The problem#

My system for automatically posting future-dated blog posts mysteriously stopped working recently. The posts would appear if I manually published the blog, but not with the automatic scheduling mechanism.

The solution#

In schedule_publish.sh, I changed the line

echo "$0" | at -q g $time

to

if [ "$(date -d "$time PST" +'%s')" -ge "$now" ]
then
    echo "$0" | at -q g -t "$(date +'%Y%m%d%H%M' -d "$time PST")"
fi

(where "PST" is the timezone of this blog; adjust as appropriate for your blog). $now is initialized with

now="$(date +'%s')"

before the call to make publish to avoid a race condition.

The details#

The problem#

I recently replaced my old Nvidia graphics card with a newer one. Upon booting up, I ran glxgears to test that 3D graphics were working properly and got an error like

X Error of failed request:  BadWindow (invalid Window parameter)
 Major opcode of failed request:  155 (NV-GLX)
 Minor opcode of failed request:  4 ()
 Resource id in failed request:  0x1200003
 Serial number of failed request:  34
 Current serial number in output stream:  34

The solution#

Either delete /etc/X11/xorg.conf or edit it and remove (or comment out) the "Files" section; that is, the lines

Section "Files"
    ...
EndSection

Cowardly tar#

$ tar c
tar: Cowardly refusing to create an empty archive
Try 'tar --help' or 'tar --usage' for more information.

That's what the GNU implementation of tar says. For comparison, Debian includes FreeBSD's implementation of tar in the bsdtar package as the bsdtar command:

$ bsdtar c
bsdtar: no files or directories specified

Clingy lynx#

In the text-only web browser lynx (not to be confused with links), if you press q to quit, it asks

Are you sure you want to quit? (y)

Most keys quit the program, but if you decide not to quit and press n for no, lynx shows the message

Excellent!!!

which goes away after a couple seconds allowing you to continue browsing.

The problem#

The Xbox 360 controller has become the defacto standard controller in PC gaming in recent years, likely due to both the popularity of the Xbox and the fact that the controller can easily be used with a computer. One downside of this is that some games assume you have one. If the game supports it and is running through Steam, then Steam's controller settings will let you use any controller, but that doesn't work for all games, and you might not be using Steam. The game that prompted this blog post actually does have Steam controller support promised in the future, but it's in early access and they are busy developing other parts of the game.¹

xboxdrv#

The solution is xboxdrv, the userspace Xbox controller driver. In addition to supporting actual Xbox controllers, it can also simulate Xbox controllers based on inputs from other devices like a PlayStation controller or some less common controller.

Linux has a tendency to give very unhelpful error messages when it is unable to create a file. I previously blogged about a few different reasons Linux might report a disk is full, but all of the reasons included the disk actually not having space for more files. Yet another reason to get similar errors is if the partition is mounted readonly (ro):

$ mount | grep -F /usr
/dev/sdc2 on /usr type ext4 (ro,nodev,noatime,data=ordered)

mount without any options lists all of the mounted partitions along with their mount options.

Many programs will show a helpful error message:

$ touch test
touch: cannot touch ‘test’: Read-only file system

But some others won't:

rtorrent: Could not lock session directory: "./session/", held by "<error>".

That error is normally caused by ./session/rtorrent.lock not being writable due to being held by another process, but in this case it's not writable due to the filesystem being readonly. rtorrent doesn't distinguish the two.

For that reason, when running into weird behavior from a program on Linux, it's a good idea to check that the directories the program might try to write to are actually writable.

The error#

I was having trouble with a USB joystick adapter (an EMS Playstation controller adapter, to be specific). When I plugged it in, it wouldn't work and checking dmesg showed the same error getting generated over and over again (at least once per second):

$ dmesg
...
[81700.968873] usbhid 6-1:1.0: can't add hid device: -71
[81700.968885] usbhid: probe of 6-1:1.0 failed with error -71
[81700.968986] usb usb6-port1: disabled by hub (EMI?), re-enabling...
[81700.968991] usb 6-1: USB disconnect, device number 53
[81701.208025] usb 6-1: new low-speed USB device number 54 using uhci_hcd
[81701.384866] usb 6-1: string descriptor 0 read error: -32
...

