A fifo(7), or "named pipe" is a character device, that when is opened by one process for reading, and another for writing, allows inter-process communication.

This is similar to how you can use a pipe(7), but does not require both processes to be started by a common ancestor.

This can be demonstrated in two terminals. In the first run:

$ mkfifo test.fifo
$ cat test.fifo

In the second run:

$ cat >test.fifo

Now when you enter lines of text into the second terminal, they will be output on the first.

fifos also have another trick up their sleeves, as when a process opens a fifo, it will block until the other end has been opened.

Using this ephemeral-launch script, we can start an echo server on a random port, and have a client block until the server is ready, and also report which port should be used.

In one terminal run:

$ mkfifo test.fifo
$ ephemeral-launch --port-file test.fifo cat

In a second terminal run:

$ port="$(cat test.fifo)"
$ nc 127.0.0.1 "$port"

Now when you enter lines of text into the second terminal, it will echo them back.

Because the processes block on the open, the cat test.fifo in the second terminal will block until the ephemeral-launch process has bound the port and written it out to the fifo.

Last time, on glorious scripting for whizz kids we discussed chunking your scripts up into function, method, subroutine, or whatever your language might call them. I also left you with a challenge to write as complex/clever a program you could, using only the syntax we'd already discussed. Sadly I hadn't given you one of the most important things you need in order to write useful programs -- namely the ability to make decisions.

The mechanisms of controlling program flow when programming are typically referred to as control structures, and each of Python, Perl, and Lua share some common structure concepts (with moderately different syntax in each case). Also, naturally, each language has some other unique control structures just for themselves.

We're going to use the most fundamental control structure (and most commonly shared one too) in order to extend our cleverness with function to be able to calculate something useful -- fibonacci numbers. For our purposes we'll define the first (1th) and second (2th) numbers in the sequence as both being 1 and that the user of our function will never ask us for the 0th or earlier. We're also going to extend our understanding of function by using a very powerful programming concept called recursion

Python

>>> def fib(n):
...     if n < 2:
...         return 1
...     else:
...         return fib(n-1) + fib(n-2)
...
>>> fib(6)
8
>>> fib(20)
6765
>>>

Perl

$_ sub fib {
>   my $n = shift;
>   if ($n <= 2) {
>     return 1;
>   } else {
>     return fib($n-1) + fib($n-2)
>   }
> }
$_fib(6)
8$_ fib(20)
6765$_

Lua

> function fib(n)
>>   if n <= 2 then
>>     return 1
>>   else
>>     return fib(n-1) + fib(n-2)
>>   end
>> end
> =fib(6)
8
> =fib(20)
6765
>

Challenge

Using the power of the simple control structure exhibited here, and the incredible versatility of recursion, most if not all other control structures and thus programs can be built. Of course that'd often be exceedingly long winded and uncomfortable to do, and as such each of the scripting languages we are discussing have plenty of other control structures. Your challenge is to root out all of the control structures you can find for the language(s) you have chosen, and practice using them ready for our next lesson.

pandoc is a tool for converting documents from one format into another.

You could use it to read an HTML document and convert it to markdown.

I prefer to use it to produce nice PDFs but using markdown instead of LaTeX.

To do this, you probably want both pandoc and texlive-full.

Unfortunately, this can be a rather large thing to install:

$ sudo apt-get install pandoc texlive-full
Reading package lists... Done
Building dependency tree       
Reading state information... Done
…
The following NEW packages will be installed
  aglfn chktex cm-super cm-super-minimal context context-modules
  dvidvi dvipng feynmf fonts-cabin fonts-comfortaa
  fonts-crosextra-caladea fonts-crosextra-carlito fonts-ebgaramond
  fonts-ebgaramond-extra fonts-font-awesome fonts-freefont-otf
  fonts-gfs-artemisia fonts-gfs-baskerville fonts-gfs-bodoni-classic
  fonts-gfs-complutum fonts-gfs-didot fonts-gfs-didot-classic
  fonts-gfs-gazis fonts-gfs-neohellenic fonts-gfs-olga
  fonts-gfs-porson fonts-gfs-solomos fonts-gfs-theokritos
  fonts-hosny-amiri fonts-inconsolata fonts-ipaexfont-gothic
  fonts-ipaexfont-mincho fonts-junicode fonts-lato
  fonts-linuxlibertine fonts-lmodern fonts-lobster fonts-lobstertwo
  fonts-oflb-asana-math fonts-roboto fonts-sil-gentium
  fonts-sil-gentium-basic fonts-sil-gentiumplus fonts-stix
  fonts-texgyre fragmaster lacheck latex-cjk-all latex-cjk-chinese
  latex-cjk-chinese-arphic-bkai00mp latex-cjk-chinese-arphic-bsmi00lp
  latex-cjk-chinese-arphic-gbsn00lp latex-cjk-chinese-arphic-gkai00mp
  latex-cjk-common latex-cjk-japanese latex-cjk-japanese-wadalab
  latex-cjk-korean latex-cjk-thai latexdiff latexmk lcdf-typetools
  libfile-homedir-perl libfile-which-perl libintl-perl libplot2c2
  libpoppler-qt4-4 libpotrace0 libpstoedit0v5 libptexenc1 libsynctex1
  libtexlua52 libtexluajit2 libtext-unidecode-perl libxml-libxml-perl
  libxml-namespacesupport-perl libxml-sax-base-perl
  libxml-sax-expat-perl libxml-sax-perl libzzip-0-13 lmodern m-tx
  musixtex pandoc pandoc-data pfb2t1c2pfb pmx prerex
  preview-latex-style prosper ps2eps pstoedit psutils purifyeps
  tex-common tex-gyre tex4ht tex4ht-common texinfo texlive-base
  texlive-bibtex-extra texlive-binaries texlive-extra-utils
  texlive-font-utils texlive-fonts-extra texlive-fonts-extra-doc
  texlive-fonts-recommended texlive-fonts-recommended-doc
  texlive-formats-extra texlive-full texlive-games
  texlive-generic-extra texlive-generic-recommended
  texlive-humanities texlive-humanities-doc texlive-lang-african
  texlive-lang-arabic texlive-lang-chinese texlive-lang-cjk
  texlive-lang-cyrillic texlive-lang-czechslovak texlive-lang-english
  texlive-lang-european texlive-lang-french texlive-lang-german
  texlive-lang-greek texlive-lang-indic texlive-lang-italian
  texlive-lang-japanese texlive-lang-korean texlive-lang-other
  texlive-lang-polish texlive-lang-portuguese texlive-lang-spanish
  texlive-latex-base texlive-latex-base-doc texlive-latex-extra
  texlive-latex-extra-doc texlive-latex-recommended
  texlive-latex-recommended-doc texlive-luatex texlive-math-extra
  texlive-metapost texlive-metapost-doc texlive-music texlive-omega
  texlive-pictures texlive-pictures-doc texlive-plain-extra
  texlive-pstricks texlive-pstricks-doc texlive-publishers
  texlive-publishers-doc texlive-science texlive-science-doc
  texlive-xetex tipa ttf-adf-accanthis ttf-adf-gillius
  ttf-adf-universalis vprerex
0 to upgrade, 161 to newly install, 0 to remove and 0 not to upgrade.
Need to get 1,782 MB of archives.
After this operation, 3,466 MB of additional disk space will be used.

You may find it convenient to define a simple make rule for turning pandoc markdown documents into PDFs, as follows:

$ cat >Makefile <<'EOF'
%.pdf : %.mdwn
    pandoc -o $@ $<
EOF

Now you may write markdown files, and convert them into PDFs, simply by running make foo.pdf to turn foo.mdwn into foo.pdf.

If you use a PDF viewer such as evince, if you have the document open, and then generate a new version, your viewer will refresh itself to view the new version.

You can make your PDF viewer show live updates to your document by combining this make rule with an inotify watch command:

$ while true; do inotifywait -e close_write,move_self foo.mdwn; make foo.pdf; done

You can write in-line LaTeX to include content that is not expressible in markdown.

markdown also lacks a way of providing metadata, but pandoc has an extension called yaml_metadata_block, which parses the header of a document as a YAML document.

---
title: Foo
author: Joe Bloggs
header-includes:
- \usepackage{bytefield}
geometry: margin=3cm
---
# Title

This is an example bytefield figure:

\begin{bytefield}{16}
  \wordbox{1}{A 16-bit field} \\
  \bitbox{8}{8 bits} & \bitbox{8}{8 more bits} \\
  \wordbox{2}{A 32-bit field. Note that text wraps within the box.}
\end{bytefield}

Without the header, you would otherwise need to write a separate file containing the \usepackage{bytefield} directive, and invoke pandoc with --template=use-bytefield.latex.

You probably know that file names that start with a ., known as dotfiles, will not show when you do ls. What you may not know that the glob * does not match dotfiles either.

This convention exists for historical reasons. In any Unix-like system you will see two entries in all directories, . and ... . represents the current directory and .. represents the parent directory. They exist for ease of navigation. In the early days of Unix somebody decided they were sick of seeing these when doing ls so quickly hacked up some code to hide any files which started with a .. Over time people realised this was a useful hack when creating files you don't always want to see, for example rc-files.

You can see dotfiles with ls by adding the --all flag, -a for short. Slightly neater is the --almost-all flag, or -A for short, which will show all your dotfiles but not show . and ... You can match dotfiles with a glob using .*. .* will show any file starting with a . except . and ... They will be listed in alphabetical order ignoring all leading dots. This will only match dotfiles so if you want to match all files you must combine two globs together, * .*.