This is gash.info, produced by makeinfo version 7.2 from gash.texi.

Copyright © 2018 Rutger EW van Beusekom
Copyright © 2018 Jan (janneke) Nieuwenhuizen
Copyright © 2019 Timothy Sample

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A
copy of the license is included in the section entitled "GNU Free
Documentation License".
INFO-DIR-SECTION Basics
START-INFO-DIR-ENTRY
* Gash: (gash).       Guile As SHell.
* gash: (gash)Invoking gash.       Running Gash, a shell written in Scheme.
END-INFO-DIR-ENTRY


File: gash.info,  Node: Top,  Next: Introduction,  Up: (dir)

Gash
****

This document describes Gash version 0.3.0, a POSIX-compatible shell
written in Guile Scheme.

* Menu:

* Introduction::                What is Gash about?
* Using Gash::                  How to use Gash as a shell.
* Hacking with Gash::           How to use Gash as a Scheme library.
* GNU Free Documentation License::  The license of this manual.


File: gash.info,  Node: Introduction,  Next: Using Gash,  Prev: Top,  Up: Top

1 Introduction
**************

Gash is a POSIX-compatible shell written in Guile Scheme.  It has two
goals: bootstrapping GNU Bash and exposing the shell to Scheme.  In the
following sections we will unpack that definition and explain all of its
constituent parts.

* Menu:

* What is a POSIX-compatible shell?::
* What is Guile Scheme?::
* What of Bash and its bootstraps?::
* What does Scheme want from the shell?::


File: gash.info,  Node: What is a POSIX-compatible shell?,  Next: What is Guile Scheme?,  Up: Introduction

1.1 What is a POSIX-compatible shell?
=====================================

When talking about operating systems, a “shell” refers to the day-to-day
user interface that a person would use to interact with the system.
That's what you might learn in computer science school, but in popular
usage "shell" refers to a very particular type of interface--the one
that developed along with the Unix operating system.  The Unix shell is
a command-line interface designed mostly for invoking programs and
connecting programs together.  Although we provide a whirlwind tour of
shell features in *note Using Gash::, this manual is not a good general
introduction to using a Unix-like shell.  For that, try Introduction to
the Command Line
(http://write.flossmanuals.net/command-line/introduction/).

   So now we know what a _Unix_ shell is, but what is a
_POSIX-compatible_ shell?  Over time, Unix inspired many similar
systems, all of which were mostly the same, but not quite compatible
with one another.  What worked on one Unix-like operating system might
not work on another.  To make it easier to write portable software,
everyone got together and described a minimal set of features that all
Unix-like systems should provide.  They called it the “Portable
Operating System Interface”, or “POSIX” for short.  Being a
_POSIX-compatible_ shell means that Gash should have at least all of the
features required by POSIX.(1)

   ---------- Footnotes ----------

   (1) Note that Gash is merely POSIX-compatible and _not_
POSIX-certified.  This means we try our best, but we have not gotten an
official stamp of approval from The Open Group, who owns the POSIX
trademark and administers a certification program.


File: gash.info,  Node: What is Guile Scheme?,  Next: What of Bash and its bootstraps?,  Prev: What is a POSIX-compatible shell?,  Up: Introduction

1.2 What is Guile Scheme?
=========================

“Scheme” is a cool programming language from the 70's.  It is a very
conceptually simple dialect of “Lisp”--a cool programming language from
the 50's.  Why are we interested in using such an old programming
language?  There are two reasons.  The first is that even by modern
standards, Scheme is a very expressive, high-level language.  Writing in
such a language is a pleasure.  The second reason is that Scheme's
simplicity makes it well-suited to the bootstrapping task (*note What of
Bash and its bootstraps?::).

   “Guile” is a particular implementation of Scheme.  It is the official
extension language of the GNU project.  There are many implementations
of Scheme, but Guile is our choice mainly out of a desire to integrate
nicely with the GNU Guix project, which is also written in Guile.


