Scripting ELinks with Lua
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## Scripting ELinks with Lua

This file documents the Lua scripting interface of the ELinks web browser.

### Introduction

#### What is it

Lua scripting capabilities permit users to customize the ELinks behaviour to unusual degree - they allow automatic rewriting of HTML code of the received documents, rewriting of the URLs entered by user etc. You can even write your own bookmarks system with Lua. See also contrib/lua/ for some examples of the possibilities of ELinks Lua support.

Please do not confuse Lua scripting with JavaScript, EcmaScript, VBScript and similar. Those are embedded in page, allowing per-document scripting related to its presentation and providing some degree of interactivity etc. On the contrary, the current Lua support permits scripts to be embedded to the browser directly, changing the behaviour of the browser, not the document.

The original Lua support (in the form of Links-Lua fork of original Links) was written by Peter Wang and Cliff Cunnington. There are some rough edges remaining, but is suitable for everyday use (I have been using it every day for a year).

#### Where to get it

The Lua scripting support comes with the stock ELinks distribution, no additional patches and tweaks should be needed.

The web site of the original Links-Lua is at http://links.sourceforge.net/links-lua/. Some older patches against regular Links are available at http://www.sourceforge.net/projects/links/, but they are not being maintained.

Lua can be found at http://www.lua.org/.

#### What it runs on

The Lua support has only been tested under Linux, although it should work under other platforms that ELinks and Lua support (perhaps with some changes to source code?).

Also, note that many of the scripts given here assume a Unix system. Your mileage will definitely vary on other platforms.

### Installing

#### Installing Lua

Before you can compile ELinks with Lua support, you must compile and install Lua. The following instructions are for a Linux system. People on other systems should try to enable `popen` support, but this is not necessary (you will lose a bit of functionality though).

1. Download and unpack the Lua `tar.gz` or `zip` somewhere.
2. `cd` into the `lua` directory.
3. Open `config` in a text editor and uncomment the `POPEN` line.
4. Optionally, change the `INSTALL_ROOT line.
5. Run `make; make so; make sobin; make install`.

On systems without shared object support, simply run `make; make install` instead.

Since ELinks 0.11.0, only version 5.0 of Lua is supported. Future versions of ELinks will probably support Lua 5.1 too; see bug 742.

#### Installing ELinks

Follow the instructions for building ELinks (it is the standard `./configure; make; make install` procedure). During the configure step make sure that Lua has been detected on your system.

#### Running ELinks with Lua

Simply start ELinks as you normally would. To check you have Lua support compiled in, open up the "Help | About" dialog box. It should list "Scripting (Lua)" under "Features". If not, make sure you do not have other copies of ELinks running, or start ELinks again with the "-no-connect" option on the command-line.

### Using ELinks with Lua

Out of the box, ELinks with Lua will do nothing different from regular ELinks. You need to write some scripts.

#### ELinks Lua additions

The Lua support is based on the idea of hooks. A hook is a function that gets called at a particular point during the execution of ELinks. To make ELinks do what you want, you can add and edit such hooks.

The Lua support also adds an extra dialog box, which you can open while in ELinks with the comma (`,`) key. Here you can enter Lua expressions for evaluation, or override it to do something different.

And finally, you can bind keystrokes to Lua functions. These keystrokes won't let you do any more than is possible with the Lua Console, but they're more convenient.

Note that this document assumes you have some knowledge of programming in Lua. For that, you should refer to the Lua reference manual (http://www.lua.org/docs.html). In fact, the language is relatively trivial, though. You could already do wonders with simply refactoring the example scripts.

#### Config file

On startup, ELinks reads in two Lua scripts. Firstly, a system-wide configuration file called `/etc/elinks/hooks.lua`, then a file in your home directory called `~/.config/elinks/hooks.lua`. From these files, you can include other Lua files with `dofile`, if necessary.

To see what kind of things you should put in here, look at `contrib/lua/hooks.lua`.

#### Hooks

The following hooks are available.

goto_url_hook (url, current_url)
  This hook is called when the user enters a string into the "Go to URL" dialog box. It is given the string entered, and the current URL (which may be `nil`). It should return a string, which is the URL that ELinks should follow, or `nil` to cancel the operation.
follow_url_hook (url)
  This hook is passed the URL that ELinks is about to follow. It should return a string (the URL modified or unmodified), or `nil` to stop ELinks following the URL
pre_format_html_hook (url, html)
  This hook gets called just before the final time an HTML document is formatted, i.e. it only gets called once, after the entire document is downloaded. It will be passed the URL and HTML text as strings, and should return the modified HTML text, or `nil` if there were no modifications.
proxy_for_hook (url)
  This hook is called when ELinks is about to load a resource from a URL. It should return "PROXY:PORT" (e.g. "localhost:8080") to use the specified proxy, "" to contact the origin server directly, or `nil` to use the default proxy of the protocol.
lua_console_hook (string)

