Category Archives: Coding

Set Up Your Own Link Shortening Service with a Raspberry Pi

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“Link shortening” happens when a short URL, such as http://bit.ly/2bo3XYY, points to the same web page as a longer link, such as https://en.wikipedia.org/wiki/BBC.  Short links are often used where there are a limited number of characters available, such as an SMS text or a Twitter post.  Short links are also quicker to type and neater than the associated full length links.

Two of the main providers of short links are Bitly and Google (Goo.gl).  For example, I used Bitly to create the short link in the above paragraph.  However, if you have a Raspberry Pi (or any kind of Linux server), you don’t need to use a provider.  You can create your own short links.  This article explains how. Continue reading →

Perl: Sort Hash Values by Key with a Hash Slice

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In Perl, hash (associative array) sorting is a common and easy practice. Sorting values by key is easy. And so is sorting by value. But how do you sort the values of a hash by key? One answer is to use a hash slice. Continue reading →

List Virtual Machines on ESXi

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ESXi is popular hypervisor product from VMware. It comes with several management GUIs including Vsphere and VCenter. Command line tools are also bundled, though they are used more rarely. This article describes a short script to list all virtual machines on the system.

The script is below. It is intended to run under the Busybox shell, the default environment when you ssh directly into the system hosting ESXi. Continue reading →

Simple Picture Gallery on Raspberry Pi

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The Raspberry Pi is a small Linux computer designed to help children learn programming. Being a full Linux System, it can also be used as a server or as the basis for various projects. The Pi’s low power consumption makes it particularly suited to the role of always-on web server.

This post describes how to create a simple photo gallery on the Pi, which can be shared over the internet with or without password protection. While not as polished as Flickr, Smugmug or similar services, it allows you to retain ownetship, control and security of the shared images. Continue reading →

Quick Script to Find Duplicate Files

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Here’s a quick script to show duplicate files on Linux. It should cope with arbitrary spaces in file names, and to save time and CPU resources, it will checksum only files of the same size.

Usage: Save the script to dups.sh or whatever, then run it with no arguments. A list of duplicated files is output.

Continue reading →

Perl – OR Pattern Match Slow, Use Two Patterns Instead

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A handy feature of regular expressions is their ability to “or” match. Searching for two strings in a file is easy with a construct like “egrep ‘root|uucp’ /etc/passwd”. The vertical bar (“|”) acts as an “or” operator. Perl supports the vertical bar too, and the same match could be achieved in Perl thus:

bash-4.2$ perl -n -e 'print $_ if /root|uucp/' < /etc/passwd root:x:0:0:root:/root:/bin/bash uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin

Weirdly though, this construction is up to 10 times slower than 2 separate matches performing the same search, as can be shown with a quick demonstration. Continue reading →

Simple Locking Wrapper for VI

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This post presents a quick way to add file locking to the vi editor.

The vi editor is found on pretty much every unix system, which means it is often used by system administrators to update configuration files. Unfortunately vi does not lock the file being edited. This can lead to two or more people inadvertently editing the same file at the same time. Edits can become duplicated, confused or lost entirely, perhaps leaving the file in a non-working state.

Editing Critical Files

It is bad enough in the case of small files such as yum.conf or ntp.conf. More serious are files like /etc/fstab, where a simple syntax error could lead to an unbootable system. Worst of all are network-wide resources like DNS zone files. A mistake here could mess up more than one machine. Continue reading →