File: gash.info,  Node: What of Bash and its bootstraps?,  Next: What does Scheme want from the shell?,  Prev: What is Guile Scheme?,  Up: Introduction

1.3 What of Bash and its bootstraps?
====================================

One of the most important functions of Gash is to bootstrap GNU Bash
(*note (bash)What is Bash?::).  “Bootstrapping” is, briefly, going from
having no software to having software.  What makes it hard is that you
usually use software to build software, and that snake has a habit of
eating its own tail.  For example, Bash uses shell scripts to describe
how it should be built, so you need a shell to build a shell.  What if
this other shell needs shell scripts too?  Then you have a problem.
Normally this problem goes unnoticed, because you already have a shell
that you got from somebody else, and that somebody got their shell from
somebody, and this process works its way back to the dawn of time--er,
the dawn of Unix time, which is the early 70's--when the first shell
emerged from the primordial soup.

   Where you really run head-long into this problem is when you have a
complete, fine-grained description of a system's software dependency
graph, like they have over at the GNU Guix project.  Then, if you follow
the arrows back as far as you can, you can't miss the fact that the
first step in building the system is "download many megabytes of
inscrutable binary programs from the Internet."  There are a handful of
people working to change this, and Gash is a small part of this larger
project.  For all the details, see the Bootstrappable Builds website
(https://bootstrappable.org/).


File: gash.info,  Node: What does Scheme want from the shell?,  Prev: What of Bash and its bootstraps?,  Up: Introduction

1.4 What does Scheme want from the shell?
=========================================

There is a long history of mixing Scheme and the shell.  Most famously,
there is Scsh (https://scsh.net/), which is a hybrid language that tries
to combine the brevity of the shell with the flexibility of Scheme.
There are a few other projects that mix Scheme or Lisp with shell
syntax, and many projects mixing the shell with other popular
programming languages.  A common goal of many of these projects is to
provide a way forward for shell scripts that have become so large they
are difficult to maintain.  Having a language that has a similar syntax
to shell scripts but provides more ways to structure code would allow
these scripts to continue to grow cleanly.

   On the other hand, it can be useful to provide a shell-like interface
for Scheme environments.  For instance, the Guix System boots into Guile
and if anything goes wrong, it presents the user with a Scheme REPL. It
would be nice to be able to drive Guile using shell syntax, since most
of what you need to do in that context is manipulate files and execute
external utilities.  Another Guix example is adding build-system phases
that are just shell scripts.(1)

   Ultimately, the future of Gash is yet to be explored, but there are
plenty of spaces it could gracefully move into.  Patches are welcome!

   ---------- Footnotes ----------

   (1) This one is a little heretical, but I imagine it being attractive
as Guix becomes more popular.


File: gash.info,  Node: Using Gash,  Next: Hacking with Gash,  Prev: Introduction,  Up: Top

2 Using Gash
************

Gash is a mostly complete POSIX-compatible shell, so it should mostly
work as you would expect.  However, it is missing a few POSIX features
(*note Missing features::), and does not really have any features beyond
POSIX. Specifically, many of the "Bashisms" that you may be familiar
with are not available.(1)

   In the following we cover the major features that work very well in
Gash.  You should feel confident using these features as you would in
any other POSIX-compatible shell.

   All kinds of command invocation should just work.  This includes
setting temporary variables, redirects, and here-documents.  All of the
POSIX-specified "special" built-ins are available except for ‘times’.
Only a handful of other built-ins are provided (like ‘cd’ and ‘pwd’).

   All of the control structures are available and full-featured.  This
means ‘for’, ‘while’, ‘until’, ‘if’, and ‘case’.  Functions and
subshells also work fine.

   Field splitting and quoting is well-tested and should work even in
exceptional cases, like ‘"$@"’ and when ‘$IFS’ is manipulated.

   You can set variables and mark them read-only or exported.  Many
special variables are available, and all of the variable operators (like
‘${VARIABLE+alternate}’) work.

   Both types of command substitution work (that is, ‘$(...)’ and
‘`...`’), and can even be nested.

   In general, Gash should not surprise you too much, and should run
most POSIX-compatible shell scripts.

* Menu:

* Invoking Gash::
* Using Gash from the Guile REPL::
* Missing features::

   ---------- Footnotes ----------

   (1) *Note (bash)Major Differences From The Bourne Shell::.


