view doc/interpreter/basics.txi @ 15546:0272bb60408a

maint: Periodic merge of stable to default.
author Rik <rik@octave.org>
date Fri, 19 Oct 2012 08:57:10 -0700
parents d8e3111b1890 6a4e79110857
children
line wrap: on
line source

@c Copyright (C) 1996-2012 John W. Eaton
@c
@c This file is part of Octave.
@c
@c Octave is free software; you can redistribute it and/or modify it
@c under the terms of the GNU General Public License as published by the
@c Free Software Foundation; either version 3 of the License, or (at
@c your option) any later version.
@c 
@c Octave is distributed in the hope that it will be useful, but WITHOUT
@c ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
@c FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
@c for more details.
@c 
@c You should have received a copy of the GNU General Public License
@c along with Octave; see the file COPYING.  If not, see
@c <http://www.gnu.org/licenses/>.

@node Getting Started
@chapter Getting Started

This chapter explains some of Octave's basic features, including how to
start an Octave session, get help at the command prompt, edit the
command line, and write Octave programs that can be executed as commands
from your shell.

@menu
* Invoking Octave from the Command Line::             
* Quitting Octave::             
* Getting Help::                
* Command Line Editing::        
* Errors::                      
* Executable Octave Programs::  
* Comments::                    
@end menu

@node Invoking Octave from the Command Line
@section Invoking Octave from the Command Line

Normally, Octave is used interactively by running the program
@samp{octave} without any arguments.  Once started, Octave reads
commands from the terminal until you tell it to exit.

You can also specify the name of a file on the command line, and Octave
will read and execute the commands from the named file and then exit
when it is finished.

You can further control how Octave starts by using the command-line
options described in the next section, and Octave itself can remind you
of the options available.  Type @samp{octave --help} to display all
available options and briefly describe their use (@samp{octave -h} is a
shorter equivalent).

@menu
* Command Line Options::        
* Startup Files::               
@end menu

@node Command Line Options
@subsection Command Line Options
@cindex Octave command options
@cindex command options
@cindex options, Octave command

Here is a complete list of the command line options that Octave
accepts.


@table @code
@item  --debug
@itemx -d
@cindex @code{--debug}
@cindex @code{-d}
Enter parser debugging mode.  Using this option will cause Octave's
parser to print a lot of information about the commands it reads, and is
probably only useful if you are actually trying to debug the parser.

@item --doc-cache-file @var{filename}
@cindex @code{--doc-cache-file @var{filename}}
Specify the name of the doc cache file to use.  The value of @var{filename}
specified on the command line will override any value of
@w{@env{OCTAVE_DOC_CACHE_FILE}} found in the environment, but not any commands
in the system or user startup files that use the @code{doc_cache_file}
function.

@item  --echo-commands
@itemx -x
@cindex @code{--echo-commands}
@cindex @code{-x}
Echo commands as they are executed.

@item --eval @var{code}
Evaluate @var{code} and exit when finished unless @option{--persist} is also
specified.

@item --exec-path @var{path}
@cindex @code{--exec-path @var{path}}
Specify the path to search for programs to run.  The value of @var{path}
specified on the command line will override any value of
@w{@env{OCTAVE_EXEC_PATH}} found in the environment, but not any commands
in the system or user startup files that set the built-in variable
@w{@env{EXEC_PATH}}.

@item --force-gui
@cindex @code{--force-gui}
Force the graphical user interface (GUI) to start.

@item  --help
@itemx -h
@itemx -?
@cindex @code{--help}
@cindex @code{-h}
@cindex @code{-?}
Print short help message and exit.

@item --image-path @var{path}
@cindex @code{--image-path @var{path}}
Add path to the head of the search path for images.  The value of 
@var{path} specified on the command line will override any value of 
@w{@env{OCTAVE_IMAGE_PATH}} found in the environment, but not any commands 
in the system or user startup files that set the built-in variable 
@w{@env{IMAGE_PATH}}.

