The network time protocol (NTP) is a really smart and useful protocol to synchronize the time of your systems, but even if we are in two-thousand-whatever there are reasons why you need to seek for alternatives...
You may now have some kind of »what the [cussword of your choice]« in mind, but I have just been in an ugly situation. All UDP traffic is dropped and I don't have permissions to adjust the firewall.. And you might have heard about the consequences of time differences between servers. Long story short, there is a good solution to sync the time via TCP, using the Time Protocol and a tool called `rdate` .
First off all you need another server having a correct time (e.g. NTP sync'ed), which can be reached at port 37. Let's call this server `$MASTER` . To enable the Time Protocol on `$MASTER` you have to enable the time service in (x)inetd. For instance to enable the TCP service for a current `xinetd` you could create a file in `/etc/xinetd.d/time` with the following contents:
Such a file may already exist, so you just have to change the value of the `disable` -key to `no` . Still using inetd? I'm sure you'll find your way to enable the time server on your system :)
On the client, which is not allowed to use NTP (wtfh!?), you need to install `rdate` :
Just call the following command to synchronize the time of the client with `$MASTER` :
Since `rdate` immediately corrects the time of your system you need to be root to run this command.
Finally, to readjust the time periodically you might want to install a cronjob. Beeing root call `crontab -e` to edit root's crontab and append a line like the following:
This will synchronize the time of your client with the time of `$MASTER` every six hours. (Don't forget to substitute `$MASTER` using your desired server IP or DNS.)
Last but not least I want you to be aware that this workaround just keeps the difference in time between both systems less than 0.5 secs. Beyond all doubt, looking at NTP that's very poor. Nevertheless, 0.5 secs delay is much better than several minutes or even hours!
If it is also not permitted to speak to port 37 you need to tunnel your connections or you have to tell the time server to listen to another, more common port (e.g. 80, 443, or 993), as long as they are not already allocated by other services..
I wanted to publish this summary about wildcards in the bash (and similar shells) some time ago, but didn’t finish it. But finally it gets published.
The shell handles words or patterns containing a wildcard as a template. Available filenames are tested to see if they fit this template. This evaluation is also called globbing.
Let’s have a look at a small example:
In this example * is replaced by appropriate characters, and the list of matching files are passed to the ls command. This set of files will be used in the following examples.
Encode for a single character: `?`
The question mark can be replaced by a single character. So if you want to get the files aaa1 , aaa2 , aaa3 and aaab you can use the following pattern:
So you see, the ? is replaced by exactly one character. That is, both aaa and aaaa1 won’t match.
Encode for a an arbitrary number of characters: `*`
To match any number of characters you can use the asterix * . It can replace 0 to n characters, n is limited by the max length of the file name and depends on the file system you’re using. Adapting the previous snippet you’ll now also get aaa and aaaa1 :
Encode for a set of characters: `[...]`
Most of the common tasks can be done with the previous templates, but there are cases when you need to define the characters that should be replaced. You can specify this set of characters using brackets, e.g.  can be replaced by 3 , 4 , 2 or 1 and is the same as [1-4] :
As you can see aaaa5 doesn’t match  , and btw. the order of the specified characters doesn’t matter. And because it would be very annoying if you want to match against any alphabetic character (you would need to type all 26 characters), you can specify character ranges using a hyphen ( a-z ). Here are some exmaples:
`x` , `y` , `z` or `1`
`C` , `D` , `E` , `F` , `c` , `d` , `e` or `f`
Any small character or digit
Any character except `b` , `c` , `d`
Case-insensitive matching of yes
Alphanumeric characters, same as `A-Za-z0-9`
Alphabetic characters, same as `A-Za-z`
Digits, same as `0-9`
Lowercase alphabetic characters, same as `a-z`
Uowercase alphabetic characters, same as `A-Z`
Whitespace characters (space, tab etc.)
Btw. the files that match such a template are sorted before they are passed to the command.
Even shell scripts may get very complex, so it is helpful to know how to debug them.
Lets explain it on a small example:
Executing it you’ll get an output like this:
To debug the execution of scripts the bash provides a debugging mode. There is one option -x to trace the execution
So you see, every line that is executed at the runtime will be printed with a leading + , comments are ignored. There is another option -v to enable verbose mode. In this mode each line that is read by the bash will be printed before it is executed:
Of course you can combine both modes, so the script is sequentially printed and the commands are traced:
These modes will help you to find some errors.
To modify the output of the tracing mode you may configure the PS4 :
This will also print the file name of the executing script, the line number of the current command that is executed and the respective function name:
if You don’t want to trace a whole script you can enable/disable tracing from within a script:
This will result in something like:
It is of course also possible to enable/disable verbose mode inside the script with set -v and set +v , respectively.
I’m currently developing some JavaServlets and one of the tasks is to create images dynamically. But where to store them accessible for users?
If you want to show the user for example a graph of some stuff that changes frequently you need to generate the image dynamically. The rendering of the graphic is one thing, but where to store the picture so that the visitor can access it from the web?
There were many options to try, and I found that getServletContext().getRealPath (".") from ServletRequest› was the result I’ve been looking for. So to spare you the tests I’ll provide the different options (download):
Let’s assume your webapps-directory is /var/lib/tomcat6/webapps/ , your servlet context is project and the user asks for the servlet test the output probably looks like:
I just bought a new printer, the Brother MFC-9120CN. It’s also able to scan and to copy documents and to send them by fax. Since the installation instructions are win/mac-only I’ll shortly explain how to setup the device in a Linux environment.
