@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
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@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@page
@node Command Line Handling
@section 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.

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.

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, @xref{getopt-long}.

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