@item --info-file @var{filename}
@cindex @code{--info-file @var{filename}}
Specify the name of the info file to use.  The value of @var{filename}
specified on the command line will override any value of
@w{@env{OCTAVE_INFO_FILE}} found in the environment, but not any commands
in the system or user startup files that use the @code{info_file}
function.

@item --info-program @var{program}
@cindex @code{--info-program @var{program}}
Specify the name of the info program to use.  The value of @var{program}
specified on the command line will override any value of
@w{@env{OCTAVE_INFO_PROGRAM}} found in the environment, but not any
commands in the system or user startup files that use the
@code{info_program} function.

@item  --interactive
@itemx -i
@cindex @code{--interactive}
@cindex @code{-i}
Force interactive behavior.  This can be useful for running Octave via a
remote shell command or inside an Emacs shell buffer.  For another way
to run Octave within Emacs, see @ref{Emacs Octave Support}.

@item --jit-debug
@cindex @code{--jit-debug}
Enable JIT compiler debugging and tracing.

@item --line-editing
@cindex @code{--line-editing}
Force readline use for command-line editing.

@item --no-gui
@cindex @code{--no-gui}
Disable the graphical user interface (GUI) and use the command line
interface (CLI) instead.

@item  --no-history
@itemx -H
@cindex @code{--no-history}
@cindex @code{-H}
Disable recording of command-line history.

@item --no-init-file
@cindex @code{--no-init-file}
Don't read the initialization files @file{~/.octaverc} and @file{.octaverc}.

@item --no-init-path
@cindex @code{--no-init-path}
Don't initialize the search path for function files to include default 
locations.

@item --no-jit-compiler
@cindex @code{--no-jit-compiler}
Disable the JIT compiler used for accelerating loops.

@item --no-line-editing
@cindex @code{--no-line-editing}
Disable command-line editing.

@item --no-site-file
@cindex @code{--no-site-file}
Don't read the site-wide @file{octaverc} initialization files.

@item --no-window-system
@cindex @code{--no-window-system}
Disable use of a windowing system including graphics.  This forces a
strictly terminal-only environment.

@item  --norc
@itemx -f
@cindex @code{--norc}
@cindex @code{-f}
Don't read any of the system or user initialization files at startup.
This is equivalent to using both of the options @option{--no-init-file}
and @option{--no-site-file}.

@item  --path @var{path}
@itemx -p @var{path}
@cindex @code{--path @var{path}}
@cindex @code{-p @var{path}}
Add path to the head of the search path for function files.  The 
value of @var{path} specified on the command line will override any value
of @w{@env{OCTAVE_PATH}} found in the environment, but not any commands in the
system or user startup files that set the internal load path through one
of the path functions.

@item --persist
@cindex @code{--persist}
Go to interactive mode after @option{--eval} or reading from a file
named on the command line.

@item  --silent
@itemx --quiet
@itemx -q
@cindex @code{--silent}
@cindex @code{--quiet}
@cindex @code{-q}
Don't print the usual greeting and version message at startup.

@item --texi-macros-file @var{filename}
@cindex @code{--texi-macros-file @var{filename}}
Specify the name of the file containing Texinfo macros for use by makeinfo.

@item  --traditional
@itemx --braindead
@cindex @code{--traditional}
@cindex @code{--braindead}
For compatibility with @sc{matlab}, set initial values for
user preferences to the following values

@example
@group
PS1                             = ">> "
PS2                             = ""
allow_noninteger_range_as_index = true
beep_on_error                   = true
confirm_recursive_rmdir         = false
crash_dumps_octave_core         = false
default_save_options            = "-mat-binary"
do_braindead_shortcircuit_evaluation = true
fixed_point_format              = true
history_timestamp_format_string = "%%-- %D %I:%M %p --%%"
page_screen_output              = false
print_empty_dimensions          = false
@end group
@end example

@noindent
and disable the following warnings

@example
@group
Octave:abbreviated-property-match
Octave:fopen-file-in-path
Octave:function-name-clash
Octave:load-file-in-path
@end group
@end example

@item  --verbose
@itemx -V
@cindex @code{--verbose}
@cindex @code{-V}
Turn on verbose output.