Decision for this printer
First of all I was searching for a printer that is in any case compatible to Linux systems. You might also have experiences with this driver f$ckup, or at least have heard about it. The manufactures often only provide drivers for Win or Mac, so you generally get bugged if you want to integrate those peripherals in your environment.
The MFC-9120CN scores at this point. It is able to print and scan via network. Drivers for the printer are available and the the scanned documents can be sent at any FTP server. So you don’t need to have special drivers for scanning, just setup a small FTP server.
This model is also a very cheap one compared to other color-laser MFP’s, and with the ADF it completely matches my criteria.
I already noticed some disadvantages. One is the speed, the printer is somewhat slow. Since I’m not printing thousands of pages it’s more or less minor to me, but you should be aware of that. Another issue is the fact, that the device always forgets the date if it is turned of for a time.. And the printer is a bit too noisy.
The printer comes with a large user manual (>200 pages). It well explains setup the fax functionality, but the installation of the network printer and scanner is only described for win/mac, so I’ll give you a small how-to for your Linux systems.
To use this device via network you have to connect it to a router. It should be able to request an IP via DHCP, but if you don’t provide a DHCP server you need to configure the network manually (my values are in parenthesis):
IP: menu->5->1->2 ( 192.168.9.9 )
Netmask: menu->5->1->3 ( 255.255.255.0 )
Gateway: menu->5->1->4 ( 192.168.9.1 )
If this is done you should be able to ping the printer:
If you browse to this IP using your web browser you’ll find a web interface for the printer. We’ll need this website later on.
Big thanks to the CUPS project, it’s very easy to setup the network-printer! If you haven’t installed cups yet, do it now:
Just browse to your CUPS server (e.g. http://localhost:631 if it is installed on your current machine) and install a new printer via Administration->add Printer (you need to be root). Recent CUPS versions will detect the new printer automatically and you’ll find it in the list of Discovered Network Printers. Just give it a name and some description, select a driver (I’m using Brother MFC-9120CN BR-Script3 (color, 2-sided printing)) and you’re done! Easy, isn’t it!? ;-)
For those of you that have an older version of CUPS: The URI of my printer is dnssd://Brother%20MFC-9120CN._printer._tcp.local/ .
As explained above, the printer is able to send scanned documents to a FTP location. That is, there is no need for a scanner driver! Just install a small FTP server, I decided for ProFTPd:
Make sure, that the /etc/proftpd/proftpd.conf contains the following lines:
and create a new virtual FTP user:
You will be asked for a password. The scanned documents will be stored in /PATH/TO/FILES . This command creates a file ftpd.passwd . Move this file to /etc/proftpd/ , if you didn’t execute the command in that directory.
You should be able to connect to your FTP server:
If that was successful, let’s configure the scanner to use this FTP account. Use your web browser to open the interface of the printer (e.g. http://192.168.9.9/) and go to Administrator Settings->FTP/Network Scan Profile (you have to authenticate, default login is admin and the password is access). Here you’ll find 10 different profiles that can be configured. Click for example on Profile Name 1 and modify the profile:
Host Address: The IP of the FTP server (e.g. 192.168.9.10 )
Username: The username of the virtual FTP user you’ve created (e.g. YourPrinter )
Password and Retype Password: The password of the virtual FTP
Store Directory: /
If you submit these values you’ll be able to scan to your FTP server. Just give it a try! ;-)
I recommend to configure your firewall to drop all packets of your printer that try to leave your own network.
I’m currently developing some GUI stuff and was wondering how to let a JScrollPane scroll automatically if it’s already on the bottom and the size of it’s content increases.
For example if you use a JTextArea to display some log or whatever, than it would be nice if the scroll bars move down while there are messages produced, but it shouldn’t scroll down when the user just scrolled up to read a specific line.
To scroll down to the end of a JTextArea can be done with just setting the carret to the end of the text:
But we first want to check whether the scroll bar is already at the bottom, and only if that’s the case it should automatically scroll down to the new bottom if another message is inserted.
To obtain the position data of the vertical scroll bar on can use the following code:
Unfortunately log.append ("some msg") won’t append the text in place, so the size of the text area will not necessarily change before we ask for the new maximum position. To avoid a wrong max value one can also schedule the scroll event:
As you can see, here a new event is put in the EventQueue, and this event is told to put another event in the queue that will do the scroll event. Correct, that’s a bit strange, but the swing stuff is very lazy and it might take a while until the new maximum position of the scroll bar is calculated after the whole GUI stuff is re-validated. So let’s be sure that our event definitely happens when all dependent swing events are processed.
For example to update the default editor for your system you need to update the alternatives system via:
update-alternatives --set editor /usr/bin/vim
There is also an interactive version available:
update-alternatives --config editor
To see available browsers you need to run
update-alternatives --list x-www-browser
However, the alternatives system is a nice idea I think, but it’s a bit confusing sometimes. And installing a new group or adding another entry to an existing group is pretty complicated and requires information from multiple other commands beforehand.
With galternatives you’ll get a graphical interface to manage all these things. That really brings light into the dark! Just install it via