This article explains how to create some fractal plots with graphics.py, a popular graphics library for Python written by John Zelle. Graphics.py is a single file containing graphics functions such as Point, Line, Circle and Rectangle. Elsewhere on this blog, it’s been used to create sinusoidal patterns, including a Mexican hat/blancmange surface. This article focuses on the creation of fractal images.
Gatting the age of a file in days is sometimes useful in scripts.
The date +%s command will give the current time in epoch seconds. It can be handily combined with -r, which causes date to use the modification time of a file rather than the current time. For example:
$ ls -l /etc/hosts
-rw-r--r-- 1 root root 3031 Mar 22 20:23 /etc/hosts
Our hosts file was last modified on March 22nd. Converting that date to "epoch" time, gives us the age of the file (since Jan 1970), in seconds:This article explains how to find the time interval between two Unix time stamps. The default time format in Unix is like this: “Sun 9 Aug 12:50:08 BST 2020”. It appears in many places on the system and is the default produced by the “date” command.
For example, it might appear in application log files, like this::
Start time: Sun 9 Aug 10:05:00 BST 2020 End Time: Sun 9 Aug 12:51:12 BST 2020
This article explains how to subtract one time from the other in Bash, and obtain the intervening time in seconds.
Continue readingA strange message was recently received from a Bash script running under Linux Mint 18:
Unescaped left brace in regex is deprecated, passed through in regex; marked by <-- HERE in m/%{ <-- HERE (.*?)}/ at /usr/bin/print line 528.
Error: no such file "test\n"
Slightly confusing, as it reads like a Perl error, rather than bash. Below is another variety of the same thing
Continue reading
This article explains how to create some simple mathematical shapes with graphics.py, a popular graphics library for Python written by John Zelle. Graphics.py is a single file containing graphics functions such as Point, Line, Circle and Rectangle. In this article though, we are just going to use it to plot single points.
At the top of the page is a blancmange like shape. The program that drew it is at the bottom of the article, if you want to jump straight there. Otherwise, a couple of simpler plots will be demonstrated first, just to show a couple of underlying principles. Continue reading
Most programming languages offer the facility for making large, compound data structures. For example C, Pascal, Perl and Python. A few simple data types are provided, out of which larger structures can be built. A programmer can store data in a whatever way is most suitable for the application.
Often, a simple list or dictionary will be enough. Read the data in, process it, and print the results out. Perfect. But for a larger or more useful application, more data, and more kinds of data, will need to be stored and processed at the same time.
This article demonstrates the building of a complex data structure in Python. Note: it is not about classes, or object oriented programming, just the syntax for handling complex data structures, made up of lists, dictionaries and simple strings and integers. Continue reading
Unix users and administrators will be familiar with the cron, unix’s built in job scheduler. It is a good way of running regular jobs eg backups, system monitoring programs or housekeeping scripts. The configuration of cron is quite particular and care is needed when setting up a new job. Your well tested script can behave differently when it is called from cron. Sometimes the differences won’t matter. But sometimes they do, and finding the cause can be tricky.
This brief article describes how many such problems can be tracked down simply by capturing the standard error output properly. In short, make sure your troublesome cron job is not quietly discarding the very information you need to fix it. Continue reading
A backup script that runs on several Linux systems recently produced the following error:
ls: cannot access '/tmp/sortrq9hq8': No such file or directory
It happens every time the backup runs. Other than the above message, there seems to be no ill effect on the backup, which completes successfully. The ls command did not find a file that it expected to be there.
This article explains how the missing file was created by the Linux sort command as a temporary storage area, how error messages about these files are likely to crop up from time to time, how to reproduce the error, and some background about the behaviour of the sort command on Linux and Solaris. Continue reading
Ansible provides a rich pattern matching ability. Modules like lineinfile can match strings based on regular expressions. Similar expressions are used in Python, Perl and older tools such as egrep, grep, sed and awk.
When attempting to match a string containing awkward characters, an escape mechanism can be used. For example, the dollar character ($) has a special meaning within in a regular expression, being the match for end-of-line. So to match a literal dollar, an escape character, usually a backslash (\), is needed. For example, the regular expression “\$1.65” will successfully match $1.65, without treating $ as end of line.
When processing a string that contains many special characters, the escape syntax can become onerous. One solution is to just “blanket” match the special character, rather than trying to match it precisely. In other words: just use a dot. Continue reading
Nextcloud is an open source software package providing remote file sharing services. It is similar to Dropbox. But with Nextcloud, you retain ownership, security and control of the shared data. This procedure describes how to build a working Nextcloud service using just 3 commands. It was tested successfully on a Raspberry Pi 4 running Raspberry Pi OS 10 (Buster) and Raspberry Pi OS 11 (Bullseye). Article updated 26/3/22.
Note: If you would rather do the installation manually, step-by-step, without the help of a script, please see my previous article “Simple Nextcloud Installation on Raspberry Pi“. It explains how to do the installation in detail, and provides more background information on Nextcloud. Both procedures achieve the same overall result, however.