@item  --version
@itemx -v
@cindex @code{--version}
@cindex @code{-v}
Print the program version number and exit.

@item @var{file}
Execute commands from @var{file}.  Exit when done unless
@option{--persist} is also specified.
@end table

Octave also includes several functions which return information 
about the command line, including the number of arguments and all of the
options.

@DOCSTRING(argv)

@DOCSTRING(program_name)

@DOCSTRING(program_invocation_name)

Here is an example of using these functions to reproduce the command 
line which invoked Octave.

@example
@group
printf ("%s", program_name ());
arg_list = argv ();
for i = 1:nargin
  printf (" %s", arg_list@{i@});
endfor
printf ("\n");
@end group
@end example

@noindent
@xref{Indexing Cell Arrays}, for an explanation of how to retrieve objects
from cell arrays, and @ref{Defining Functions}, for information about the 
variable @code{nargin}.

@node Startup Files
@subsection Startup Files
@cindex initialization
@cindex startup

When Octave starts, it looks for commands to execute from the files in
the following list.  These files may contain any valid Octave commands,
including function definitions.

@cindex startup files

@table @code
@item @var{octave-home}/share/octave/site/m/startup/octaverc
@cindex site startup file
where @var{octave-home} is the directory in which Octave is installed
(the default is @file{/usr/local}).
This file is provided so that changes to the default Octave environment 
can be made globally for all users at your site for all versions of Octave
you have installed.  Care should be taken when making changes to this file 
since all users of Octave at your site will be affected.  The default file 
may be overridden by the environment variable @w{@env{OCTAVE_SITE_INITFILE}}.

@item @var{octave-home}/share/octave/@var{version}/m/startup/octaverc
@cindex version startup file
where @var{octave-home} is the directory in which Octave is
installed (the default is @file{/usr/local}), and @var{version}
is the version number of Octave.  This file is provided so that changes
to the default Octave environment can be made globally for all users of
a particular version of Octave.  Care should be taken when making
changes to this file since all users of Octave at your site will be
affected.  The default file may be overridden by the environment variable
@w{@env{OCTAVE_VERSION_INITFILE}}.

@item ~/.octaverc
@cindex personal startup file
@cindex @code{~/.octaverc}
This file is used to make personal changes to the default 
Octave environment.

@item .octaverc
@cindex project startup file
@cindex @code{.octaverc}
This file can be used to make changes to the default Octave environment
for a particular project.  Octave searches for this file in the current
directory after it reads @file{~/.octaverc}.  Any use of the @code{cd}
command in the @file{~/.octaverc} file will affect the directory where
Octave searches for @file{.octaverc}.

If you start Octave in your home directory, commands from the file
@file{~/.octaverc} will only be executed once.
@end table

A message will be displayed as each of the startup files is read if you
invoke Octave with the @option{--verbose} option but without the
@option{--silent} option.

The @code{dump_prefs} function is useful for determining what customizations
to Octave are possible and which are in effect.

@DOCSTRING(dump_prefs)

@node Quitting Octave
@section Quitting Octave
@cindex exiting octave
@cindex quitting octave

@DOCSTRING(quit)

@DOCSTRING(atexit)

@node Getting Help
@section Commands for Getting Help
@cindex online help
@cindex help, online

The entire text of this manual is available from the Octave prompt
via the command @kbd{doc}.  In addition, the documentation for
individual user-written functions and variables is also available via
the @kbd{help} command.  This section describes the commands used for
reading the manual and the documentation strings for user-supplied
functions and variables.  @xref{Function Files}, for more information
about how to document the functions you write.

