Possibly the last blog post

It's been a long journey. I would still define myself a beginner in the immensity of the Linux world, but I surely learned a few things. There's a lot which I did not share here and the list of blog post ideas was growing too big, while available time to write them was getting too short, so: I am regrettably closing this blog down. I hope the old posts will be helpful for visitors in the future. Thank you for your attention!

$RANDOM

I recently discovered a variable in BASH which is going to stick as one of my favourite variables (everybody has one).
That is the $RANDOM variable.

It generates a pseudorandom number between 0 and 32767. This is an awkward choice of limits, but it can be adjusted by just taking a modulus:

$ echo $((RANDOM%2))

outputs 0 or 1, or:

$ echo $((RANDOM%10))

outputs a number between 0 and 9.

This inspired me to do some simple calculations on random numbers which you can find in my other blog post.
If you want the details of what and how I did them (apologies if most of it is just quick and dirty scripting), keep reading!


I produced the random distribution of points in 2D, by creating two lists of numbers (doubles) between 0 and 1, joining them and use gnuplot to plot them:

$ for i in {1..5000} ; do echo "$((RANDOM%10000)) / 10000" | bc -l; done > random.txt
$ sort --random-sort random.txt > random2.txt
$ paste random.txt random2.txt > randomjoin.txt
gnuplot> plot 'randomjoin.txt' using 1:2 pt 7 ps 0.4 notitle

Funnily enough, the uniform distribution - which I generated by applying a small "wobble" to a grid of evenly spaced points - was more complicated to produce:

$ for i in {0..70} ; do for j in {0..70}; do echo "$i $j" ; done ; done | awk '{print $1/70 " " $2/70}' > uniform.txt
$ sed 's/\./\.00/g' random.txt > wobble1.txt
$ sed 's/\./\.00/g' random2.txt > wobble2.txt
$ paste wobble1.txt wobble2.txt > wobblejoin.txt
$ paste uniform.txt wobblejoin.txt > uniformjoin.txt
$ awk '{print $1-$3 " " $2-$4}' uniformjoin.txt > uniformwobble.txt
gnuplot> plot 'uniformwobble.txt' using 1:2 pt 7 ps 0.4 notitle

The 1D plots have been generated with the same data and some re-binning tricks in gnuplot:

gnuplot> binwidth=0.1
gnuplot> bin(x,width)=width*floor(x/width)
gnuplot> plot 'random.txt' using (bin($1,binwidth)):(1.0) smooth freq with boxes notitle


For the coin tosses example, I generated the sequences using some BASH:

$ for i in {1..100000} ; do for i in {1..15}; do echo -n $((RANDOM%2)); done | sed 's/0/H/g' | sed 's/1/T/g' && echo; done > tosses.txt

And analyzed, counting the repetition (or better, the non-repetitions) in it with:

$ echo "100000 - `grep -v 'HHH' tosses.txt | grep -v 'TTT' | wc -l`" | bc -l

As a bonus, the same set can also be used to explore the number of head (or tails) for each occurrence. Given that for a fair coin there is a 50% chance of getting head (and tails), the result should be having about half heads and half tails for most cases. Below are listed the number of instances (first column) for a given number of heads (second column):

$ while read line ; do echo $line | grep -o H | wc -l ; done < tosses.txt > freq.txt
$ more freq.txt | sort | uniq -c

      3       0
     51      1
    312     2
   1359    3
   4189    4
   9105    5
  15338   6
  19608   7
  19806   8
  15175   9
   9145    10
   4095    11
   1393    12
    362     13
     55      14
      4       15

This is a binomial distribution centered around 7.5, as expected.

Use sed to retreive links from a Firefox bookmark file

Sed is a great text manipulation tool. It allows to manipulate text through simple substitution commands.

It did occur to me that I had a few backup .json files in which Firefox bookmarks were stored. As I did not want to overwrite the existing bookmarks, but I wanted to retreive just a few links, I tried to open the file with a text editor.

The result was not pleasant:

I decided to use some CLI magic to transform this in a list of URLs which is easily readable, instead. It can be done in one line:

$ sed "s/\"uri\"\:\"/\\`echo -e '\n\r'`/g" bookmarks.json | sed "s/\"[^@]*$//" > new_bookmarks.txt

Where:   bookmarks.json -> bookmark file name
            new_bookmarks.txt -> new file name where the links will be saved

Enjoy!