File: gash.info,  Node: Invoking Gash,  Next: Using Gash from the Guile REPL,  Up: Using Gash

2.1 Invoking Gash
=================

The ‘gash’ command is the sh interpreter.

     gash OPTION... FILE

   The OPTIONs can be among the following:

‘-c, --command=STRING’
     Evaluate STRING, and then exit.

‘-e, --errexit’
     Exit immediately on any error.

‘-h, --help’
     Display help on invoking Gash, and then exit.

‘-p, --parse’
     Rather than evaluate a script, parse it and output its syntax tree.

‘-v, --version’
     Display the current version of Gash, and then exit.

‘-x, --xtrace’
     Print each command that is executed.


File: gash.info,  Node: Using Gash from the Guile REPL,  Next: Missing features,  Prev: Invoking Gash,  Up: Using Gash

2.2 Using Gash from the Guile REPL
==================================

Gash defines a language specification that extends Guile, allowing you
to use shell syntax from the REPL. This is accomplished by using the
‘language’ REPL command:

     scheme@(guile-user)> ,language sh
     Happy hacking with Guile as Shell!  To switch back, type `,L scheme'.
     sh@(guile-user)> echo "Hello Gash!"
     Hello Gash!
     $1 = 0


File: gash.info,  Node: Missing features,  Prev: Using Gash from the Guile REPL,  Up: Using Gash

2.3 Missing features
====================

In this section, we cover some of the features that are currently
missing in Gash.  Particularly, we look at the features specified by
POSIX that have not yet been implemented.  This list is not exhaustive,
but covers the most glaring omissions.

   • Job control.

   • Alias creation and substitution.

   • Certain special variables.  This includes ‘$ENV’; all the
     localization variables like ‘$LANG’, ‘$LC_*’, and ‘$NLSPATH’; the
     parent process ID variable (‘$PPID’); and the prompt variables
     (‘$PS*’).

   • Tilde expansion.

   • Multi-line commands from the readline interface.  If you press
     <Enter> in the middle of a command (e.g., from within an unfinished
     quotation) Gash will not ask for more input but rather treat it as
     a syntax error and exit with an error message.

   These features will be added to a future version of Gash.  If you
would like to contribute to Gash by implementing one of these features,
please get in touch by writing to gash-devel@nongnu.org
(mailto:gash-devel@nongnu.org).


File: gash.info,  Node: Hacking with Gash,  Next: GNU Free Documentation License,  Prev: Using Gash,  Up: Top

3 Hacking with Gash
*******************

Have you ever wanted to write Scheme code that can understand and
manipulate shell scripts on the fly?  Have you ever wanted your Scheme
scripts to have the breezy convenience of a shell script?  You've come
to the right place!  In this chapter we will look at all the tools that
Gash offers for exploring and illuminating the dark canyon that
separates Scheme and the shell.

* Menu:

* Parsing shell scripts::
* Using shell-like constructs from Scheme::


File: gash.info,  Node: Parsing shell scripts,  Next: Using shell-like constructs from Scheme,  Up: Hacking with Gash

3.1 Parsing shell scripts
=========================

If you're reading this, you probably have some familiarity with Scheme,
and you have probably heard of Scheme's ‘read’ procedure.  Gash exports
a very similar function from the ‘(gash parser)’ module.  It's called
‘read-sh’, and as you may have guessed, it reads shell code instead of
Scheme code.

   With Scheme code, the internal and external representations of the
code are basically the same, but this is not true with shell code.  You
will have to learn and understand Gash's particular internal
representation of shell code to use it effectively.  Fortunately, it is
designed to be familiar to a Scheme programmer, and we provide many
examples here to get you up to speed quickly.  At the end of this
section, a formal definition of the internal representation is given for
reference.

 -- Function: read-sh [port]
     Read a complete shell command from the input port PORT if it is
     specified, or from the current input port if PORT is not specified.
     A “complete command” is essentially something an interactive shell
     would be able to execute immediately without prompting for more
     input.  Technically, it corresponds to the ‘complete_command’
     production rule defined in the POSIX specification.

   For convenience, we also export the following procedure.

 -- Function: read-sh-all [port]
     Read all complete shell commands from the input port PORT if it is
     specified, or from the current input port if PORT is not specified.
     This works much like writing your own loop using ‘read-sh’, but it
     should be more efficient.  As a bonus, it ensures that the result
     is always a list of commands.

