Display Git’s and Current Directory on Terminal Bar

I typically have more than ten windows open on my desktop and rely on their names to select them. Being a command-line aficionado, most of them are terminals. I have them configured to display the current directory by setting the bash PROMPT_COMMAND environment variable to 'printf "\033]0;%s:%s\007" "${HOSTNAME%%.*}" "${PWD/#$HOME/~}"'. The problem is that the directory I’m often in has a generic name, such as src or doc, so the terminal’s name isn’t very useful.

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The Psychology of the AWS Outage

Unless you’ve been living on another planet, you’re certainly aware that over the past couple of hours Amazon’s AWS S3 service has experienced a serious outage, which has affected thousands of sites and services around the world. For reasons I will elaborate in this post, the coverage of this outage has been blown completely out of proportion. So, what’s the difference between the perceived risk associated with the AWS outage and the actual risk of this outage?

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The Road to Debugging Success

A colleague recently asked me how to debug a Linux embedded system that crashed in the Unix shell (and only there), when its memory got filled through the buffer cache. He added that when he emptied the buffer cache the crash no longer occurred.

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Debugging PCSecrets Synchronization

A reader of my Effective Debugging book commented that debugging is learned through experience. I think he’s partly right, so I’ll periodically describe here techniques and tools I use when debugging. A problem I faced today was the inability of the PC-based PCSecrets program to sync with the Secrets for Android counterpart. Here is how I troubleshot and solved the problem.

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As it Happened: Leap Second 37

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Netdata on a Raspberry Pi

A couple of days ago I had the privilege to see a demo of netdata by Costa Tsaousis, the person behind this project. The project offers comprehensive real time monitoring of a Linux computer with low overhead in a single easily-installed and self-contained system. I thought this was too good to be true, but the number of users and installations hinted that this could well be the case. I therefore decided to install the system on a Raspberry Pi I’m configuring to replace an ancient 20 year old IBM PS/2 server.

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Debugging a File Synchronization Problem

In Effective Debugging I write that if a web search doesn’t return you any useful results, then maybe you’re barking at the wrong tree. Here’s an example.

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Service Orchestration with Rundeck

Increasingly, software is provided as a service. Managing and controlling the service’s provision is tricky, but tools for service orchestration, such as Rundeck, can make our lives easier. Take software deployment as an example. A well-run IT shop will have automated both the building of its software using tools like make, Ant, and Maven and the configuration of the hosts the software runs on with CFEngine, Chef, or Puppet (see the post “Don’t Install Software by Hand”). Furthermore, version control tools and continuous integration will manage the software and the configuration recipes, handling developer contributions, reviews, traceability, branches, logging, and sophisticated workflows. However, these tools still leave a gap between the software that has been built and is ready to deploy, and the server that has been configured with the appropriate components and libraries and is ready to run the software.

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Developing in the Cloud

Running a top-notch software development organization used to be a capital-intensive endeavor, requiring significant technical and organizational resources, all managed through layers of bureaucracy. Not anymore. First, many of the pricey systems and tools that we developers need to work effectively are usually available for free as open source software. More importantly, cheap, cloud-based offerings do away with the setup, maintenance, and user support costs and complexity associated with running these systems. Here are just a few of the services and providers that any developer group can easily tap into (you can find many more listed here):

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How to Create Your Own Git Server

Although I'm a happy (also paying) user of GitHub's offerings, there are times when I prefer to host a private repository on a server I control. Setting up your own Git server can be useful if you're isolated from the public internet, if you're subject to inflexible regulations, or if you simply want features different from those offered by GitHub (and other similar providers). Setting up a Git server on a Unix (Linux, Mac OS X, *BSD, Solaris, AIX) machine isn't difficult, but there are many details to observe. Here is a complete guide.

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Virtualize Me

The virtual machine (VM) is the most dazzling comeback in information technology. IBM implemented a VM platform architecture in the late 1960s in its CP/CMS operating system. The company’s goal was to provide the time-sharing capabilities that its batch-oriented System/360 lacked. Thus a simple control program (CP) created a VM environment where multiple instances of the single-user CMS operating system could run in parallel. Thirty years later, virtualization was rediscovered when companies like VMware found ways to virtualize the less accommodating Intel x86 processor architecture. The popularity of Intel’s platform and the huge amount of software running on it made virtualization an attractive proposition, spawning within a decade tens of proprietary and open source virtualization platforms.

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Don't Install Software by Hand

An IT system’s setup and configuration is a serious affair. It increasingly affects us developers mainly due to the proliferation and complexity of internet-facing systems. Fortunately, we can control and conquer this complexity by adopting IT-system configuration management tools.

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Why are AWS Command-Line Tools so Slow?

Amazon's Elastic Compute Cloud command-line tools are useful building blocks for creating more complex shell scripts. They allow you to start and stop instances, get their status, add tags, manage storage, IP addresses, and so on. They have one big disadvantage: they take a long time to run. For instance, running ec2-describe-instances for six instances takes 19 seconds on an m1.small AWS Linux instance. One answer given, is that this is caused by JVM startup overhead. I found that hard to believe, because on the same machine a Java "hello world" program executes in 120ms, and running ec2-describe-instances --help takes just 321ms. So I set out to investigate, and, using multiple tracing tools and techniques, this is what I found.

