Archives for December, 2011

Notes on shell scripting

published on December 29, 2011.

Yesterday I did some shell scripting and thought about writing down the few things learned along the way. Amazing how little needs to be done to learn a lot :)

Result of a command to a variable

First thing I learned is how to “save” the result of a shell command to a local variable:

PHP_BINPATH=$(which php)

By enclosing the command in parenthesis and putting a dollar sign in front of it, will put the result of that command in the variable.

An empty variable

Turns out, a variable can be empty, null. Nothing strange with that, until one tries to do something with that variable. For example:

if [ $PHP_BINPATH == "foo" ]
    echo "It's foo"

will die with a strange error: “line 2: [: =: unary operator expected”. Problem is that when evaluating it will see if [ == “foo” ] and turns out [ is some reserved command or some such. The fix is to wrap $PHP_BINPATH in double quotes:

if [ "$PHP_BINPATH" == "foo" ]
    echo "It's foo"

Pass all the arguments!

When calling some other command from within the shell script, and all the arguments which are passed to the shell script need to be passed to that other command, use "$@“ for that:

$PHP_BINPATH usher.php "$@"

This will pass all the arguments to the PHP script which is called from within the shell script.

Happy hackin’!

Listening to Dbus signals with PHP

published on December 26, 2011.

In my previous post I described (tried, at least) how to communicate with Pidgin from PHP, by using the Dbus PHP extension.

The good part is that not can we only call different methods against Pidgin’s libpurple API, we can also listen to different signals on different events, that are sent via Dbus. Some of the events that are signalled are when a chat message is recieved, a friend comes online, a file is sent, or any other from a list of some 110 different events.

The PHP Dbus extension allows us to watch for one exact signal on an interface, or for all signals on an interface. Of course, we can add watches on multiple interfaces at once.

Watching for signals

Once we know the interface and/or the specific signal we’re interested in, we can add a watch on it. This is done by calling the addWatch method on the Dbus object, were the first parameter is the interface, and the second, optional parameter is the exact signal we want to listen to.


$dbus = new Dbus(Dbus::BUS_SESSION);

// watching for a specific signal
$dbus->addWatch("im.pidgin.purple.PurpleInterface", "ReceivedImMsg");
// or watching on an entire interface
// $dbus->addWatch("im.pidgin.purple.PurpleInterface");
// also can listen to different interfaces at the same time

Next, we need a way to actually get these signals when the events occur. For this we are using the waitLoop method of the Dbus class. That method accepts a number as a parameter, which is the number of miliseconds it should wait between requests. If an event happened on the interface we’re watching, it will return the signal, which is a DbusSignal; otherwise we’ll get a null:

do {
    $signal = $dbus->waitLoop(1000);

    if ($signal instanceof DbusSignal) {
        // even if we watch only for one signal on one interface
        // we still can get rubbish, so making sure this is what we need
            // data is in this weird DbusSet object thingy
            $data = $signal->getData()->getData();
            echo "Got stuff!\n";
} while (true);

Once we got the signal, to make sure that the signal is really the one we’re interested in, we call the matches method on it. The first parameter is the interface and the second is the signal.

Each event has (can have? not sure yet) additional data associated with it. To get to it, for some odd reason, we need to call getData()->getData() on the signal; note that this is in case of listening on libpurple’s interfaces, not sure about others. Experiment. Also, what kind of data is returned, again, depends on the interface and/or the event - some return arrays, some strings.

Have a look at the Github repo for some more examples (the dbus-signals* files).

In the third, and probably last post in this dbus mini-series, I’ll try to build a bot with which I can communicate and issue out commands to.

Happy hackin’!

Configuring 2 monitors with xrandr

published on December 25, 2011.

My current, most used set up, includes a laptop and a second screen attached to it. The laptop is always to the left of the second monitor and together they give one big screen with a total resolution of 3046x1050. From time to time, X11 gets confused and shows the same image, with the same resolution, on both monitors.

The tool which can help fix this is xrandr.

First, query X11 to find out what monitors there are:

$ xrandr -q

Once the monitor IDs are known, this fixes things for me:

$ xrandr --output VGA1 --auto --right-of LVDS1

Where LVDS1 is the laptop’s screen and VGA1 is the second screen.

Happy hackin’!

Tags: monitors, x11, xrandr.
Categories: Software.

A quick note on Dojo's data grids and

published on December 23, 2011.

I’m spending this day trying to create an “universal” administration dashboard with which I’ll finally be happy with. I’m using Dojo to spice up the UI, because I think it’s awesome and it has a lot of stuff in it and plays well with Zend Framework. This post is dedicated to the future stupid me.