* Menu:

* Internal representation examples::
* Internal representation definition::


File: gash.info,  Node: Internal representation examples,  Next: Internal representation definition,  Up: Parsing shell scripts

3.1.1 Internal representation examples
--------------------------------------

The easiest way to understand Gash's internal representation of shell is
through examples, so let's take a look at a few.  To simplify the
examples, we are going abuse notation a little bit.  Instead of writing

     (call-with-input-string "SHELL-EXAMPLE" read-sh)
     ⇒
     RESULT

   we will simply write

     SHELL-EXAMPLE
     ⇒
     RESULT

   The major benefit of doing this is that we do not have to escape the
shell examples to be proper Scheme strings.

Simple commands
...............

The parser can read simple commands and split them into their
constituent parts:

     echo foo
     ⇒
     (<sh-exec> "echo" "foo")

     echo $FOO
     ⇒
     (<sh-exec> "echo" (<sh-ref> "FOO"))

     echo foo$BAR baz
     ⇒
     (<sh-exec> "echo" ("foo" (<sh-ref> "BAR")) "baz")

   It preserves quotes, since they are significant for field splitting
and escaping special characters:

     echo "$FOO"
     ⇒
     (<sh-exec> "echo" (<sh-quote> (<sh-ref> "FOO")))

     echo *
     ⇒
     (<sh-exec> "echo" "*")

     echo '*'
     ⇒
     (<sh-exec> "echo" (<sh-quote> "*"))

   It represents redirects and temporary variable assignments with a nod
to Scheme:

     CC=gcc CFLAGS='-O2 -g' make
     ⇒
     (<sh-exec-let> (("CC" "gcc")
                     ("CFLAGS" (<sh-quote> "-O2 -g")))
       "make")

     wc < gash.texi
     ⇒
     (<sh-with-redirects> ((< 0 "gash.texi"))
       (<sh-exec> "wc"))

     POSIXLY_CORRECT=y sed N < file
     ⇒
     (<sh-with-redirects> ((< 0 "file"))
       (<sh-exec-let> (("POSIXLY_CORRECT" "y"))
         "sed" "N"))

     ;; Watch out for this one, though!
     echo foo >| file
     ⇒
     (<sh-with-redirects> ((>! 1 "file"))
       (<sh-exec> "echo" "foo"))

   Here documents get processed by the parser, so they only have one
form:

     cat <<EOF > hello.scm
     (display "Hello world!\n")
     EOF
     ⇒
     (<sh-with-redirects> ((<< 0 (<sh-quote>
                                  "(display \"Hello world!\\n\")\n"))
                           (> 1 "hello.scm"))
       (<sh-exec> "cat"))

     cat <<EOF > hello.sh
     hi=Howdy; echo $hi world!
     EOF
     ⇒
     (<sh-with-redirects> ((<< 0 (<sh-quote>
                                  ("hi=Howdy; echo " (<sh-ref> "hi")
                                   " world!\n")))
                           (> 1 "hello.sh"))
       (<sh-exec> "cat"))

     ;; That's not what we meant!  Let's try again.
     cat <<\EOF > hello.sh
     hi=Howdy; echo $hi world!
     EOF
     ⇒
     (<sh-with-redirects> ((<< 0 (<sh-quote> "hi=Howdy; echo $hi world!\n"))
                           (> 1 "hello.sh"))
       (<sh-exec> "cat"))

Compound commands
.................

Loops look like they do on the shell:

     while test $GASH = confusing
     do
         read-the-manual
     done
     ⇒
     (<sh-while> (<sh-exec> "test" (<sh-ref> "GASH") "=" "confusing")
       (<sh-exec> "read-the-manual"))

     until cows come home; do wait; done
     ⇒
     (<sh-until> (<sh-exec> "cows" "come" "home")
       (<sh-exec> "wait"))

     for cat in Murray Flash Boots
     do
         pet $cat
     done
     ⇒
     (<sh-for> ("cat" ("Murray" "Flash" "Boots"))
       (<sh-exec> "pet" (<sh-ref> "cat")))