@DOCSTRING(help)

@DOCSTRING(doc)

@DOCSTRING(lookfor)

To see what is new in the current release of Octave, use the @code{news}
function.

@DOCSTRING(news)

@DOCSTRING(info)

@DOCSTRING(warranty)

The following functions can be used to change which programs are used
for displaying the documentation, and where the documentation can be
found.

@DOCSTRING(info_file)

@DOCSTRING(info_program)

@DOCSTRING(makeinfo_program)

@DOCSTRING(texi_macros_file)

@DOCSTRING(doc_cache_file)

@DOCSTRING(suppress_verbose_help_message)

The following functions are principally used internally by Octave for
generating the documentation.  They are documented here for completeness
and because they may occasionally be useful for users.

@DOCSTRING(gen_doc_cache)

@DOCSTRING(get_help_text)

@DOCSTRING(get_help_text_from_file)

@DOCSTRING(get_first_help_sentence)

@node Command Line Editing
@section Command Line Editing
@cindex command-line editing
@cindex editing the command line

Octave uses the GNU Readline library to provide an extensive set of
command-line editing and history features.  Only the most common
features are described in this manual.  In addition, all of the editing
functions can be bound to different key strokes at the user's discretion.  
This manual assumes no changes from the default Emacs bindings.  See the GNU 
Readline Library manual for more information on customizing Readline and 
for a complete feature list.

To insert printing characters (letters, digits, symbols, etc.), simply
type the character.  Octave will insert the character at the cursor and
advance the cursor forward.

Many of the command-line editing functions operate using control
characters.  For example, the character @kbd{Control-a} moves the cursor
to the beginning of the line.  To type @kbd{C-a}, hold down @key{CTRL}
and then press @key{a}.  In the following sections, control characters
such as @kbd{Control-a} are written as @kbd{C-a}.

Another set of command-line editing functions use Meta characters.  To 
type @kbd{M-u}, hold down the @key{META} key and press @key{u}.  Depending
on the keyboard, the @key{META} key may be labeled @key{ALT} or
even @key{WINDOWS}.  If your terminal does not have a @key{META} key, you
can still type Meta characters using two-character sequences starting
with @kbd{ESC}.  Thus, to enter @kbd{M-u}, you would type
@key{ESC} @key{u}.  The @kbd{ESC} character sequences are also allowed on
terminals with real Meta keys.  In the following sections, Meta
characters such as @kbd{Meta-u} are written as @kbd{M-u}.


@menu
* Cursor Motion::               
* Killing and Yanking::         
* Commands For Text::           
* Commands For Completion::     
* Commands For History::        
* Customizing readline::        
* Customizing the Prompt::      
* Diary and Echo Commands::     
@end menu

@node Cursor Motion
@subsection Cursor Motion

The following commands allow you to position the cursor.

@table @kbd
@item C-b
Move back one character.

@item C-f
Move forward one character.

@item @key{BACKSPACE}
Delete the character to the left of the cursor.

@item @key{DEL}
Delete the character underneath the cursor.

@item C-d
Delete the character underneath the cursor.

@item M-f
Move forward a word.

@item M-b
Move backward a word.

@item C-a
Move to the start of the line.

@item C-e
Move to the end of the line.

@item C-l
Clear the screen, reprinting the current line at the top.

@item C-_
@itemx C-/
Undo the last action.  You can undo all the way back to an empty line.

@item M-r
Undo all changes made to this line.  This is like typing the `undo'
command enough times to get back to the beginning.
@end table

The above table describes the most basic possible keystrokes that you need
in order to do editing of the input line.  On most terminals, you can
also use the left and right arrow keys in place of @kbd{C-f} and @kbd{C-b}
to move forward and backward.

Notice how @kbd{C-f} moves forward a character, while @kbd{M-f} moves
forward a word.  It is a loose convention that control keystrokes
operate on characters while meta keystrokes operate on words.

