Akom's Tech Ruminations

Various tech outbursts - code and solutions to practical problems

Linux Getting a systemd unit to read your .bashrc file for its environment

Posted by Admin • Thursday, June 11. 2015 • Category: Linux
Although it'd be nice to have all of your services not rely on their shell environment, sometimes it is unavoidable. In my case, some of the systems are still on init.d (redhat <=65, centos <=65, etc), but some are on systemd (7.0's, suse 12, etc). The old init scripts rely on the .bashrc file. In order to make the service run consistently on both flavors, I had to teach systemd to read the user's bashrc file. This is what it looks like.

# Process the normal environment files for this user by starting a login shell
# and outputting it all in a temp file.  This makes it compatible
# with the non-systemd init scripts that still rely on .bashrc
ExecStartPre=/bin/bash --login -c 'env > /tmp/.magic-environment-file'
The "-" before the filename seems to make systemd ignore the error if the file is not there (for example, on the first run of the pre command). Now, obviously, you could just set your ~some-user/.bashrc as the EnvironmentFile - but your mileage will vary. systemd will not interpolate variables and it will ignore lines starting with "export"...

Linux Wifi doesn't work after resume from suspend in Ubuntu 15.04 (Dell Latitude E4310)

Posted by Admin • Sunday, May 17. 2015 • Category: Linux
Although toggling the hardware Wi-Fi switch usually fixed this issue, it was decidedly annoying for the non-technical user of this laptop. Googling for a well-known solution turned up several, but none of these worked. I found that either running
nmcli r wifi off
nmcli r wifi on
usually worked, so I tried to stick that into /etc/pm/sleep.d - but that script was never used. I then discovered that in 15.04 Ubuntu switched to systemd, which requires a service file in /etc/systemd/system/ In the end, it turns out that the problem is intermittent, therefore it is not possible to simply toggle networking blindly and hope for the best. Half the time it will not work. The ultimate solution turned out to be a combination of a toggle and a check, and both could be done via network manager, as follows:

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Linux Building RPMs for an older version of CentOS

Posted by Admin • Thursday, November 20. 2014 • Category: Linux
If you build RPMs on Centos 6.X (6.5 in my case) and then try to install them on Centos 5.X (5.10 in my case), bad things happen. Ironically, bad things happen even though my RPM contains a single jar file, and is thus entirely platform independent. Here is what I see:
Running rpm_check_debug
ERROR with rpm_check_debug vs depsolve:
rpmlib(FileDigests) is needed by my-rpm-1.0.x86_64
rpmlib(PayloadIsXz) is needed by my-rpm-1.0.x86_64
If you google for a solution, most people suggest running a virtual with Centos 5 just so you can build the RPMs, but this is apparently not necessary in this simple case, as you can simply specify a few flags. Basically, Centos 5 can't decompress the default archive format, and doesn't support the new digest algorithm.

Since I am using maven-rpm-plugin to build RPMs, my modifications look as follows:


                      <!-- skipping irrelevant items -->

                            <!-- don't strip jar files, it takes forever and is useless -->
                            <defineStatement>__os_install_post %{nil}</defineStatement>
                            <!-- for Centos6 -> Centos5 forwards compatibility -->
                            <defineStatement>_source_filedigest_algorithm md5</defineStatement>
                            <defineStatement>_binary_filedigest_algorithm md5</defineStatement>
                            <defineStatement>_source_payload w9.bzdio</defineStatement>
                            <defineStatement>_binary_payload w9.bzdio</defineStatement>

If you're using fpm or rpmbuild, you can just take these lines and make them %define's in your spec.

Linux Backing up cPanel 11 hosted account with wget and dav/rsync

Posted by Admin • Thursday, August 21. 2014 • Category: Linux
I want to back up my hosting account regularly by retrieving everything onto my box somewhere else (my home server). By regularly, I mean every day. I want this to happen automatically. cPanel makes that hard to do, but there is always a way to script things.

First, let's break down what we want to back up:
  1. Files
  2. Databases and email forwarders
Files are fairly easy. Obviously we don't want to use the full backup functionality of cPanel because we'd be transferring your entire storage space each and every time, even if nothing has changed. In order to do it efficiently, you have a few options:
  1. FTP account using recursive wget (create an FTP account with required access and teach an ftp client of your choice to recursively transfer everything. Hopefully this client skips unchanged files)
  2. WebDAV using rsync (this is what I'm using). Just mount, back up, unmount
Databases and Email stuff is not as easy, as we do have to log into cPanel. The trick to logging in to cPanel is:
  1. Submit your login to the log-in page and save cookies
  2. Parse the resulting file, find the backup link which includes your session name in the URL and hit that
  3. Only accept .gz files (DB backups and email stuff), but avoid hitting /logout, and don't start spidering the entire website
So, how do we do this? Here is the plan

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Linux mssh Bash completion

Posted by Admin • Friday, February 28. 2014 • Category: Linux
Clusterssh (cssh) is great, but I was getting a little fed up with unmanageable terminal windows. They either go all over my monitors or get lost, and they are hard to move and resize.

