vncserver won't start on CentOS 5

CentOS/RHEL has a vncserver script which is configured to start instances for users specified in /etc/sysconfig/vncservers. The format is documented in the comments in that file.

I copied this from a host that I was making a copy of, ran /etc/init.d/vncserver start and it failed. Running the script again with sh -vx /etc/init.d/vncserver start showed me that it was failing on the first user at this line:

runuser -l ${USER} -c "cd ~${USER} && [ -f .vnc/passwd ] && vncserver :${DISP} ${VNCUSERARGS}"

Of course! I had forgotten to copy the .vnc/passwd file for each user across so the servers were failing to start. If this is a new machine, then you need to create passwords for each user first. It's a pity that the script doesn't inform you what really happened.

As an aside, I modified this line:

[ "$RETVAL" -ne 0 ] && break

to:

[ "$RETVAL" -ne 0 ] && echo "Warning: ${USER}:${DISP} not started"

so that the script would not die at the first user that caused an error.

But the script is not very sophisticated and doesn't allow for example starting VNC for a single user that you've added to the configuration.

You don't need iTunes

Well, with a provocative title like that I should explain that I have tricked you a bit. You don't need iTunes to reset a first generation iPod Shuffle (gumstick) if it has gone out of its mind. Mine was flashing the green and orange LEDs alternately.

The usual advice is to install iTunes and to repair the iPod filesystem. However if you don't mind resetting everything, and there is little to lose on a 512MB or 1GB Shuffle, you can go straight to this reset utility from Apple and not have to download and install the big iTunes software package. At the same time this will update the software to the final version released. As the article states, you may need to try a few times. Mine worked on the second attempt.

I don't think this tip has a large audience as most first generation Shuffles are dead now as those models were released in 2005. Mine's still going strong though. I wonder when the flash memory and rechargeable cells will wear out.

Incidentally I can highly recommend the open source shuffle DB software. With this Python script installed in the Shuffle, and a Python interpreter on the host, you don't need complicated tune management software. Just copy the files onto the Shuffle and then run the program to build a playlist that the Shuffle will understand.

Is Trackerbird maxing out your CPU? Here's how to remove it

After my recent OS upgrade I found that Thunderbird would pop up a dialog box every now and then saying that a script had stopped responding. A top command showed Thunderbird was consuming all of one CPU. I have multiple cores but this would be a problem for people with a single core, and in addition isn't good for power consumption. Also the status bar at the bottom showed either thousands of items remaining to be indexed, or Initializing...

A search showed somebody with the same issue. My situation was worse. Even though I disabled Trackerbird from the Addons Manager, I could not shut Thunderbird down properly to run without this addon. Every time I started up Thunderbird, it said I had to restart to disable Trackerbird. In addition, there was no Remove button. A search showed that the addon is a system one, and comes from tracker-miner-thunderbird. Removing the package with zypper fixed the problem once and for all. No more items remaining to index, no more maxing out one CPU. The package may be named different in your distro; mine is openSUSE.

I surmise that some of my mailboxes, being POPS or IMAPS connections that are not always connected, caused Trackerbird to spin its wheels trying to index the mailboxes.

As a personal gripe, why do Linux desktops foist indexing tools upon me without asking me if I want my data indexed or not? I have no use for indexing since I know where my documents are. I've also disabled akonadi, nepomuk and strigi because they were eating up CPU for no benefit to me. Answers to me on the back of a postage stamp.

sox, rec, and play stopped working with ALSA?

I upgraded my openSUSE installation from 12.1 to 12.2 recently and to my chagrin, my audio recording script stopped working. I use references to ALSA devices such as hw:1. The error message was cryptic:

rec FAIL formats: can't open output file `hw:1': can not open audio device: Invalid argument

After a lot of searching, I found the answer. The base cause is that in libsox2, which openSUSE 12.2 uses instead of libsox1, has separated out the Linux specific ALSA support into a separate shared object which is not loaded unless you set the environment variable AUDIODRIVER=alsa. So this is what I had to do in my script:

export AUDIODRIVER=alsa
export AUDIODEV=hw:1
rec ... program.wav

You are supposed to be able to use -t alsa in the options of rec but I had no luck with that.

Similarly for the play command which is just a link to sox like rec.

If you are just using the default sound device, you may not hit this bug as it may fall back to pulseaudio support. It's only when you must target a specific ALSA sound card that this bug manifests itself.

Here is the bug report that was the crucial clue.

Blacklist a command in bash

I have an alias called nf. Sometimes due to fat fingers I end up typing mf and this starts up Metafont, which is installed because I use TeX, then I would have to exit it. I got tired of this and added this alias to $HOME/.alias:

alias mf='echo "Use \\mf if you really want metafont"'

If I really want to run mf, which is rarely, I can type \mf at the command line as the \ stops alias expansion. Invocations from shell scripts and Makefiles are not affected as $HOME/.alias is only read in by interactive shells.

BTW, please do not use this technique to block shell users from executing certain commands by aliasing them to something else. It's trivial to bypass in just the way I've shown.

cd to directory containing a file

Many times I have run a program on a file and then immediately, wanted to change to the directory containing it, to do more operations. For example:

vim path/to/some/dir/file

then I next want to do:

cd path/to/some/dir

I used to do

cd !$

and then use the up arrow key to get the previous failed command to edit out the trailing filename and re-execute. Then I decided I should write a shell alias or function to do this. It has to be an alias or a function and not a script as cd needs to work in the current shell.

This is what I came up with:

cdf() { cd ${1%/[^/]*}; }

This uses the remove matching suffix operation of parameter substitution, see the bash manual page for details. So now I can do:

cdf !$

and I will end up in the directory containing the file I just worked on.

I picked cdf as an abbreviation for change to directory of file, but you may prefer some other name.

VirtualBox host/guest conflct over audio device

I started a VirtualBox instance on my workstation that contains a CentOS instance to develop software. A bit later on I found this error from a periodic cron job I run on the workstation to record a radio program using the sound card:

rec FAIL formats: can't open input `hw:0': snd_pcm_open error: Device or resource busy

That error message comes from the rec program, part of the sox package. The first time it happened I thought the audio hardware had locked up so I rebooted it and the error went away. The next morning it happened again. I realised that the reason it went away the first time was because the CentOS guest had been shutdown by the reboot.

I disabled the audio device in the guest OS and had no more failures recording from the sound card. In general if you do not need audio in the guest, disable the device so that the guest does not interfere with the host OS's use of it.

You can also disable by editing the XML config fie, but only when the guest OS is not running. This is the relevant line:

<AudioAdapter controller="AC97" driver="ALSA" enabled="false"/>