@cindex clearing the screen

The function @code{clc} will allow you to clear the screen from within
Octave programs.

@DOCSTRING(clc)

@node Killing and Yanking
@subsection Killing and Yanking

@dfn{Killing} text means to delete the text from the line, but to save
it away for later use, usually by @dfn{yanking} it back into the line.
If the description for a command says that it `kills' text, then you can
be sure that you can get the text back in a different (or the same)
place later.

Here is the list of commands for killing text.

@table @kbd
@item C-k
Kill the text from the current cursor position to the end of the line.

@item M-d
Kill from the cursor to the end of the current word, or if between
words, to the end of the next word.

@item M-@key{DEL}
Kill from the cursor to the start of the previous word, or if between
words, to the start of the previous word. 

@item C-w
Kill from the cursor to the previous whitespace.  This is different than
@kbd{M-@key{DEL}} because the word boundaries differ.
@end table

And, here is how to @dfn{yank} the text back into the line.  Yanking
means to copy the most-recently-killed text from the kill buffer.

@table @kbd
@item C-y
Yank the most recently killed text back into the buffer at the cursor.

@item M-y
Rotate the kill-ring, and yank the new top.  You can only do this if
the prior command is @kbd{C-y} or @kbd{M-y}.
@end table

When you use a kill command, the text is saved in a @dfn{kill-ring}.
Any number of consecutive kills save all of the killed text together, so
that when you yank it back, you get it in one clean sweep.  The kill
ring is not line specific; the text that you killed on a previously
typed line is available to be yanked back later, when you are typing
another line.

@node Commands For Text
@subsection Commands For Changing Text

The following commands can be used for entering characters that would
otherwise have a special meaning (e.g., @key{TAB}, @kbd{C-q}, etc.), or
for quickly correcting typing mistakes.

@table @kbd
@item C-q
@itemx C-v
Add the next character that you type to the line verbatim.  This is
how to insert things like @kbd{C-q} for example.

@item M-@key{TAB}
Insert a tab character.

@item C-t
Drag the character before the cursor forward over the character at the
cursor, also moving the cursor forward.  If the cursor is at the end of
the line, then transpose the two characters before it.

@item M-t
Drag the word behind the cursor past the word in front of the cursor
moving the cursor over that word as well.

@item M-u
Uppercase the characters following the cursor to the end of the current
(or following) word, moving the cursor to the end of the word.

@item M-l
Lowercase the characters following the cursor to the end of the current
(or following) word, moving the cursor to the end of the word.

@item M-c
Uppercase the character following the cursor (or the beginning of the
next word if the cursor is between words), moving the cursor to the end
of the word.
@end table

@node Commands For Completion
@subsection Letting Readline Type For You
@cindex command completion

The following commands allow Octave to complete command and variable
names for you.

@table @kbd
@item @key{TAB}
Attempt to do completion on the text before the cursor.  Octave can
complete the names of commands and variables.

@item M-?
List the possible completions of the text before the cursor.
@end table

@DOCSTRING(completion_append_char)

@DOCSTRING(completion_matches)

@node Commands For History
@subsection Commands For Manipulating The History
@cindex command history
@cindex input history
@cindex history of commands

Octave normally keeps track of the commands you type so that you can
recall previous commands to edit or execute them again.  When you exit
Octave, the most recent commands you have typed, up to the number
specified by the variable @code{history_size}, are saved in a file.
When Octave starts, it loads an initial list of commands from the file
named by the variable @code{history_file}.

Here are the commands for simple browsing and searching the history
list.

@table @kbd
@item @key{LFD}
@itemx @key{RET}
Accept the current line regardless of where the cursor is.  If the line is
non-empty, add it to the history list.  If the line was a history
line, then restore the history line to its original state.

@item C-p
Move `up' through the history list.

@item C-n
Move `down' through the history list.

@item M-<
Move to the first line in the history.

