's value.

If the option @code{--} appears in the argument list, argument parsing
stops there and subsequent arguments are returned as ordinary arguments,
even if they resemble options.  So, with the command line

@example
$ ./foo.scm --apples "Granny Smith" -- --blimp Goodyear
@end example

@noindent
@code{getopt-long} will recognize the @code{--apples} option as having
the value "Granny Smith", but will not treat @code{--blimp} as an
option.  The strings @code{--blimp} and @code{Goodyear} will be returned
as ordinary argument strings.


@node getopt-long Reference
@subsection Reference Documentation for @code{getopt-long}

@deffn {Scheme Procedure} getopt-long args grammar
Parse the command line given in @var{args} (which must be a list of
strings) according to the option specification @var{grammar}.

The @var{grammar} argument is expected to be a list of this form:

@code{((@var{option} (@var{property} @var{value}) @dots{}) @dots{})}

where each @var{option} is a symbol denoting the long option, but
without the two leading dashes (e.g. @code{version} if the option is
called @code{--version}).

For each option, there may be list of arbitrarily many property/value
pairs.  The order of the pairs is not important, but every property may
only appear once in the property list.  The following table lists the
possible properties:

@table @asis
@item @code{(single-char @var{char})}
Accept @code{-@var{char}} as a single-character equivalent to
@code{--@var{option}}.  This is how to specify traditional Unix-style
flags.
@item @code{(required? @var{bool})} 
If @var{bool} is true, the option is required.  @code{getopt-long} will
raise an error if it is not found in @var{args}.
@item @code{(value @var{bool})}
If @var{bool} is @code{#t}, the option accepts a value; if it is
@code{#f}, it does not; and if it is the symbol @code{optional}, the
option may appear in @var{args} with or without a value.
@item @code{(predicate @var{func})}
If the option accepts a value (i.e. you specified @code{(value #t)} for
this option), then @code{getopt-long} will apply @var{func} to the
value, and throw an exception if it returns @code{#f}.  @var{func}
should be a procedure which accepts a string and returns a boolean
value; you may need to use quasiquotes to get it into @var{grammar}.
@end table
@end deffn

@code{getopt-long}'s @var{args} parameter is expected to be a list of
strings like the one returned by @code{command-line}, with the first
element being the name of the command.  Therefore @code{getopt-long}
ignores the first element in @var{args} and starts argument
interpretation with the second element.

@code{getopt-long} signals an error if any of the following conditions
hold.

@itemize @bullet
@item
The option grammar has an invalid syntax.

@item
One of the options in the argument list was not specified by the
grammar.

@item
A required option is omitted.

@item
An option which requires an argument did not get one.

@item
An option that doesn't accept an argument does get one (this can only
happen using the long option @code{--opt=@var{value}} syntax).

@item
An option predicate fails.
@end itemize


@node option-ref Reference
@subsection Reference Documentation for @code{option-ref}

@deffn {Scheme Procedure} option-ref options key default
Search @var{options} for a command line option named @var{key} and
return its value, if found.  If the option has no value, but was given,
return @code{#t}.  If the option was not given, return @var{default}.
@var{options} must be the result of a call to @code{getopt-long}.
@end deffn

@code{option-ref} always succeeds, either by returning the requested
option value from the command line, or the default value.

The special key @code{'()} can be used to get a list of all
non-option arguments.


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