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Package Management Systems

DLL hell was a condition that often afflicted unfortunate users of old Microsoft Windows versions. Under it, the installation of one program would render others unusable due to incompatibilities between dynamically linked libraries. Suffering users would have to carefully juggle their conflicting DLLs to find a stable configuration. Similar problems distress any administrator manually installing software that depends on incompatible versions of other helper modules.

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Using the HP 4470c Scanner Under Windows 7

Neither Hewlett Packard nor Microsoft Windows 7 offer native support for my HP 4470c scanner. Throwing a working scanner away to buy a new one only because some software was missing seemed like a waste, so I looked for an alternative solution. This is how I made it work using SANE, an open source framework for scanners.

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Sane vim Editing of Unicode Files

Being able to use plain alphabeitc keys as editing commands is for many of us a great strength of the vi editor. It allows us to edit without hunting for the placement of the various movement keys on each particular keyboard, and, most of the time, without having to juggle in order to combine particular keys with ctrl or alt. However, this advantage can turn into a curse when editing files using a non-ASCII keyboard layout. When the keyboard input method is switched to another script (Greek in my case, or, say, Cyrillic for others) vi will stop responding to its normal commands, because it will encounter unknown characters. Here is how I've dealt with this problem.

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Batch Files as Shell Scripts Revisited

Four years ago I wrote about a method that could be used to have the Unix Bourne shell interpret Windows batch files. I'm using this trick a lot, because programming using the Windows/DOS batch files facilities is decidedly painful, whereas the Bourne shell remains a classy programming environment. There are still many cases where the style of Unix shell programming outshines and outperforms even modern scripting languages.

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Useful Polyglot Code

Four years ago I blogged about an incantation that would allow the Windows command interpreter (cmd) to execute Unix shell scripts written inside plain batch files. Time for an update.

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The Risk of Air Gaps

As some readers of this blog know, from this month onward I'm on a leave of absence from my academic post to head the Greek Ministry of Finance General Secretariat of Information Systems. The job's extreme demands explain the paucity of blog postings here. I'll describe the many organizational and management challenges of my new position in a future blog post. For now let me concentrate on a small but interesting technical aspect: the air gap we use to isolate the systems involved in processing tax and customs data from the systems used for development and production work.

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Madplay on an Intel Mac

Numerous MP3 players around my house pull music from a central file server. The hardware I'm using is extremely diverse and many devices can nowadays be politely described as junk: they include 100MHz Pentiums with 16MB RAM, and an ARM-based prototype lacking support for floating point operations. For the sake of simplicity I've standardized the setups around a web server running on each machine to list static HTML pages containing the available music files, and simple shell-based CGI clients that invoke madplay to play the music. When I added an Intel-based Mac to the mix I found that madplay refused to work, producing only a white noise hiss.

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Parallelizing Jobs with xargs

With multi-core processors sitting idle most of the time and workloads always increasing, it's important to have easy ways to make the CPUs earn their money's worth. My colleague Georgios Gousios told me today how the Unix xargs command can help in this regard.

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A Well-Tempered Pipeline

I am studying the use of open source software in industry. One way to obtain empirical data is to look at the operating systems and browsers used by the Fortune 1000 companies by examining browser logs. I obtained a list of the Fortune 1000 domains and wrote a pipeline to summarize results by going through this site's access logs.

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Monitor Process Progress on Unix

I often run file-processing commands that take many hours to finish, and I therefore need a way to monitor their progress. The Perkin-Elmer/Concurrent OS32 system I worked-on for a couple of years back in 1993 (don't ask) had a facility that displayed for any executing command the percentage of work that was completed. When I first saw this facility working on the programs I maintained, I couldn't believe my eyes, because I was sure that those rusty Cobol programs didn't contain any functionality to monitor their progress.

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An Inadvertent Denial of Service Attack

If you're wondering why this blog was down for the past few hours, here is the story. In an earlier blog post I listed a small script I'm using to lock-away door knockers who attempt to break into our group's computer by trying various passwords. If you like puzzles, read the script again and think how it could be used against us by isolating our computer from the entire world.

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Suspend Windows from the Command Line

I used to leave my computer up all night, but I've come to realize that this is ecologically unsound. Now I suspend it before going to sleep, but this missed running a daily job that used to run at 03:00 am. The job marks my students' exercises and send me email with the next day's appointments. I thus decided to schedule the task to wakeup my computer at 3:00 am, run the job, and then suspend it again. The Windows scheduler allows you to specify a wakeup option, but not a subsequent suspend. Furthermore, it seems that Windows lacks a way to suspend from the command line (while maintaining the ability to hibernate), and the only free tools on the web are distributed in executable form, so I ended writing a small tool myself.

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LTO Tape Drive Compression Considered Harmful

I used to think that tape drive compression was a silly marketing trick used by manufacturers to inflate the advertised capacity of their tape drives. Apparently it is worse than that.