@item M->
Move to the end of the input history, i.e., the line you are entering!

@item C-r
Search backward starting at the current line and moving `up' through
the history as necessary.  This is an incremental search.

@item C-s
Search forward starting at the current line and moving `down' through
the history as necessary.
@end table

On most terminals, you can also use the up and down arrow keys in place 
of @kbd{C-p} and @kbd{C-n} to move through the history list.

In addition to the keyboard commands for moving through the history
list, Octave provides three functions for viewing, editing, and
re-running chunks of commands from the history list.

@DOCSTRING(history)

@DOCSTRING(edit_history)

@DOCSTRING(run_history)

@noindent
Octave also allows you customize the details of when, where, and how history
is saved.

@DOCSTRING(saving_history)

@DOCSTRING(history_control)

@DOCSTRING(history_file)

@DOCSTRING(history_size)

@DOCSTRING(history_timestamp_format_string)

@DOCSTRING(EDITOR)

@node Customizing readline
@subsection Customizing @code{readline}
@cindex @code{~/.inputrc}
@cindex customizing @code{readline}
@cindex @code{readline} customization

Octave uses the GNU Readline library for command-line editing and
history features.  Readline is very flexible and can be modified through
a configuration file of commands (See the GNU Readline library for the
exact command syntax).  The default configuration file is normally
@file{~/.inputrc}.

Octave provides two commands for initializing Readline and thereby changing
the command line behavior.

@DOCSTRING(read_readline_init_file)

@DOCSTRING(re_read_readline_init_file)

@node Customizing the Prompt
@subsection Customizing the Prompt
@cindex prompt customization
@cindex customizing the prompt

The following variables are available for customizing the appearance of
the command-line prompts.  Octave allows the prompt to be customized by
inserting a number of backslash-escaped special characters that are
decoded as follows:

@table @samp
@item \t
The time.

@item \d
The date.

@item \n
Begins a new line by printing the equivalent of a carriage return
followed by a line feed.

@item \s
The name of the program (usually just @samp{octave}).

@item \w
The current working directory.

@item \W
The basename of the current working directory.

@item \u
The username of the current user.

@item \h
The hostname, up to the first `.'.

@item \H
The hostname.

@item \#
The command number of this command, counting from when Octave starts.

@item \!
The history number of this command.  This differs from @samp{\#} by the
number of commands in the history list when Octave starts.

@item \$
If the effective UID is 0, a @samp{#}, otherwise a @samp{$}.

@item \nnn
The character whose character code in octal is @var{nnn}.

@item \\
A backslash.
@end table

@DOCSTRING(PS1)

@DOCSTRING(PS2)

@DOCSTRING(PS4)

@node Diary and Echo Commands
@subsection Diary and Echo Commands
@cindex diary of commands and output
@cindex command and output logs
@cindex logging commands and output
@cindex echoing executing commands
@cindex command echoing

Octave's diary feature allows you to keep a log of all or part of an
interactive session by recording the input you type and the output that
Octave produces in a separate file.

@DOCSTRING(diary)

Sometimes it is useful to see the commands in a function or script as
they are being evaluated.  This can be especially helpful for debugging
some kinds of problems.

@DOCSTRING(echo)

@DOCSTRING(echo_executing_commands)

@node Errors
@section How Octave Reports Errors
@cindex error messages
@cindex messages, error

Octave reports two kinds of errors for invalid programs.

A @dfn{parse error} occurs if Octave cannot understand something you
have typed.  For example, if you misspell a keyword,

@example
octave:13> function y = f (x) y = x***2; endfunction
@end example

@noindent
Octave will respond immediately with a message like this:

@example
@group
parse error:

  syntax error

>>> function y = f (x) y = x***2; endfunction
                              ^
@end group
@end example

@noindent
For most parse errors, Octave uses a caret (@samp{^}) to mark the point
on the line where it was unable to make sense of your input.  In this
case, Octave generated an error message because the keyword for
exponentiation (@code{**}) was misspelled.  It marked the error at the
third @samp{*} because the code leading up to this was correct but the final
@samp{*} was not understood.