   If-statements and case-statements are a bit more like Scheme:

     if test $FLAVOR = chocolate
     then
         echo hooray!
     elif test $FLAVOR = vanilla
     then
         echo yum!
     else
         echo too fancy!
     fi
     ⇒
     (<sh-cond>
      ((<sh-exec> "test" (<sh-ref> "FLAVOR") "=" "chocolate")
       (<sh-exec> "echo" "hooray!"))
      ((<sh-exec> "test" (<sh-ref> "FLAVOR") "=" "vanilla")
       (<sh-exec> "echo" "yum!"))
      (<sh-else>
       (<sh-exec> "echo" "too" "fancy!")))

     case $FLAVOR
     in
         *mint*) echo blech! ;;
         *peanut*) echo so good! ;;
         *maple*) echo maybe! ;;
         *) echo sure why not! ;;
     esac
     ⇒
     (<sh-case> (<sh-ref> "FLAVOR")
       (("*mint*") (<sh-exec> "echo" "blech!"))
       (("*peanut*") (<sh-exec> "echo" "so" "good!"))
       (("*maple*") (<sh-exec> "echo" "maybe!"))
       (("*") (<sh-exec> "echo" "sure" "why" "not!")))

   Brace groups and subshells are pretty straight-forward:

     { echo jump to the left;
       echo step to the right; }
     ⇒
     (<sh-begin>
      (<sh-exec> "echo" "jump" "to" "the" "left")
      (<sh-exec> "echo" "step" "to" "the" "right"))

     (cd tmp; cat ephemera.txt)
     ⇒
     (<sh-subshell>
      (<sh-exec> "cd" "tmp")
      (<sh-exec> "cat" "ephemera.txt"))

Function definitions
....................

Function definitions look more like Lisp:

     dance() {
       echo swing your hips now!
     }
     ⇒
     (<sh-defun> "dance"
       (<sh-exec> "echo" "swing" "your" "hips" "now!"))

     ;; Even this weird thing works!
     dance_to_the_beat() {
       sed 's/beat/swing hips/g'
     } < beats.txt
     ⇒
     (<sh-defun> "dance_to_the_beat"
       (<sh-with-redirects> ((< 0 "beats.txt"))
         (<sh-exec> "sed" (<sh-quote> "s/beat/move hips/g"))))

Pipelines and asynchronous commands
...................................

Pipelines collect commands into a list:

     source | sink
     ⇒
     (<sh-pipeline> (<sh-exec> "source") (<sh-exec> "sink"))

     cut -d ' ' -f 4 < ice-cream.txt \
       | grep chocolate \
       | sed 's/mint/peanut butter/g'
     ⇒
     (<sh-pipeline>
      (<sh-with-redirects> ((< 0 "ice-cream.txt"))
        (<sh-exec> "cut" "-d" (<sh-quote> " ") "-f" "4"))
      (<sh-exec> "grep" "chocolate")
      (<sh-exec> "sed" (<sh-quote> "s/mint/peanut butter/g")))

   Asynchronous commands are simply regular commands wrapped with
‘<sh-async>’:

     helpful-daemon &
     ⇒
     (<sh-async> (<sh-exec> "helpful-daemon"))

     ;; So-so dancing.
     swing-hips ; flail-arms
     ⇒
     (<sh-begin>
      (<sh-exec> "swing-hips")
      (<sh-exec> "flail-arms"))

     ;; Great dancing.
     swing-hips & flail-arms
     ⇒
     (<sh-begin>
      (<sh-async> (<sh-exec> "swing-hips"))
      (<sh-exec> "flail-arms"))

Boolean expressions
...................

The logical "and" and "or" operators are binary, left-associative and
have equal precedence:

     foo && bar || baz
     ⇒
     (<sh-or> (<sh-and> (<sh-exec> "foo")
                        (<sh-exec> "bar"))
              (<sh-exec> "baz"))

     foo || bar && baz
     ⇒
     (<sh-and> (<sh-or> (<sh-exec> "foo")
                        (<sh-exec> "bar"))
               (<sh-exec> "baz"))

   The logical "not" operator wraps a command or pipeline:

     ! false
     ⇒
     (<sh-not> (<sh-exec> "false"))

     ! cat flavors.txt | grep mint
     ⇒
     (<sh-not> (<sh-pipeline>
                (<sh-exec> "cat" "flavors.txt")
                (<sh-exec> "grep" "mint")))

Variables and command substitution
..................................

