PulseAudio is what most modern Linux distributions use as a sound server, the part of the sound subsystem that sits between applications and the sound driver supporting features like allowing multiple applications to output sound simultaneously. PulseAudio adds various features not present in other Linux sound servers like per-application volume controls and easily outputting to different audio devices (for instance, using HDMI audio instead of the normal audio jack).

PulseAudio can be controlled using pavucontrol, which is a GUI audio mixer. It shows a volume meter and control for every application producing sound as well as an option to choose which audio device it is outputting to. It additionally lists all of the hardware input and output devices, as you would expect from an audio mixer.

Fixing problems#

Restarting PulseAudio#

If PulseAudio is not working properly, you can restart it by running

$ killall -9 pulseaudio

No, really, that's what Debian's PulseAudio page says to do.

When I initially installed PulseAudio, it didn't have my sound cards listed and just had the default null outuput, making it not very useful. Running that command to restart it fixed it.

Muted devices#

PulseAudio seems to mute my sound card all by itself. Currently, I just go into pavucontrol and unmute it.

X forwarding#

Normally with X, it's easy to run an application on a remote computer just by using X forwarding:

local:~$ ssh -X host
host:~$ echo $DISPLAY
localhost:20.0
host:~$ xterm

The xterm will appear on your local computer.

But if you want to continue working with that application on a different remote computer (or once you are physically in front of the computer it is running on), then you're out of luck.

For an application running in the terminal, you can start it inside a GNU Screen (or tmux) session which you can detach and then attach to again on another ssh connection.

GNU Screen for X#

xpra (X Persistent Remote Applications) lets you move graphical applications from one computer to another in addition to fixing other problems with X forwarding. If you instead use xpra for the forwarding, then you can detach and reattach to the session at will.

Dice in shell#

Today I was borrowing a board game from the lending library at Emerald City Comicon and it was missing its dice. We could have gotten some physical dice somewhere, but instead we decided to use the materials we had on hand. The people I was playing with agreed that we did not want to drain our phone batteries by using a dice app on our phones, but I had a laptop with me. So I wrote a dice app for the shell:

while true
do
    reset
    seq 1 6 | shuf -n1
    seq 1 6 | shuf -n1
    read
done

This rolls two six-sided dice every time you hit enter and clears the screen before showing the result using reset.

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

X clipboard#

The X Window System, the basis for the GUI on most desktop Linux systems, defines how the clipboard works for copying and pasting between applications in Linux. One notable quark of X clipboard is that there's actually two clipboards in common use: the one you expect explicitly accessed via Copy and Paste menu items or key shortcuts called the CLIPBOARD and another one where you copy by selecting text and paste by pressing the middle mouse button called the PRIMARY selection.

X clipboard utilities#

Occasionally it is useful to be able to read or write the clipboard at the command-line. For most uses, your terminal emulator's copy and paste options are probably enough. The primary use case I have for using a command-line program to interact with the clipboard is when I am uploading a file as a Gist:

<file xclip

The xclip utility will copy the contents of the file onto the clipboard (PRIMARY, not CLIPBOARD, by default) and then I can paste it on the Gist website.

My system also has xsel which is very similar to xclip. Wikipedia actually lists several such programs, including the unfortunately named xcopy, not to be confused with XCOPY.

GNU Screen copy mode#

GNU Screen provides its own clipboard for copying information between the different windows of a screen session. ctrl+a, [ enters copy mode. In copy mode you can move the cursor using the arrow keys and page up/page down keys. Screen keeps a history (of configurable size), so you can scroll back pretty far. In fact, I use Screen's copy mode far more often for viewing the history in a terminal than for actually copying anything. You can exit copy mode either by using esc to cancel or enter once to mark the start of the selection and again to mark the end of it. Once you have copied something, ctrl+a, ] pastes the contents of the clipboard.

Vim plugins#

The real power of Vim and the other text editor comes not from their complicated key combinations, but their programmability. Vim uses its own language Vim script while Emacs uses a dialect of Lisp called Emacs Lisp. Both have large communities who have authors many plugins.

A great source for Vim plugins is [Vim.org][vim-scipts], which also has a wiki about Vim with additional information on some of the plugins and lots of tips on how to better use Vim.

Plugin managers#

While, Vim does have basic support for plugins via .vba files, also known as vimball archives, it's easier to use a plugin manager. I recommend Vundle, which is easy to install and use.

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

Calculating π the hard way#

In honor of Pi Day, I was going to try to write a script that computed π in shell, but given the lack of floating point support, I decided it would be too messy. If you want to see hard to follow code to generate π, I highly recommend the IOCCC entry westley.c from 1998, the majority of which is an ASCII art circle which calculates its own area and radius in order to estimate π. The hint file suggests looking at the output of

$ cc -E westley.c

The 2012 entry, endoh2 is also a pretty amazing π calculator.

Getting π#

Instead, I will just generate π the shell way: using another program.

$ python -c 'import math; print(math.pi)'
3.14159265359

The problem#

After using Vim as my primary editor for a while, I find myself trying to use vi-style keyboard shortcuts in other contexts. Usually resulting in :w in middle of whatever I was writing as saving is a natural thing to do when pausing.

There's a few ways to fix this:

1. Get used to the fact that not everything supports vi-style keyboard shortcuts.
2. Change the keyboard shortcuts of the programs I do use.
3. Use Vim for everything.

The first option is suboptimal because vi-style keyboard shortcuts are very useful. Luckily, in many cases there's ways to get them.

The goal#

Given a series of images, present them in a video with a blend effect between the images. The frames between the input frames should gradually transition between the previous and next image. This, and other transition effects, could likely be implemented using a slideshow generator, but it can also be done quite easily using a shell script.

The script#

The final script I wrote is blend.sh. The following commands fetch and run it:

$ wget https://gist.githubusercontent.com/dperelman/2b4b86233aa13d13c0ab/raw/91c7988e27288af8aeb25ade11d0cab90355702f/blend.sh
$ chmod +x blend.sh
$ ./blend.sh slideshow.mkv first.png second.jpg third.gif
$ mplayer slideshow.mkv

The input images may be in any format. The extension of slideshow.mkv will be used by FFmpeg to guess the desired video format (H.264 in a Matroska Multimedia Container for .mkv).