Another class of error message occurs at evaluation time.  These
errors are called @dfn{run-time errors}, or sometimes
@dfn{evaluation errors}, because they occur when your program is being
@dfn{run}, or @dfn{evaluated}.  For example, if after correcting the
mistake in the previous function definition, you type

@example
octave:13> f ()
@end example

@noindent
Octave will respond with

@example
@group
error: `x' undefined near line 1 column 24
error: called from:
error:   f at line 1, column 22
@end group
@end example

@noindent
This error message has several parts, and gives quite a bit of
information to help you locate the source of the error.  The messages
are generated from the point of the innermost error, and provide a
traceback of enclosing expressions and function calls.

In the example above, the first line indicates that a variable named
@samp{x} was found to be undefined near line 1 and column 24 of some
function or expression.  For errors occurring within functions, lines
are counted from the beginning of the file containing the function
definition.  For errors occurring outside of an enclosing function, 
the line number indicates the input line number, which is usually displayed 
in the primary prompt string.

The second and third lines of the error message indicate that the error 
occurred within the function @code{f}.  If the function @code{f} had been 
called from within another function, for example, @code{g}, the list of 
errors would have ended with one more line:

@example
error:   g at line 1, column 17
@end example

These lists of function calls make it fairly easy to trace the
path your program took before the error occurred, and to correct the
error before trying again.

@node Executable Octave Programs
@section Executable Octave Programs
@cindex executable scripts
@cindex scripts
@cindex batch processing
@cindex self contained programs
@cindex program, self contained
@cindex @samp{#!}

Once you have learned Octave, you may want to write self-contained
Octave scripts, using the @samp{#!} script mechanism.  You can do this
on GNU systems and on many Unix systems @footnote{The @samp{#!}
mechanism works on Unix systems derived from Berkeley Unix, System V
Release 4, and some System V Release 3 systems.}.

Self-contained Octave scripts are useful when you want to write a
program which users can invoke without knowing that the program is
written in the Octave language.  Octave scripts are also used for batch
processing of data files.  Once an algorithm has been developed and tested
in the interactive portion of Octave, it can be committed to an executable
script and used again and again on new data files.

As a trivial example of an executable Octave script, you might create a
text file named @file{hello}, containing the following lines:

@example
@group
#! @var{octave-interpreter-name} -qf
# a sample Octave program
printf ("Hello, world!\n");
@end group
@end example

@noindent
(where @var{octave-interpreter-name} should be replaced with the full
path and name of your Octave binary).  Note that this will only work if
@samp{#!} appears at the very beginning of the file.  After making the
file executable (with the @code{chmod} command on Unix systems), you can
simply type:

@example
hello
@end example

@noindent
at the shell, and the system will arrange to run Octave as if you had
typed:

@example
octave hello
@end example

The line beginning with @samp{#!} lists the full path and filename of an
interpreter to be run, and an optional initial command line argument to
pass to that interpreter.  The operating system then runs the
interpreter with the given argument and the full argument list of the
executed program.  The first argument in the list is the full file name
of the Octave executable.  The rest of the argument list will either be
options to Octave, or data files, or both.  The @samp{-qf} options are
usually specified in stand-alone Octave programs to prevent them from
printing the normal startup message, and to keep them from behaving
differently depending on the contents of a particular user's
@file{~/.octaverc} file.  @xref{Invoking Octave from the Command Line}.