So I switched to mssh, which solves all that because all the terminals are in one window.... but, it doesn't read my /etc/clusters file! In fact, nobody seemed to even know what file it does read.

What to do? First of all, for the record, it reads its aliases (-a) from ~/.mssh_clusters This file is exactly like /etc/clusters, except for a colon, like so:
alias1: host host host
alias2: host host host

Now that we got that straight, let's make a bash completion file

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Linux Upgrading pfSense fails due to low disk space

Posted by Admin • Tuesday, October 22. 2013 • Category: Linux
I have an embedded installation of pfSense 2.0.2-RELEASE running from a 512MB compact flash card via an IDE adapter. At the time this was what I had on hand, and it seemed like plenty of space - but now that I tried to upgrade to 2.1, the process failed because there wasn't enough room to download the new image...

I tried all sorts of ways including both webconfigurator upgrade and trying to scp the image to the box, but either way there simply wasn't enough space. "OK" I said, "let me install 2.1 on a new (larger) card and restore the config.xml" .... bad idea. The firewall becomes unbootable when you do this - I think that no upgrade processing is performed on the configuration, it is assumed to be a match for your software version. Not to mention that I tried a 4GB card, but the old Pentium III running the firewall can't handle anything over 2GB!

Ultimately, this is the process:
  1. Download and install the same version as what you're running on the larger card
  2. Restore your config.xml backup onto the new instance of pfSense. You can do this through webconfigurator, or if you are (like me) running on a temporary rig without multiple NIC's, you can scp it over manually
  3. Confirm that the system boots, but don't make any changes (to the interfaces, for instance). Put the card into your real system
  4. Now you can use the auto-updater as normal, and your old config.xml will be upconverted

Linux amixer toggle mutes but does not unmute

Posted by Admin • Monday, July 22. 2013 • Category: Linux
It seems that the common approach to mute the audio in Ubuntu 13.04 is amixer -q sset Master toggle ... which works great when muting, but fails to unmute. This command is, for example, the default for openbox's handling of XF86AudioPlay shortcut.

The problem, it would appear, is that muting the Master channel causes other channels (eg Headphone and Front on my machine) to be muted as well. I have no idea why this works this way and why it doesn't undo what it did... and, frankly, I don't really care that much. It's much easier to code around this, see the following workaround script:

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Linux motion drops MySQL support, but you can get around it with a shell script

Posted by Admin • Monday, September 24. 2012 • Category: Linux
I use motion as a surveillance system for a sizable office (9 cameras). Motion is good. But the current package of motion in Ubuntu 12.04 is no longer compiled with MySQL support (used to have it in 10.04). Compiling motion from source really didn't sound appealing... but I do want to add events to a database for my own in-house motion history browser. What to do?

Motion supports command execution on events, such as on_movie_end and on_picture_save... which makes it really easy to do a one-liner like this:
on_movie_end bash -c "echo insert into security(camera, filename, frame, file_type, time_stamp, event_time_stamp, event_id) values('%t', '%f', '%q', '%n', '%Y-%m-%d %T', '%C', '%v') | mysql -uUSER -pPASS"
if you'd like to do something fancier, you can break it out into a shell script:


logger -t test "$0 executing with $*"
sql="insert into security(camera, filename, frame, file_type, time_stamp, event_time_stamp, event_id) values('$1', '$2', '$3', '$4', '$5', '$6', '$7')"
echo $sql | mysql -u$U -p$P motion
and invoke like this:
on_movie_end U=USER P=PASS bash /usr/local/sbin/motion-insert-event.sh '%t' '%f' '%q' '%n' '%Y-%m-%d %T' '%C' '%v

(same for on_picture_save, if needed). You can also daisy-chain commands with semicolon if you have multiple. Also, note that the query above is for my table, yours may have different columns. The rest are my specific examples of usage:

Continue reading "motion drops MySQL support, but you can get around it with a shell script"

Linux Splunk high CPU utilization starting July 1, 2012

Posted by Admin • Thursday, July 5. 2012 • Category: Linux
This has to be written down for posterity: My single server installation of splunk started using all of the CPU about then. Nagios caught it, but I had no time to debug it. I tried disabling indexes, apps, and inputs - none of that made any difference. I reduced an index size, which again had no effect. I cleaned it out and installed it fresh (empty database), only to see the now familiar steady CPU usage.

In desperation I went to open a question on splunk support forums where it suggested I read a related thread which in fact had the solution: http://splunk-base.splunk.com/answers/52109/universal-forwarder-high-cpu-after-leap-second-correction Apparently the culprit is the leap year second inserted at midnight on June 30. Why that makes splunk go nuts I'm not quite sure, but i did see a warning about it in my dmesg:
[3073222.768708] Clock: inserting leap second 23:59:60 UTC

Apparently this is the same problem that took down Amazon. The solution involves running a perl command to alter the system date ever so slightly in the absence of ntp.