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The Relativity of Performance Improvements

Today, after receiving a 1.7MB daily security log message containing thousands of ssh failed login attempts from bots around the world, I decided I had enough. I enabled IPFW to a FreeBSD system I maintain, and added a script to find and block the offending IP addresses. In the process I improved the script's performance. The results of the improvement were unintuitive.

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The Memory Savings of Shared Libraries

A recent thread in the FreeBSD ports mailing list discusses the benefits and drawbacks of static builds. How can we measure the memory savings of shared libraries?

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Handling Traffic

In an earlier blog entry I described ACM's imaginative way to handle web site downtime. Today I noticed that the web site of the Berlin Philharmonic uses an equally imaginative (and low-tech) way to handle excessive web traffic.

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The Treacherous Power of Extended Regular Expressions

I wanted to filter out lines containing the word "line" or a double quote from a 1GB file. This can be easily specified as an extended regular expression, but it turns out that I got more than I bargained for.

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Location-Based Dictionary Attacks

I get daily security reports from the hosts I manage. Typically these contain invalid user attempts for users like guest, www, and root. (Although FreeBSD doesn't allow remote logins for root, I was surprised to find out that many Linux distributions allow them.)

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Handling Downtime

Ideally web sites should be up on a 24 by 7 basis. This is however a difficult and often an expensive proposition. Today I saw on the ACM Portal site an innovative alternative.

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Breaking into a Virtual Machine

Say you're running your business on a rented virtual private server. How secure is your setup? I wouldn't expect it to be more secure than the system your server runs on, and a simple experiment confirmed it.

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I Spy

Knowledge is power.

—Sir Francis Bacon

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A Humbling Upgrade

Yesterday I upgraded one of the servers I maintain from FreeBSD 4.11, which had reached its end of life, into the latest production release 6.2. It was a humbling experience.

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Software Rejuvenation is Counterproductive

In the February issue of the Computer magazine Grottke and Trivedi propose four strategies for fighting bugs that are difficult to detect and reproduce. Retrying an operation and replicating software are indeed time-honored and practical solutions. When coupled with appropriate logging, they may allow an application to continue functioning, while also alerting its maintainers that something is amiss. On the other hand, the proposal to restart applications at regular intervals (rejuvenation as the authors call it), doesn't allow us to find latent bugs, sweeping them instead under the carpet. This lowers the bar on the quality we expect from software, and will doubtless result in a higher density of bugs and increasingly complicated failure modes.

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Open Source and Professional Advancement

Doing really first-class work, and knowing it, is as good as wine, women (or men) and song put together.

— Richard Hamming

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What Can System Administrators Learn from Programmers?

Although we often hear about program bugs and techniques to get rid of them, we seldom see a similar focus in the field of system administration. This is unfortunate, because increasingly the reliability of an IT system depends as much on the software comprising the system as on the support infrastructure hosting it.

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Efficient Human Multitasking

I sometimes hear colleagues complaining that they can't get anything done, because they have too many tasks in their head. I've found that in order to increase the efficiency of my work I need a moderately large selection of pending tasks. This allows me to match the type of work I can do at a given moment with a task in the most optimal way.

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Batch Files as Shell Scripts

Although the Unix Bourne shell offers a superb environment for combining existing commands into sophisticated programs, using a Unix shell as an interactive command environment under Windows can be painful.

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Surprising Findings on Software Reuse

Kevin DeSouza and his colleagues in a recent article in the Communications of the ACM published some surprising findings regarding software reuse: reuse happens more by novices rather than by experts, within projects rather than across them, and in transient teams rather than permanent ones. The statement regarding the higher propensity of rookies to reuse compared to older professionals rang particularly true to my ears.

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Project Asset Portability

It's said that real computer scientists don't program in assembler; they don't write in anything less portable than a number two pencil. Joking aside, at the end of the 1970s, the number of nonstandard languages and APIs left most programs tied to a very specific and narrow combination of software and hardware. Entire organizations were locked in for life to a specific vendor, unable to freely choose the hardware and software where their code and data would reside. Portability and vendor independence appeared to be a faraway, elusive goal.

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Hard Disk Failure

I tell everybody that the question is not whether your hard drive will fail, but when it will fail. My laptop's drive started emmitting a loud grinding sound last Saturday.

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System administration stories: The Revolt

Can a small embedded system the size of a paperback lead a group of machines into revolt? Apparently yes.

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Detective Work and Dropped TCP Connections

I had problems with TCP connections (mostly long-lasting ssh sessions) getting dropped on my ADSL line. In the end, I found that the problem had two different roots. The detective work behind establishing them is, I believe, interesting. It also shows how accessible source code, and the will to use it, can be a tremendous boost to difficult system administration problems.

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Optimizing ppp and Code Quality

The Problem

While debugging a problem of my ppp connection I noticed that ppp was apparently doing a protocol lookup (with a file open, read, close sequence) for every packet it read. This is an excerpt from the strace log, one of my favourite debugging tools.

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