Note that some operating systems may place a limit on the number of
characters that are recognized after @samp{#!}.  Also, the arguments 
appearing in a @samp{#!} line are parsed differently by various 
shells/systems.  The majority of them group all the arguments together in one 
string and pass it to the interpreter as a single argument.  In this case, the
following script:

@example
@group
#! @var{octave-interpreter-name} -q -f # comment
@end group
@end example

@noindent
is equivalent to typing at the command line:

@example
@group
octave "-q -f # comment"
@end group
@end example

@noindent
which will produce an error message.  Unfortunately, it is
not possible for Octave to determine whether it has been called from the 
command line or from a @samp{#!} script, so some care is needed when using the
@samp{#!} mechanism.

Note that when Octave is started from an executable script, the built-in
function @code{argv} returns a cell array containing the command line
arguments passed to the executable Octave script, not the arguments
passed to the Octave interpreter on the @samp{#!} line of the script.
For example, the following program will reproduce the command line that
was used to execute the script, not @samp{-qf}.

@example
@group
#! /bin/octave -qf
printf ("%s", program_name ());
arg_list = argv ();
for i = 1:nargin
  printf (" %s", arg_list@{i@});
endfor
printf ("\n");
@end group
@end example

@node Comments
@section Comments in Octave Programs
@cindex comments
@cindex use of comments
@cindex documenting Octave programs

A @dfn{comment} is some text that is included in a program for the sake
of human readers, and which is NOT an executable part of the program.  
Comments can explain what the program does, and how it works.  Nearly all
programming languages have provisions for comments, because programs are
typically hard to understand without them.

@menu
* Single Line Comments::
* Block Comments::
* Comments and the Help System::                    
@end menu

@node Single Line Comments
@subsection Single Line Comments
@cindex @samp{#}
@cindex @samp{%}

In the Octave language, a comment starts with either the sharp sign
character, @samp{#}, or the percent symbol @samp{%} and continues to the
end of the line.  Any text following the sharp sign or percent symbol is
ignored by the Octave interpreter and not executed.  The following example
shows whole line and partial line comments.

@example
@group
function countdown
  # Count down for main rocket engines 
  disp (3);
  disp (2);
  disp (1);
  disp ("Blast Off!");  # Rocket leaves pad
endfunction
@end group
@end example

@node Block Comments
@subsection Block Comments
@cindex block comments
@cindex multi-line comments
@cindex @samp{#@{}
@cindex @samp{%@{}

Entire blocks of code can be commented by enclosing the code between 
matching @samp{#@{} and @samp{#@}} or @samp{%@{} and @samp{%@}} markers.  
For example,

@example
@group
function quick_countdown
  # Count down for main rocket engines 
  disp (3);
 #@{
  disp (2);
  disp (1);
 #@}
  disp ("Blast Off!");  # Rocket leaves pad
endfunction
@end group
@end example

@noindent
will produce a very quick countdown from '3' to 'Blast Off' as the
lines "@code{disp (2);}" and "@code{disp (1);}" won't be executed.

The block comment markers must appear alone as the only characters on a line
(excepting whitespace) in order to be parsed correctly.

@node Comments and the Help System
@subsection Comments and the Help System
@cindex documenting functions
@cindex documenting user scripts
@cindex help, user-defined functions

The @code{help} command (@pxref{Getting Help}) is able to find the first
block of comments in a function and return those as a documentation
string.  This means that the same commands used to get help
on built-in functions are available for properly formatted user-defined
functions.  For example, after defining the function @code{f} below,

@example
@group
function xdot = f (x, t)

# usage: f (x, t)
#
# This function defines the right-hand
# side functions for a set of nonlinear
# differential equations.

  r = 0.25;
  @dots{}
endfunction
@end group
@end example

@noindent
the command @kbd{help f} produces the output

@example
@group
 usage: f (x, t)

 This function defines the right-hand
 side functions for a set of nonlinear
 differential equations.
@end group
@end example

Although it is possible to put comment lines into keyboard-composed,
throw-away Octave programs, it usually isn't very useful because the
purpose of a comment is to help you or another person understand the
program at a later time.

The @code{help} parser currently only recognizes single line comments
(@pxref{Single Line Comments}) and not block comments for the initial 
help text.