Linux Fu: Better Bash Scripting

It is easy to dismiss bash — the typical Linux shell program — as just a command prompt that allows scripting. Bash, however, is a full-blown programming language. I wouldn’t presume to tell you that it is as fast as a compiled C program, but that’s not why it exists. While a lot of people use shell scripts as an analog to a batch file in MSDOS, it can do so much more than that. Contrary to what you might think after a casual glance, it is entirely possible to write scripts that are reliable and robust enough to use in many embedded systems on a Raspberry Pi or similar computer.

I say that because sometimes bash gets a bad reputation. For one thing, it emphasizes ease-of-use. So while it has features that can promote making a robust script, you have to know to turn those features on. Another issue is that a lot of the functionality you’ll use in writing a bash script doesn’t come from bash, it comes from Linux commands (or whatever environment you are using; I’m going to assume some Linux distribution). If those programs do bad things, that isn’t a problem specific to bash.

One other limiting issue to bash is that many people (and I’m one of them) tend to write scripts using constructs that are compatible with older shells. Often times bash can do things better or neater, but we still use the older ways. For example:

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Linux-Fu: Running Commands

One of the things that makes Linux and Unix-like systems both powerful and frustrating is that there are many ways to accomplish any particular goal. Take something simple like running a bunch of commands in sequence as an example. The obvious way is to write a shell script which offers a tremendous amount of flexibility. But what if you just want some set of commands to run? It sounds simple, but there are a lot of ways to issue a sequence of commands ranging from just typing them in, to scheduling them, to monitoring them the way a mainframe computer might monitor batch jobs.

Let’s jump in and take a look at a few ways you can execute sequences from bash (and many other Linux shells). This is cover the cron and at commands along with a batch processing system called task spooler. Like most things in Linux, this isn’t even close to a complete list, but it should give you some ideas on ways to control sequences of execution.

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Free as in Beer, or the Story of Windows Viruses

Whenever there’s a new Windows virus out there wreaking global havoc, the Linux types get smug. “That’ll never happen in our open operating system,” they say. “There are many eyes looking over the source code.” But then there’s a Heartbleed vulnerability that keeps them humble for a little while. Anyway, at least patches are propagated faster in the Linux world, right?

While the Linuxers are holier-than-thou, the Windows folks get defensive. They say that the problem isn’t with Windows, it’s just that it’s the number one target because it’s the most popular OS. Wrong, that’d be Android for the last few years, or Linux since forever in the server space. Then they say it’s a failure to apply patches and upgrade their systems, because their users are just less savvy, but that some new update system will solve the problem.

There’s some truth to the viruses and the patching, but when WannaCry is taking over hospitals’ IT systems or the radiation monitoring network at Chernobyl, it’s not likely to be the fault of the stereotypical naive users, and any automatic patch system is only likely to help around the margins.

So why is WannaCry, and variants, hitting unpatched XP machines, managed by professionals, all over the world? Why are there still XP machines in professional environments anyway? And what does any of this have to do with free software? The answer to all of these questions can be found in the ancient root of all evil, the want of money. Linux is more secure, ironically, at least partly because it’s free as in beer, and upgrading to a newer version is simply cheaper.

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EV3DEV Lego Linux Updated

The ev3dev Linux distribution got an update this month. The distribution targets the Lego EV3 which is a CPU Lego provides to drive their Mindstorm robots. The new release includes the most recent kernel and updates from Debian 8.8. It also contains tools needed for some Wi-Fi dongles and other updates.

If you haven’t seen ev3dev before, it is quite simply Linux that boots on the EV3 hardware using an SD card. You don’t have to reflash the computer and if you want to return to stock, just take out the SD card. You can also use ev3dev on a Raspberry Pi or BeagleBone, if you like. There’s a driver framework included for handling sensors, motors, and other items using the file system.

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VIM Normalization

Linux users–including the ones at the Hackaday underground bunker–tend to fall into two groups: those that use vi and those that use emacs. We aren’t going to open that debate up again, but we couldn’t help but notice a new item on GitHub that potentially negates one of the biggest complaints non-vi users have, at least for vim which is the most common variant of vi in use on most modern systems. The vim keybinding makes vim behave like a “normal” editor (and to forestall flames, that’s a quote from the project page).

Normally vi starts out in a command mode that it calls normal mode. Pressing a key will execute an editing command, unlike most other modern editors which just insert characters into the open file. For example, pressing x will delete a character. This surprises most people who aren’t familiar with vi. In all fairness, there are other older editors that work this way, but they usually were not screen-oriented.

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Btrfs for the Pi

File systems are one of those things that typical end users don’t think much about. Apparently, [seaQueue] isn’t a typical end user. He’s posted some instructions on how to run an alternate file system–btrfs–on the Raspberry Pi.

The right file system can make a big difference when it comes to performance and maintainability of any system that deals with storage. Linux, including most OSs for the Raspberry Pi, uses one of the EXT file systems. These are battle-hardened and well understood. However, there are other file systems, many of which have advanced features superior to the default file system for some applications.

Btrfs, often pronounced “butter eff ess”, begin life at Oracle and was born from an idea in an IBM paper. It offers advanced features like pooling, snapshots, and the ability to fuse multiple devices into one logical device. One notable feature the file system offers is copy-on-write. That means file copies can share common blocks as long as they stay common.  Compression is available, as is seeding a file system with read-only storage, which could be very useful in some embedded systems. You can also configure several types of RAID using nothing but btrfs. You can see a video presentation about features of btrfs below.

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