Anyway, when using as a store for a (or any other grid, really), pay attention to the definition of the columns structure which is passed to the grid. I was doing a really stupid thing which cost me a bunch of hours until I finally figured out what was going on.

Let’s take for example this is the given HTML table:

<table id="datastore">
    <!-- body comes here -->

I was defining the structure for the columns as:

var struct = [[
   { field: 'id', name: 'ID', width: 'auto' },
   { field: 'name', name: 'Name', width: 'auto'}

which was wrong. This is the correct one:

var struct = [[
   { field: 'ID', name: 'ID', width: 'auto' },
   { field: 'Name', name: 'Name', width: 'auto'}

Use what’s in the TH tags for the field properties! I was trying to be clever and use the name of the fields in the database. The worst part is that there will be no errors, the grid will render correctly the header row and a correct number of rows for the data, but! it will show “…” in each column, instead of the actual data.

Happy hackin’!

Communicating with Pidgin from PHP via D-Bus

published on December 18, 2011.

Earlier this week I got an idea of trying to communicate with Pidgin, a chat client, via the terminal. Sounded like a fun thing to hack on, plus, could be made useful (in my head, at least), for things like logging from a web application directly to IM, or, heck, even creating something like Github’s Hubot, commanding a server or an application just via chat. Surely I wasn’t the first one to come up with this idea and after a bit of a googling found out that Pidgin’s libpurple has a nice API for that, exposed via D-Bus.

I first planned to write some scripts for this in Python or C, but when I finally sat down over the weekend to hack on this, realized there is a PHP D-Bus extension, thanks to Derick Rethans! As I rarely have the opportunity/need to play with more “obscure” PHP extensions, decided to give this one a spin… (Note: apart from that D-Bus is used for processes communicating with each other, I have zero knowledge about it, so I might be wrong with some things down below.)

Installing D-Bus for PHP

As the extension requires the dbus-devel package, first make sure it is installed:

$ yum install dbus-devel

The installation of the extension itself is pretty easy:

$ pecl install dbus-beta

Add the line to your php.ini, restart Apache if needed and have a look at the phpinfo();, there should be an entry for D-Bus listed.

Note that there is no documentation for this extension at the moment, but, luckily, the sources include an examples directory full of goodies! After fiddling around with those for an hour or so, got the basics of the extension figured out.

One thing I haven’t figured out completely, is how to run scripts which use the Dbus extension via the browser, as in those cases the scripts are dying with a terrible X11 error message. So, run Dbus scripts from the console and all should be fine.

The a-ha! moment

What I learned by looking at the examples, is that the Dbus class is used for creating a proxy, which can be used to call methods on the service/application we’re interested in. But, what methods are available, what arguments do those methods accept (if any), and what will they return as a result?

This can easily be found out by introspection. Create a proxy to an object which implements the Introspectable interface and call the Introspect method on that proxy:


$dbus = new Dbus;

$proxy = $dbus->createProxy("im.pidgin.purple.PurpleService",

$data = $proxy->Introspect();

file_put_contents('introspect.xml', $data);

As the result of the introspection is returned in an XML and can be quite big, putting it in a file for easier viewing.

By looking at the XML file, it’s pretty easy to figure out what’s going on; method names, method arguments, their names and types, and the returned result:

<method name="PurpleAccountGetUsername">
  <arg name="account" type="i" direction="in"></arg>
  <arg name="RESULT" type="s" direction="out"></arg>

With all this information at our disposal, it’s easy to write a script which does something useful, like, listing all the connected accounts and the protocols they are using:


$dbus = new Dbus;

$proxy = $dbus->createProxy("im.pidgin.purple.PurpleService",

$accounts = $proxy->PurpleAccountsGetAllActive();

foreach ($accounts->getData() as $account) {
    if ($proxy->PurpleAccountIsConnected($account)) {
        $username = $proxy->PurpleAccountGetUsername($account);
        $protocolId = $proxy->PurpleAccountGetProtocolId($account);
        $protocolName = $proxy->PurpleAccountGetProtocolName($account);
        echo $username . " is connected on the " . $protocolName
                       . " (" . $protocolId . ") protocol.\n";

A sample output would be something like: “ is connected on the IRC (prpl-irc) protocol.”

Next steps

Of course, this is far from a chat bot which can execute commands on a remote server, but at least we have some foundation to build on. In the coming days I’ll try to figure out how to create a loop which can be used to listen to different purple events - when a contact comes online, a chat is sent, or received, etc.

Also, it is quite fun trying to figure out a PHP extension just by looking at examples and the C source itself. One can learn a lot this way.

Happy hackin’!

P.S.: Code samples are up on Github!

Update 2011-12-26: the 2nd post, listening to dbus signals with php, is published!

Robert Basic

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