Variable operators have names inspired by Scheme:

     echo ${FOO+bar}
     ⇒
     (<sh-exec> "echo" (<sh-ref-and> "FOO" "bar"))

     echo ${FOO:+bar}
     ⇒
     (<sh-exec> "echo" (<sh-ref-and*> "FOO" "bar"))

     echo ${FOO-bar}
     ⇒
     (<sh-exec> "echo" (<sh-ref-or> "FOO" "bar"))

     echo ${FOO=bar}
     ⇒
     (<sh-exec> "echo" (<sh-ref-or!> "FOO" "bar"))

   Command substitution is a simple wrapper:

     echo $(cat message.txt)
     ⇒
     (<sh-exec> "echo" (<sh-cmd-sub> (<sh-exec> "cat" "message.txt")))

   That's it for examples.  By this point, you should have a pretty good
sense for what Gash is going to give you.  If you need to know more, you
can always do your own experiments or read the formal definition in the
next section.


File: gash.info,  Node: Internal representation definition,  Prev: Internal representation examples,  Up: Parsing shell scripts

3.1.2 Internal representation definition
----------------------------------------

The internal representation of shell code in Gash has the following
form.  (If Gash ever returns something that does not have this form,
please file a bug!)

     sync  ::= exp
             | ('<sh-async> exp)

     exp   ::= pipe
             | ('<sh-and> exp1 exp2)
             | ('<sh-or> exp1 exp2)
             | ('<sh-not> pipe)

     pipe  ::= cmd*
             | ('<sh-pipeline> cmd* ...)

     cmd*  ::= cmd
             | ('<sh-defun> name sync ...)

     cmd   ::= ('<sh-exec> word ...)
             | ('<sh-exec-let> ((var var-word) ...) word ...)
             | ('<sh-with-redirects> (redir ...) [cmd])
             | ('<sh-set!> (var word) ...)
             | ('<sh-subshell> sync ...)
             | ('<sh-for> (name (word ...)) sync ...)
             | ('<sh-case> word ((pattern-word ...) sync ...) ...)
             | ('<sh-cond> (list sync ...) ... [('<sh-else> sync ...)])
             | ('<sh-while> list sync ...)
             | ('<sh-until> list sync ...)
             | ('<sh-begin> sync ...)

     redir ::= ('> fdes word)
             | ('< fdes word)
             | ('>& fdes word)
             | ('<& fdes word)
             | ('>> fdes word)
             | ('<> fdes word)
             | ('>! fdes word)
             | ('<< fdes word)

     word  ::= string
             | (word ...)
             | ('<sh-quote> word)
             | ('<sh-cmd-sub> sync ...)
             | ('<sh-arithmetic> word)
             | ('<sh-ref> var)
             | ('<sh-ref-or> var [word])
             | ('<sh-ref-or*> var [word])
             | ('<sh-ref-or!> var [word])
             | ('<sh-ref-or!*> var [word])
             | ('<sh-ref-assert> var [word])
             | ('<sh-ref-assert*> var [word])
             | ('<sh-ref-and> var [word])
             | ('<sh-ref-and*> var [word])
             | ('<sh-ref-except-min> var [word])
             | ('<sh-ref-except-max> var [word])
             | ('<sh-ref-skip-min> var [word])
             | ('<sh-ref-skip-max> var [word])
             | ('<sh-ref-length> var)

     var   ::= string
             | ("LINENO" integer)


File: gash.info,  Node: Using shell-like constructs from Scheme,  Prev: Parsing shell scripts,  Up: Hacking with Gash

3.2 Using shell-like constructs from Scheme
===========================================

Gash also has some features for writing Scheme code with shell-like
semantics.  Indeed, this is how Gash works internally.  It interprets
shell code into calls into its shell interface.  The two modules that
provide this interface are ‘(gash shell)’ and ‘(gash environment)’.  The
shell module provides control structures with shell semantics.  For
instance, you can use ‘sh:for’ to update a given shell variable and run
a thunk for each element of a list of strings.  The environment module
allows you to manipulate shell variables and functions, as well as a
number of other things.