  This hook is passed the string that the user entered into the "Lua Console" dialog box. It should return two values: the type of action to take (`run`, `eval`, `goto-url` or `nil`), and a second argument, which is the shell command to run or the Lua expression to evaluate. Examples:

  - `return "run", "someprogram"` will attempt to run the program `someprogram`.
  - `return "eval", "somefunction(1+2)"` will attempt to call the Lua function `somefunction` with an argument, 3.
  - `return "goto_url", "http://www.bogus.com"` will ask ELinks to visit the URL "http://www.bogus.com".
  - `return nil` will do nothing.
quit_hook ()
  This hook is run just before ELinks quits. It is useful for cleaning up things, such as temporary files you have created.

#### Functions

As well as providing hooks, ELinks provides some functions in addition to the standard Lua functions.

### Note

The standard Lua function `os.setlocale` affects ELinks' idea of the system locale, which ELinks uses for the "System" charset, for the "System" language, and for formatting dates. This may however have to be changed in a future version of ELinks, in order to properly support terminal-specific system locales.

current_url ()
  Returns the URL of the current page being shown (in the ELinks session that invoked the function).
current_link ()
  Returns the URL of the currently selected link, or `nil` if none is selected.
current_title ()
  Returns the title of the current page, or `nil` if none.
current_document ()
  Returns the current document as a string, unformatted.
current_document_formatted ([width])
  Returns the current document, formatted for the specified screen width. If the width is not specified, then the document is formatted for the current screen width (i.e. what you see on screen). Note that this function does not guarantee all lines will be shorter than `width`, just as some lines may be wider than the screen when viewing documents online.
pipe_read (command)
  Executes `command` and reads in all the data from stdout, until there is no more. This is a hack, because for some reason the standard Lua function `file:read` seems to crash ELinks when used in pipe-reading mode.
execute (string)
  Executes shell commands `string` without waiting for it to exit. Beware that you must not read or write to stdin and stdout. And unlike the standard Lua function `os.execute`, the return value is meaningless.
tmpname ()

  Returns a unique name for a temporary file, or `nil` if no such name is available. The returned string includes the directory name. Unlike the standard Lua function `os.tmpname`, this one generates ELinks-related names (currently with "elinks" at the beginning of the name).

  ### Warning

  The `tmpname` function creates the file but does not guarantee exclusive access to it: another process may delete the file and recreate it. This exposes you to symlink attacks by other users. To avoid the risk, use `io.tmpfile` instead; unfortunately, it does not tell you the name of the file.

bind_key (keymap, keystroke, function)
  Currently, `keymap` must be the string `"main"`. Keystroke is a keystroke as you would write it in the ELinks config file `~/.config/elinks/elinks.conf`. The function `function` should take no arguments, and should return the same values as `lua_console_hook`.
edit_bookmark_dialog (cat, name, url, function)
  Displays a dialog for editing a bookmark, and returns without waiting for the user to close the dialog. The return value is `1` if successful, `nil` if arguments are invalid, or nothing at all if out of memory. The first three arguments must be strings, and the user can then edit them in input fields. There are also OK and Cancel buttons in the dialog. If the user presses OK, ELinks calls `function` with the three edited strings as arguments, and it should return similar values as in `lua_console_hook`.
xdialog (string [, more strings…], function)
  Displays a generic dialog for editing multiple strings, and returns without waiting for the user to close the dialog. The return value is `1` if successful, `nil` if arguments are invalid, or nothing at all if out of memory. All arguments except the last one must be strings, and ELinks places them in input fields in the dialog. There can be at most 5 such strings. There are also OK and Cancel buttons in the dialog. If the user presses OK, ELinks calls `function` with the edited strings as arguments, and it should return similar values as in `lua_console_hook`.
set_option (option, value)
  Sets an ELinks option. The first argument `option` must be the name of the option as a string. ELinks then tries to convert the second argument `value` to match the type of the option. If successful, `set_option` returns `value`, else `nil`.
get_option (option)
  Returns the value of an ELinks option. The argument `option` must be the name of the option as a string. If the option does not exist, `get_option` returns `nil`.

#### Variables

elinks_home
  The name of the ELinks home directory, as a string. Typically this is the .elinks subdirectory of the user's home directory.

#### User protocol

There is one more little thing which Links-Lua adds, which will not be described in detail here. It is the fake "user:" protocol, which can be used when writing your own addons. It allows you to generate web pages containing links to "user://blahblah", which can be intercepted by the `follow_url_hook` (among other things) to perform unusual actions. For a concrete example, see the bookmark addon.