The wrong fix#

I decided the sensible thing to do was to reload the driver:

$ sudo modprobe -r usbhid  # Bad idea, don't run this

The problem#

Trying to install the python-numpy package in Debian Unstable ("Sid") I got the following error on the liblapack3 package (seen also in this bug):

$ sudo apt-get install -f
Reading package lists... Done
Building dependency tree       
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 36 not upgraded.
4 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Setting up liblapack3 (3.5.0-4) ...
update-alternatives: error: alternative liblapack.so.3gf can't be slave of liblapack.so.3: it is a master alternative
dpkg: error processing package liblapack3 (--configure):
 subprocess installed post-installation script returned error exit status 2
dpkg: dependency problems prevent configuration of python-numpy:
 python-numpy depends on liblapack3 | liblapack.so.3; however:
  Package liblapack3 is not configured yet.
  Package liblapack.so.3 is not installed.
  Package lapack3 which provides liblapack.so.3 is not installed.
  Package atlas3-base which provides liblapack.so.3 is not installed.
  Package liblapack3 which provides liblapack.so.3 is not configured yet.
  Package libatlas3-base which provides liblapack.so.3 is not installed.

The solution#

Run the following command to fix the error:

$ sudo update-alternatives --remove-all liblapack.so.3gf

And then rerun the install:

$ sudo apt-get install -f

The problem#

$ ls > file

doesn't do what you expect:

$ touch foo
$ touch bar
$ ls > filelist
$ cat filelist
bar
filelist
foo

You probably didn't expect, or want, filelist to be listed in filelist.

The solution#

$ filelist=$(ls); echo "$filelist" >filelist

The problem#

When developing code or creating visual artifacts in non-WYSIWYG systems, it is very useful to constantly be aware of the output of the compiler and the appearance of the artifact you are creating, whether it is a GUI, a chart, a graph, or a paper. The common way of doing this is to have an IDE specialized for the system you are using; for example, LyX provides a WYSIWYG editor for LaTeX. Similarly, there may be plugins for your text editor to support whatever kind of development you are doing. On the other hand, we can use the shell to create a solution independent of the text editor and the availability of plugins for the particular system being developed for.

In my previous post on SSH multiplexing, I gave the following to add to your ~/.ssh/config file without explaining what it actually means:

Host *
ControlMaster auto
ControlPath ~/.ssh/connections/%r_%h_%p

The documentation for ~/.ssh/config can be found at ssh_config(5). Four options are relevant to this post:

Host

The config file is broken up in to sections based on which hosts the configuration options apply to. Host * means these options apply to all connections. If you wanted the options to apply only when connecting to example.com, you could change that line to Host example.com.

Actually, you can also limit configuration options by things other than just the host using the Match directive. For example, this configuration has options for connecting with the username git, presumably due to having multiple git servers that use that username.

ControlMaster

Tells ssh to use multiplexing. Specifically, the default is no, which means it will look for an already open master connection. To actually open a master connection, yes or ask can be used, the latter means that a password prompt will appear when connecting to that master connection. The more useful options for a config file are the auto and autoask options which will use an already open connection if exists, but fall back to acting like yes and ask respectively otherwise.

ControlPath

In order to connect to the master connection, ssh needs a way to communicate with it. This is handled by the master creating a Unix socket which future ssh instances look for. Unix sockets are an IPC mechanism which allows two processes on the same machine to communicate via a connection initiated by one process creating a socket identified by a filename and another using that special file to connect. In comparison with TCP, every server needs its own port number that the client needs to know and any client can connect as long as it knows the port number.

Unix sockets are identified by filenames and ControlPath specifies the filename to use for the socket The %r, %h, %p parts mean the filename should include the remote username, hostname, and port number in order to identify which ssh session is which.

This should usually be enough, but if your home directory is shared among multiple computers, as is common in some university and other large organization setups, then you will also need %l to identify which host you are connecting from. Otherwise ssh may get confused by master connections created by a different host. Luckily, ssh provides a shortcut, which is the %C option which is a hash of all 4 (although it is not available on older versions of ssh):

ControlPath ~/.ssh/connections/%C

or, if you are a disto which does not have %C yet like the latest Ubuntu LTS:

ControlPath ~/.ssh/connections/%L_%h_%p_%r

I used %L for the short version of the local hostname (for example, if %l is foo.example.com, %L would be just foo) because when I used %l, my system complained the filename was too long.

Keep in mind that the socket file is security critical because it is used to piggyback on your existing ssh sessions without authenticating (unless you use the ask or autoask options for ControlMaster), so make sure your ~/.ssh/connections/ directory is readable only by you:

chmod 700 ~/.ssh/connections

ControlPersist

Not used above, the ControlPersist option lets you control when the master connection actually closed. When set to no, it closes with the initial connect. When set to yes it stays open until explicitly closed with ssh -O exit. It can also be set to a length of time to stay open after the last connection is closed.

While the default of ControlPersist is not clearly stated in the documentation, I checked the source code to confirm it does default to no: the default value is set to 0 here if it is still set to its initial value of -1, which is the same value it is given if the configuration file says no.