   Truthfully, these interfaces are not ready for public consumption
yet.  They are in need of some careful redesign to be useful outside of
Gash itself.  If this sounds interesting to you, please get in touch by
writing to gash-devel@nongnu.org (mailto:gash-devel@nongnu.org).


File: gash.info,  Node: GNU Free Documentation License,  Prev: Hacking with Gash,  Up: Top

Appendix A GNU Free Documentation License
*****************************************

                     Version 1.3, 3 November 2008

     Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document “free” in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of "copyleft", which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.  We
     recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it can
     be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     "Document", below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as "you".  You accept
     the license if you copy, modify or distribute the work in a way
     requiring permission under copyright law.

     A "Modified Version" of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A "Secondary Section" is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document's overall
     subject (or to related matters) and contains nothing that could
     fall directly within that overall subject.  (Thus, if the Document
     is in part a textbook of mathematics, a Secondary Section may not
     explain any mathematics.)  The relationship could be a matter of
     historical connection with the subject or with related matters, or
     of legal, commercial, philosophical, ethical or political position
     regarding them.

     The "Invariant Sections" are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in the
     notice that says that the Document is released under this License.
     If a section does not fit the above definition of Secondary then it
     is not allowed to be designated as Invariant.  The Document may
     contain zero Invariant Sections.  If the Document does not identify
     any Invariant Sections then there are none.

     The "Cover Texts" are certain short passages of text that are
     listed, as Front-Cover Texts or Back-Cover Texts, in the notice
     that says that the Document is released under this License.  A
     Front-Cover Text may be at most 5 words, and a Back-Cover Text may
     be at most 25 words.

     A "Transparent" copy of the Document means a machine-readable copy,
     represented in a format whose specification is available to the
     general public, that is suitable for revising the document
     straightforwardly with generic text editors or (for images composed
     of pixels) generic paint programs or (for drawings) some widely
     available drawing editor, and that is suitable for input to text
     formatters or for automatic translation to a variety of formats
     suitable for input to text formatters.  A copy made in an otherwise
     Transparent file format whose markup, or absence of markup, has
     been arranged to thwart or discourage subsequent modification by
     readers is not Transparent.  An image format is not Transparent if
     used for any substantial amount of text.  A copy that is not
     "Transparent" is called "Opaque".

     Examples of suitable formats for Transparent copies include plain
     ASCII without markup, Texinfo input format, LaTeX input format,
     SGML or XML using a publicly available DTD, and standard-conforming
     simple HTML, PostScript or PDF designed for human modification.
     Examples of transparent image formats include PNG, XCF and JPG.
     Opaque formats include proprietary formats that can be read and
     edited only by proprietary word processors, SGML or XML for which
     the DTD and/or processing tools are not generally available, and
     the machine-generated HTML, PostScript or PDF produced by some word
     processors for output purposes only.

     The "Title Page" means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the
     material this License requires to appear in the title page.  For
     works in formats which do not have any title page as such, "Title
     Page" means the text near the most prominent appearance of the
     work's title, preceding the beginning of the body of the text.

     The "publisher" means any person or entity that distributes copies
     of the Document to the public.

     A section "Entitled XYZ" means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
     stands for a specific section name mentioned below, such as
     "Acknowledgements", "Dedications", "Endorsements", or "History".)
     To "Preserve the Title" of such a section when you modify the
     Document means that it remains a section "Entitled XYZ" according
     to this definition.

     The Document may include Warranty Disclaimers next to the notice
     which states that this License applies to the Document.  These
     Warranty Disclaimers are considered to be included by reference in
     this License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
     applies to the Document are reproduced in all copies, and that you
     add no other conditions whatsoever to those of this License.  You
     may not use technical measures to obstruct or control the reading
     or further copying of the copies you make or distribute.  However,
     you may accept compensation in exchange for copies.  If you
     distribute a large enough number of copies you must also follow the
     conditions in section 3.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document's license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
     these Cover Texts: Front-Cover Texts on the front cover, and
     Back-Cover Texts on the back cover.  Both covers must also clearly
     and legibly identify you as the publisher of these copies.  The
     front cover must present the full title with all words of the title
     equally prominent and visible.  You may add other material on the
     covers in addition.  Copying with changes limited to the covers, as
     long as they preserve the title of the Document and satisfy these
     conditions, can be treated as verbatim copying in other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto
     adjacent pages.

