Perl – OR Pattern Match Slow, Use Two Patterns Instead

Posted on April 22, 2012 by Jim

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 →

Repairing the MySQL WordPress Database

Posted on March 27, 2012 by Jim

You can recover a running MySQL database with the mysqlcheck command.

After a power cut, I found these errors in /var/log/daemon.log. This was on Debian Squeeze running WordPress 3.3.1 and MySQL 5.1.61.

Mar 22 11:22:16 debian /etc/mysql/debian-start[2153]: Triggering myisam-recover for all MyISAM tables Mar 22 11:22:17 debian mysqld: 120322 11:22:17 [ERROR] /usr/sbin/mysqld: Table './wordpress/wp_commentmeta' is marked as crashed and should be repaired Continue reading →

Process Substitution and Pipes

Posted on March 22, 2012 by Jim

Command substitution is a widely used feature of the Bash and Korn shells, allowing the output of one command to be captured and used in another. Like this:

$ echo "Backup started at $(date)" Backup started at Fri Mar 16 15:35:14 GMT 2012

Command substitution is not to be confused with that less well known (and, to be honest, less useful) shell feature, process substitution. Despite being rarely used, process substitution is worth knowing about, if only because it illuminates other fundamental unix features – the shell, sub processes, named and unnamed pipes.

This post discusses process substitution, command substitution and the vertical bar (|). Three very different shell features, but all making use of unnamed pipes, and so not as different as they first appear. The examples are from Linux but also work on Solaris 10 and, due to the ubiquity of pipes, are likely to work on other unixes too. Continue reading →

Large files / small backups on Debian LAMP

Posted on February 21, 2012 by Jim

Backups on a LAMP server were much smaller than expected, and the cause was excessively large log files. This post explains the apparent paradox and the steps taken to trace and fix the problem.

The server in question runs the usual set of grandfather-father-son backups. Before each backup, the software prints the expected size of the backup into a log file. On other servers the estimate is usually bang-on. On this server, the backup turned out to be about 12 times smaller than the estimate. Not a problem in itself, but puzzling and indicative of something untoward on the system. Continue reading →

Fedora Dual Boot Convoluted Boot Path

Posted on December 18, 2011 by Jim

This post describes a problem with receiving Linux kernel updates after installing Fedora 16 in a dual boot environment. The Fedora installation process allows the user to rewrite Grub in the master boot record (MBR) or to leave the MBR untouched. I chose to leave it untouched, leading to a later kernel update problem which was fairly easily solved with some Grub 2 commands. Continue reading →

Open Source Software and GNU/Linux

Posted on December 18, 2011 by Jim

Introduction

This article discusses some of the workings of open source software with particular reference to the Linux kernel and GNU. It will explain some of the processes involved for those of us who know Linux but are not software engineers. Perhaps you are an Linux administrator or enthusiast, but find yourself unsure of terms such as upstream, downstream, mainline, patch, merge and other words developers love to bandy ? Read on.

Open Source Software

If you are a Linux user, you might be familiar with installing software using yum or apt-get. For example Continue reading →

Simple Locking Wrapper for VI

Posted on November 5, 2011 by Jim

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 →

http_proxy in Red Hat 5

Posted on October 23, 2011 by Jim

This post is about setting the http_proxy environment variable in Red Hat 5. Newer versions of the OS allow the variable to be set in either of two ways, that is:

# export http_proxy=http://192.168.1.100:8080 # export http_proxy=192.168.1.100:8080

Both will work. A subsequent call to yum will read the http_proxy environment variable and act on it, using the named proxy to obtain a network connection to the relevant repository. I have tested this successfully on Red Hat 5.7.

Yum Failures

Older versions of Red Hat 5 are more fussy. RHEL 5.4 will allow the first form above (export http_proxy=http://192.168.1.100:8080), but use the second form and yum will barf extravagantly, leaving you with a lengthy Python trace back. Continue reading →

Sorting a Directory in the FAT File System

Posted on September 24, 2011 by Jim

The venerable fat16 and fat32 file systems are still in widespread use today. Devices such as digital cameras, satellite navigation systems, memory sticks and mp3 players all make use of FAT.

The files in a FAT file system are arranged in a strict order. This can affect the way that some devices behave. For example, some MP3 players will play songs only in the order in which they are arranged on the device, rather than the more convenient alphabetical or alphanumeric order. Music players that use USB memory sticks and in-car USB systems can be affected in the same way. The only way to get the songs to play in a more sensible order is to sort the directory (folder) in which they are located. Continue reading →

Upgrading the Sheevaplug to Debian 6 Squeeze

Posted on July 10, 2011 by Jim

The Sheevaplug is is a very small computer made by Globalscale. It is designed to offer standard computing functions in a very small space. Consuming just a few watts of power, the Sheevaplug makes a good home server, with applications ranging from network storage to web hosting and home security.

This article explains how to upgrade a Sheevaplug to Debian 6 using a pre-built image available from New IT. It is a quick and simple procedure, requiring less technical knowledge than, for example, installing Debian from scratch. Continue reading →

Unix etc.

Unix, Linux and related technologies.

Category Archives: Linux