Messing Around with Graphics.py

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

Complex Data Structures in Python

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

Bash Script Behaves Differently When Called From Cron

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

How to Identify the Build Time of a Red Hat System

To determine when a Red Hat or CentOS server was first built, use rpm -qi basesystem:

[fred@rhel7 ~]$ rpm -qi basesystem
Name        : basesystem
Version     : 10.0
Release     : 7.el7
Architecture: noarch
Install Date: Thu 16 Apr 2015 18:16:04 CEST    <----- here
Group       : System Environment/Base
Size        : 0
License     : Public Domain
Signature   : RSA/SHA256, Tue 01 Apr 2014 15:23:16 CEST, Key ID 199e3a91fd554c52
Source RPM  : basesystem-10.0-7.el7.src.rpm
Build Date  : Fri 27 Dec 2013 18:22:15 CET
Build Host  : xxxxxxxxxx
Relocations : (not relocatable)
Packager    : Red Hat, Inc. 
Vendor      : Red Hat, Inc.
Summary     : The skeleton package which defines a simple Red Hat Enterprise Linux system
Description :
Basesystem defines the components of a basic Red Hat Enterprise Linux
system (for example, the package installation order to use during
bootstrapping). Basesystem should be in every installation of a system,
and it should never be removed.

Note: The date returned, 16th April in the example above, reflects the time that the operating system was installed. This is usually the same as the system creation time, except in cases where a system has been cloned or built from a pre-installed image. In the last case, the date returned will reflect the build date of the source image, rather than the target system.

/tmp/sortXXXXXX files

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

How to Convert a Website from HTTP to HTTPS

An http website is not encrypted. That is to say, the data comprising the site is not encrypted as it flows from the web server to the device (pc, phone, tablet) on which the user is viewing the page. Anyone able to “listen in” on the network could read that data, which is a security risk. An https web site is different. Data is encrypted. The web server encrypts each web page before transmission, and the user’s browser decrypts it, providing end-to-end protection from eavesdropping.

This article explains how to convert an existing basic website to https by obtaining a free digital certificate from Let’s Encrypt. It is based on a Raspberry Pi running the “Apache” web server, but will also work on other Linux systems. It is intended for home users and people running small-scale web sites, and as a learning aid. Continue reading

Ansible: Match Special Characters Without Escaping

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

Automatic Nextcloud Installation on Raspberry Pi

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 Nexcloud service using just 3 commands.

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. Continue reading

SSH Proxying and Agent Forwarding

SSH allows secure connections from one host to another. All traffic is encrypted. Authentication is usually by means of a key pair, where the private key resides on your local machine, and the public key is imported to the remote system. SSH keys have become particularly important for cloud computing, where users need to access cloud servers over a potentially hostile Internet.

Sometimes, the requirement is to access one system via another. You “hop” through the first system to reach the second. The following article shows how to do that, in a secure way, without having to place a private SSH key onto the middle system. Continue reading

Renaming a Bitbucket Repository

Bitbucket is a paid-for version of Github. Along with Jira and Confluence, it forms the Atlassian framework, a suite of devops tools in widespread use.

Using the Bitbucket web interface, a repository can easily be renamed. However, this causes a change in the URL, which breaks the link from existing clones of the repo. They can be deleted and re-cloned, or renamed. This post explains how to do the rename. Continue reading