     If you publish or distribute Opaque copies of the Document
     numbering more than 100, you must either include a machine-readable
     Transparent copy along with each Opaque copy, or state in or with
     each Opaque copy a computer-network location from which the general
     network-using public has access to download using public-standard
     network protocols a complete Transparent copy of the Document, free
     of added material.  If you use the latter option, you must take
     reasonably prudent steps, when you begin distribution of Opaque
     copies in quantity, to ensure that this Transparent copy will
     remain thus accessible at the stated location until at least one
     year after the last time you distribute an Opaque copy (directly or
     through your agents or retailers) of that edition to the public.

     It is requested, but not required, that you contact the authors of
     the Document well before redistributing any large number of copies,
     to give them a chance to provide you with an updated version of the
     Document.

  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with the
     Modified Version filling the role of the Document, thus licensing
     distribution and modification of the Modified Version to whoever
     possesses a copy of it.  In addition, you must do these things in
     the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of previous
          versions (which should, if there were any, be listed in the
          History section of the Document).  You may use the same title
          as a previous version if the original publisher of that
          version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document's
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on the
          Title Page.  If there is no section Entitled "History" in the
          Document, create one stating the title, year, authors, and
          publisher of the Document as given on its Title Page, then add
          an item describing the Modified Version as stated in the
          previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in the
          "History" section.  You may omit a network location for a work
          that was published at least four years before the Document
          itself, or if the original publisher of the version it refers
          to gives permission.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          Preserve the Title of the section, and preserve in the section
          all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document, unaltered
          in their text and in their titles.  Section numbers or the
          equivalent are not considered part of the section titles.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option designate
     some or all of these sections as invariant.  To do this, add their
     titles to the list of Invariant Sections in the Modified Version's
     license notice.  These titles must be distinct from any other
     section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end of
     the list of Cover Texts in the Modified Version.  Only one passage
     of Front-Cover Text and one of Back-Cover Text may be added by (or
     through arrangements made by) any one entity.  If the Document
     already includes a cover text for the same cover, previously added
     by you or by arrangement made by the same entity you are acting on
     behalf of, you may not add another; but you may replace the old
     one, on explicit permission from the previous publisher that added
     the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination all
     of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the documents
     in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow this
     License in all other respects regarding verbatim copying of that
     document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of a
     storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense, or distribute it is void,
     and will automatically terminate your rights under this License.

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly and
     finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from you
     under this License.  If your rights have been terminated and not
     permanently reinstated, receipt of a copy of some or all of the
     same material does not give you any rights to use it.

  10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     <http://www.gnu.org/copyleft/>.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If the
     Document does not specify a version number of this License, you may
     choose any version ever published (not as a draft) by the Free
     Software Foundation.  If the Document specifies that a proxy can
     decide which future versions of this License can be used, that
     proxy's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Document.

  11. RELICENSING

     "Massive Multiauthor Collaboration Site" (or "MMC Site") means any
     World Wide Web server that publishes copyrightable works and also
     provides prominent facilities for anybody to edit those works.  A
     public wiki that anybody can edit is an example of such a server.
     A "Massive Multiauthor Collaboration" (or "MMC") contained in the
     site means any set of copyrightable works thus published on the MMC
     site.

     "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
     license published by Creative Commons Corporation, a not-for-profit
     corporation with a principal place of business in San Francisco,
     California, as well as future copyleft versions of that license
     published by that same organization.

     "Incorporate" means to publish or republish a Document, in whole or
     in part, as part of another Document.

     An MMC is "eligible for relicensing" if it is licensed under this
     License, and if all works that were first published under this
     License somewhere other than this MMC, and subsequently
     incorporated in whole or in part into the MMC, (1) had no cover
     texts or invariant sections, and (2) were thus incorporated prior
     to November 1, 2008.

     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.


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