### Example recipes

This chapter contains some example scripts that you can use. All of them come from `contrib/lua/hooks.lua`. I really recommend you to see it directly instead of copying code out of this document. Also, not everything in there is covered here.

If you would like to contribute scripts, that would be great! Please send them to me at tjaden@users.sourceforge.net. Cliff and I plan to start a script repository, provided we get some contributions. As for script ideas, you'll just have to be a little creative :-)

Also take a look at the `contrib/lua/` directory in the ELinks distribution. Note that Peter and Cliff don't maintain the Lua support intensively anymore, thus it would be probably nice to Cc me (pasky@ucw.cz) if you want to contribute some patch, so that I would be able to add it to the ELinks distribution.

#### Go to URL on steroids

There are some web sites that I visit often. Bookmarks are okay, but they are separate from the "Go to URL" dialog box, so I keep forgetting to use them. Also, when I visit a search engine home page, all I really want to do is enter a search term.

The following script allows me to type certain strings into the "Go to URL" dialog box, and it will convert them to the URL I actually want to visit. As a bonus, it allows me perform some searches on sites like Google without loading up the front page first.

### Tip

The “URI rewriting” feature of ELinks handles many of the same tasks as the Lua hook shown here, and you can conveniently configure it via the option manager. It is not quite as versatile, though.

    function match (prefix, url)
        return string.sub (url, 1, string.len (prefix)) == prefix
    end

    function strip (str)
        return string.gsub (str, "^%s*(.-)%s*$", "%1")
    end

    function plusify (str)
        return string.gsub (str, "%s", "+")
    end

    function goto_url_hook (url, current_url)
        -- Google search (e.g. ,gg unix browsers).
        if match (",gg", url) then
            url = plusify (strip (string.sub (url, 4)))
            return "http://www.google.com/search?q="..url.."&btnG=Google+Search"

        -- Freshmeat search.
        elseif match (",fm", url) then
            url = plusify (strip (string.sub (url, 4)))
            return "http://www.freshmeat.net/search/?q="..url

        -- Dictionary.com search (e.g. ,dict congenial).
        elseif match (",dict", url) then
            url = plusify (strip (string.sub (url, 6)))
            return "http://www.dictionary.com/cgi-bin/dict.pl?db=%2A&term="..url

        -- RPM search (e.g. ,rpm links).
        elseif match (",rpm", url) then
            url = plusify (strip (string.sub (url, 5)))
            return "http://www.rpmfind.net/linux/rpm2html/search.php?query="
                    ..url.."&submit=Search+..."

        -- Netcraft.com search (e.g. ,whatis www.google.com).
        elseif match (",whatis", url) then
            url = plusify (strip (string.sub (url, 8)))
            return "http://uptime.netcraft.com/up/graph/?host="..url

        -- LinuxToday home page.
        elseif match (",lt", url) then
            return "http://linuxtoday.com/"

        -- Weather forecast for Melbourne, Australia.
        elseif match (",forecast", url) then
            return "http://www.bom.gov.au/cgi-bin/wrap_fwo.pl?IDV10450.txt"

        -- Unmatched
        else
            return url
        end
    end

#### Expanding ~ (tilde)

By adding an extra snippet of code to the previous example, we can make ELinks expand pathnames such as `~/foo/bar` and `~user/zappo`, like in the shell and other Unix programs.

    function goto_url_hook (url, current_url)
                    .
                    .

        -- Expand ~ to home directories.
        elseif match ("~", url) then
            if string.sub(url, 2, 2) == "/" then    -- ~/foo
                return os.getenv ("HOME")..string.sub(url, 2)
            else                                    -- ~foo/bar
                return "/home/"..string.sub(url, 2)
            end

                    .
                    .

#### Filtering crap

Many web pages nowadays have columns to the left and right of the text, which are utterly useless. If you happen to be viewing the page in a 80x25 screen, the text you want to read ends up crammed into a tiny space in the centre. We use ELinks Lua support to manipulate the HTML before it reaches the parser.

##### linuxtoday.com

### Note

This recipe is out of date for the web site.