Linux VirtualBox Extension Pack on Ubuntu Precise Pangolin 12.04

Posted by Admin • Thursday, May 31. 2012 • Category: Linux
Not sure why this so unintuitive, but here goes:
  1. You want to use USB 2.0 with Virtual Box on Ubuntu 12.04 (or recent version
  2. You're told to install the extension pack from this page
  3. The download is not the same version as your version of virtual box itself
  4. It doesn't work

Do not despair:
  1. Shorten the extension pack download URL to http://download.virtualbox.org/virtualbox/
  2. Find a directory matching your version ( dpkg -i virtualbox )
  3. Download the extension pack


Linux Autodesk Maya 2012 in Virtualbox

Posted by Admin • Wednesday, February 22. 2012 • Category: Linux
Maya does not appear to have a Linux version, so where I have a single machine, I want to run it in VirtualBox - in either Windows XP or Windows 7. I've successfully installed it in both, 32 and 64 bit, and it installs and starts up just fine. The fun comes later. There are two serious problems:
  1. Alt+Mouse doesn't work. Well, this has to do with my window manager more than anything - Alt+Mouse is usually used in Linux. I'm not willing to change that - but fortunately, Alt-Shift+Mouse does something fairly similar in Maya
  2. 3D repainting is completely messed up. In fact, there are two things that don't work immediately - selection on the 3d pane (pointer appears to be below the screen) and the Spacebar popup menu (also below the screen). Fortunately, this can be fixed with a bit of sacrifice: turn off 3d (I turned off 3d and 2d) acceleration in Virtualbox settings.

  3. And it works! (not horribly slow either)

Linux Using dual widescreen monitors on old hardware with Ubuntu

Posted by Admin • Tuesday, February 21. 2012 • Category: Linux
My primary development machine is probably 10 years old - it's a Thinkpad T42 in a docking station (Dock II). This is fine - it's fast enough for most purposes, it runs dual monitors, and it uses very little power so I can leave it on at all times.

Not so fine once I got a 24" monitor to go with the standard 20".

First, the DVI hardware in the docking station can't handle high resolutions - it worked fine at 1280x1024, but not so great at 1920x1080. That's OK, I can drive the widescreen with VGA, I can't tell the difference. But now everything on the screen repaints very, very slowly - I mean really slowly - 1/2 a frame per second. This is bad. OK so the video chip in the laptop wasn't intended to drive a combined 3200 x 1080 resolution - but what can I do about it?

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Linux Figuring out the card number of a cheap SAA7130 capture card

Posted by Admin • Sunday, June 19. 2011 • Category: Linux
So you bought yourself a cheap Philips SAA7130 (or SAA713X or SAA7134) capture card to record motion, you plugged it in and you are getting a black or grey screen. Looking in dmesg you see the famous saa7134[0]: Huh, no eeprom present (err=-5)? And
saa7134: Congratulations! Your TV card vendor saved a few saa7134: cents for a eeprom, thus your pci board has no saa7134: subsystem ID and I can't identify it automatically saa7134: I feel better now. Ok, here are the good news: saa7134: You can use the card= insmod option to specify saa7134: which board do you have. The list:
What do you do? How do you try 175 card numbers and figure out which one works? Write a script!

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Linux Backing up cPanel without hitting logout

Posted by Admin • Thursday, April 21. 2011 • Category: Linux
cPanel based hosting presents some challenges for automatic backups - there isn't an automatic way of creating local backups or any standard way of triggering their creation remotely. Numerous scripts exist yet none were quite the solution I was looking for - I am primarily interested in the databases and mail forwarders, though files wouldn't hurt either. Moreover - you can do this in a single line!

My Goal therefore is: Create and retrieve backups nightly. I don't want cPanel to push files to my off-site box, I'd rather initiate everything remotely and not have to open up access to some other system. cPanel does permit one to do this using a browser, hence it can be scripted. Really, scripting isn't even necessary - wget is all that is required! That said, I had a very hard time convincing wget not to visit the logout link on each page - once you log out, you're not getting anywhere anymore. So here is how I did it.

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Linux MySQL replication dies with fatal error 1236: Client requested master to start replication from impossible position

Posted by Admin • Friday, February 4. 2011 • Category: Linux
This time, my master-master setup stopped replicating with no explanation at all - running START SLAVE yielded no warnings or errors. Inspecting the log yielded the aforementioned error (on Ubuntu, mysql logs to /var/log/syslog by default):
[ERROR] Error reading packet from server: Client requested master to start replication from impossible position ( server_errno=1236) [ERROR] Got fatal error 1236: 'Client requested master to start replication from impossible position' from master when reading data from binary log [4654]: 110204 10:09:20 [Note] Slave I/O thread exiting, read up to log 'mysqld-bin.000742', position 4274

So at first I thought this was a weird bug or a stunnel issue, but looking at 'mysqld-bin.000742' on the master it turns out that it's only 3980 bytes - position 4274 is indeed impossible. But how did it get this way?

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