Linux Fu: Counter Rotate Keys!

June 1, 2018 by Al Williams 22 Comments

If you’ve done anything with a modern Linux system — including most variants for the Raspberry Pi — you probably know about sudo. This typically allows an authorized user to elevate themselves to superuser status to do things.

However, there is a problem. If you have sudo access, you can do anything — at least, anything the sudoers file allows you to do. But what about extremely critical operations? We’ve all seen the movies where launching the nuclear missile requires two keys counter-rotated at the same time and third firing key. Is there an equivalent for Linux systems?

It isn’t exactly a counter-rotating key, but the sudo_pair project — a prelease open-source project from Square — gives you something similar. The project is a plugin for sudo that allows you to have another user authorize a sudo request. Not only do they authorize it, but they get to see what is happening, and even abort it if something bad is happening.

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Linux Fu: File Aliases, Links, And Mappings

March 16, 2018 by Al Williams 37 Comments

Have you heard it said that everything in Linux is a file? That is largely true, and that’s why the ability to manipulate files is crucial to mastering Linux Fu.

One thing that makes a Linux filesystem so versatile is the ability for a file to be many places at once. It boils down to keeping the file in one place but using it in another. This is handy to keep disk access snappy, to modify a running system, or merely to keep things organized in a way that suits your needs.

There are several key features that lend to this versatility: links, bind mounts, and user space file systems immediately come to mind. Let’s take a look at how these work and how you’ll often see them used.

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Linux Fu: Regular Expressions

March 9, 2018 by Al Williams 37 Comments

If you consider yourself a good cook, you may or may not know how to make a souffle or baklava. But there are certain things you probably do know how to do that form the basis of many recipes. For example, you can probably boil water, crack an egg, and brown meat. With Linux or Unix systems, you can make the same observation. You might not know how to set up a Wayland server or write a kernel module. But there are certain core skills like file manipulation and editing that will serve you no matter what you do. One of the pervasive skills that often gives people trouble is regular expressions. Many programs use these as a way to specify search patterns, usually in text strings such as files.

If you aren’t comfortable with regular expressions, that’s easy to fix. They aren’t that hard to learn and there are some great tools to help you. Many tools use regular expressions and the core syntax is the same. The source of confusion is that the details beyond core syntax have variations.

Let’s look at the foundation you need to understand regular expression well.

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Linux Fu: A Little Help For Bash

February 2, 2018 by Al Williams 23 Comments

It isn’t uncommon these days for a programmer’s editor to offer you help about what you are typing, ranging from a pop up with choices to a full-blown code template. If you have written a million lines of code in the language, this might even annoy you. However, if you use it only occasionally, these can be very helpful. I’ve used Unix and Linux for many years, but I realize that there are people who don’t use it every day. With the Raspberry Pi, Linux servers, and Windows 10 having a bash shell, there are more people using a shell “every once in a while” than ever before. Could you use a little help? If so, you might try bashelp: a little something I put together while writing about bash completion.

There’s good news and bad news. The good news is that Unix has a built-in help command — man — and has for some time. The bad news is that you need to stop what you are typing and enter a man command to use it. Man, by the way, is short for manual.

There are GUI front-ends to man (like yelp, on the left) and you can even use a web browser locally or remotely. However, none of these are connected to what you are typing. You have to move to another window, enter your search term, then go back to your typing. That got me to thinking about how to get a sort of context-sensitive inline help for bash.

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Linux Fu: Custom Bash Command Completion

January 19, 2018 by Al Williams 24 Comments

If you aren’t a Linux user and you watch someone who knows what they are doing use Bash — the popular command line interpreter — you might get the impression they type much faster than they actually do. That’s because experienced Linux users know that pressing the tab key will tend to complete what they are typing, so you can type just a few characters and get a much longer line of text. The feature is very smart so you may not have realized it, but it knows a good bit about what you could type. For example, if you try to unzip a file, it knows the expected file name probably has a .zip extension.

How does that happen? At first, you might think, “who cares how it happens?” The problem is when you write a shell script or a program that runs on Linux, the completion gets dumb. Someone has to make Bash smart about each command line program and if you are the author then that someone is you.

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Linux Fu: System Administration Made Easier

November 9, 2017 by Al Williams 47 Comments

Linux can have a somewhat split personality. If you use it as a desktop OS, it has a lot of GUI tools, although sometimes you still need to access the command line. If you use it as a headless server, though, you probably ought to know your way around the command line pretty well. This is especially true if you don’t want to litter up your hard drive (and CPU) with X servers and other peculiarities of the graphical user interface.

Personally, I like the command line, but I am realistic enough to know that not everyone shares that feeling. I’ll also admit that for some tasks — especially those you don’t do very often — it is nice to have some helpful buttons and menus. There are several administration tools that you might be interested in using to handle administration tasks on your Linux machines. I’m going to look at two of them you might want to experiment with that both use a Web browser to provide their interface.

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Linux Fu: X Command

September 21, 2017 by Al Williams 44 Comments

Text-based Linux and Unix systems are easy to manipulate. The way the Unix I/O system works you can always fake keyboard input to another program and intercept its output. The whole system is made to work that way. Graphical X11 programs are another matter, though. Is there a way to control X11 programs like you control text programs? The answer to that question depends on exactly what you want to do, but the general answer is yes.

As usual for Linux and Unix, though, there are many ways to get to that answer. If you really want fine-grained control over programs, some programs offer control via a special mechanism known as D-Bus. This allows programs to expose data and methods that other programs can use. In a perfect world your target program will use D-Bus but that is now always the case. So today we’ll look more for control of arbitrary programs.

There are several programs that can control X windows in some way or another. There’s a tool called xdo that you don’t hear much about. More common is xdotool and I’ll show you an example of that. Also, wmctrl can perform some similar functions. There’s also autokey which is a subset of the popular Windows program AutoHotKey.

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