Linux Today has two problems when viewed in ELinks: the useless columns on the left and the right and all the text appears in cyan. Here is a quick recipe to fix that:

    -- Plain string.find (no metacharacters)
    function sstrfind (s, pattern)
        return string.find (s, pattern, 1, true)
    end

    function pre_format_html_hook (url, html)
        -- Strip the left and right columns from Linux Today pages
        -- and change the font colour to white.
        if sstrfind (url, "linuxtoday.com") then
            if sstrfind (url, "news_story") then
                html = string.gsub (html, '<TABLE CELLSPACING="0".-</TABLE>', '', 1)
                html = string.gsub (html, '<TR BGCOLOR="#FFF.-</TR></TABLE>', '', 1)
            else
                html = string.gsub (html, 'WIDTH="120">\n<TR.+</TABLE></TD>', '>', 1)
            end
            html = string.gsub (html, '<A HREF="http://www.internet.com.-</A>', '')
            html = string.gsub (html, "<IFRAME.-</IFRAME>", "")
            -- emphasis in text is lost
            return string.gsub (html, 'text="#002244"', 'text="#001133"', 1)
        end

        return nil
    end

##### linuxgames.com

### Note

This recipe is out of date for the web site.

Here is a simpler example, for http://www.linuxgames.com/.

    function pre_format_html_hook (url, html)
                    .
                    .

        elseif string.find (url, "linuxgames.com", 1, true) then
            return string.gsub (html, "<CENTER>.-</center>", "", 1)

                    .
                    .

#### Reading gzipped files

### Note

ELinks already supports gzipped files natively.

Sometimes documents come gzipped in order to save space, but then you need to uncompress them to read them with ELinks. Here is a recipe to handle gzipped files on a Unix system.

### Warning

This recipe opens a temporary file insecurely.

    function pre_format_html_hook (url, html)
                    .
                    .

        -- Handle gzip'd files within reasonable size.
        if string.find (url, "%.gz$") and string.len (html) < 65536 then
            local name = tmpname ()
            local file = io.open (name, "wb")
            file:write (html)
            file:close ()
            html = pipe_read ("(gzip -dc "..name.." || cat "..name..") 2>/dev/null")
            os.remove (name)
            return html
        end

                    .
                    .

#### Printing

Printing a web page with ELinks usually involves quite a few steps: Save the current document onto disk. Run it through ELinks on the command-line (so it fits into 80 columns) to generate a plain text version. Remove the 80th column from the text version, as it will make printers wrap down to the next line. Finally, run the processed file through `lpr', then delete it.

The following functions allow you to print web pages directly from ELinks, using `lpr' or `enscript'. Type `lpr()` or `enscript()` in the Lua Console to run them. (In the `hooks.lua`, I have also made it so you can just type `lpr` or `enscript`.)

### Note

The `io.popen` function is not available on all platforms.

    function pipe_formatted_to (program)
        local lp, errmsg = io.popen (program, "w")
        if lp == nil then
            error (errmsg)
        else
            lp:write (current_document_formatted (79))
            lp:close ()
        end
    end

    -- Send the current document to `lpr'.
    function lpr ()
        pipe_formatted_to ("lpr")
    end

    -- Send the current document to `enscript'.
    function enscript ()
        pipe_formatted_to ("enscript -fCourier8")
    end

#### Deferring to Netscape

If you come across a brain-dead web page that is totally unreadable with ELinks, you'd probably want to open it with a graphical browser. The following function opens the current document in Netscape.

### Tip

You can also use the built-in “URI passing” feature for this.

    -- When starting Netscape: Set to `nil' if you do not want
    -- to open a new window for each document.
    netscape_new_window = 1

    -- Open current document in Netscape.
    function netscape ()
        local new = netscape_new_window and ",new_window" or ""
        execute ("( netscape -remote 'openURL("..current_url ()..new..")'"
                 .." || netscape '"..current_url ().."' ) 2>/dev/null &")
    end

#### Alternative bookmark system

Many people would like to have a bookmark system with categories (note that ELinks already supports that, marketing name Hierarchical bookmarks), and also to be able to view them and search for them in an HTML page. I have written an alternative bookmark system (for ELinks), which some people may like better than the standard bookmark system.

#### More ideas

- The Lua interface needs to be redesigned to provide more flexible, coherent and usable interface to the scripts.
- Cliff Cunnington had a neat idea of clipping text that you see in web pages (you enter a regexp that will match the start and end of the text you want to clip), and saving the text to disk, along with the URL and timestamp. This would help if you find that you can't ever remember where you had seen a piece of text, or if you want to keep a piece of information but don't need to save the entire page.
- People who use download management programs could write a function to send the current link to their favourite downloading program.
- If you wrote a small C program to put text into the X11 selection clipboard, you could pass the current link or URL to that program, to make it easier to paste URLs into other windows. It might be possible to do the same with GPM, or the KDE/GNOME equivalents.
- Send the current page to Babelfish for translation.
- Look for stupid JavaScript URLs and convert them to something usable.
- More things are possible, I'm sure. If you have an idea that requires another hook or function, contact me (Peter Wang) and I'll see what I can do.

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