Bash

Boost Up Productivity in Bash - Tips and Tricks

When spending most of your day around bash shell, it is not uncommon to waste time typing the same commands over and over again. This is pretty close to the definition of insanity. Luckily, bash gives us several ways to avoid repetition and increase productivity. Today, we will explore the tools we can leverage to optimize what I love to call “shell time”.

The Bash Trap Command

  If you've written any amount of bash code, you've likely come across the trap command. Trap allows you to catch signals and execute code when they occur. Signals are asynchronous notifications that are sent to your script when certain events occur. Most of these notifications are for events that you hope never happen, such as an invalid memory access or a bad system call. However, there are one or two events that you might reasonably want to deal with. There are also "user" events available that are never generated by the system that you can generate to signal your script. Bash also provides a psuedo-signal called "EXIT", which is executed when your script exits; this can be used to make sure that your script executes some cleanup on exit.

Bash Shell Games: Let's Play Go Fish!

How to begin developing a computer version of the popular card game. Between the previous 163 columns I've written here in Linux Journal and the dozens of games I programmed and explored during the creation of my Wicked Cool Shell Scripts book, I've written a lot of Bash shell games. The challenge is to find one that's simple enough where a shell script will work, but isn't so simple that it ends up being only a half-dozen lines.

Job Control: the Bash Feature You Only Think You Don't Need

There are basically three types of people in the world: those who know little or nothing about bash job control, those who know enough to believe that it's nothing that they would ever use, and those who can just skim the rest of this post. Now, don't get me wrong, I'm not saying that bash's job control is going to change your world, but there are a couple simple everyday scenarios where job control can be useful, and often, it even can eliminate an "oh crap" moment.

What The @#$%&! (Heck) is this #! (Hash-Bang) Thingy In My Bash Script

  You've seen it a million times—the hash-bang (#!) line at the top of a script—whether it be Bash, Python, Perl or some other scripting language. And, I'm sure you know what its purpose is: it specifies the script interpreter that's used to execute the script. But, do you know how it actually works? Your initial thought might be that your shell (bash) reads that line and then executes the specified interpreter, but that's not at all how it works. How it actually works is the main focus of this post, but I also want to introduce how you can create your own version of "hash-bang" if you're so inclined.

Pattern Matching In Bash

Wildcards have been around forever. Some even claim they appear in the hieroglyphics of the ancient Egyptians. Wildcards allow you to specify succinctly a pattern that matches a set of filenames (for example, *.pdf to get a list of all the PDF files). Wildcards are also often referred to as glob patterns (or when using them, as "globbing"). But glob patterns have uses beyond just generating a list of useful filenames. The bash man page refers to glob patterns simply as "Pattern Matching". First, let's do a quick review of bash's glob patterns. In addition to the simple wildcard characters that are fairly well known, bash also has extended globbing, which adds additional features. These extended features are enabled via the extglob option.

Cat-Proofing Your Screen Locker with Bash

  I have a computer in my bedroom. I also have cats. Unfortunately, cats and screen lockers don't mix well, particularly at night. To be accurate, it's more a problem with the display power management than the actual screen locking. Here's the way it works: I run a script to "shut the lights off at night" (that is, lock the screen and force the display to power down), and that works great, until one of the cats jumps on the desk and causes the mouse to move and turn the display back on. And the cats don't even have to touch the mouse; the slight movement of the desk is enough to cause the mouse to react. Recently, I'd had enough of it and figured there had to be a way to disable the mouse and "refactor" the script.

Bash's Built-in printf Function

  Even if you're already familiar with the printf command, if you got your information via "man printf" you may be missing a couple of useful features that are provided by bash's built-in version of the standard printf(1) command.

Weekend Reading: All Things Bash

Bash is a shell and command language. It is distributed widely as the default login shell for most Linux distributions. We've rounded up some of the most popular Bash-related articles for your weekend reading.   Writing More Compact Bash Code By Mitch Frazier In most programming languages, non-scripting ones at least, you want to avoid uninitialized variables. In bash, using uninitialized variables can often simplify your code.  

More Roman Numerals and Bash

When in Rome: finishing the Roman numeral converter script. In my last article, I started digging in to a classic computer science puzzle: converting Roman numerals to Arabic numerals. First off, it more accurately should be called Hindu-Arabic, and it's worth mentioning that it's believed to have been invented somewhere between the first and fourth century—a counting system based on 0..9 values.

Removing Duplicate PATH Entries: Reboot

  In my first post on removing duplicate PATH entries I used an AWK one-liner. In the second post I used a Perl one-liner, or more accurately, I tried to dissect a Perl one-liner provided by reader Shaun. Shaun had asked that if I was willing to use AWK (not Bash), why not use Perl? It occurred to me that one might also ask: why not just use Bash? So, one more time into the void.

Removing Duplicate PATH Entries, Part II: the Rise of Perl

  With apologies to Arnold and the Terminator franchise for the title, let's look one more time at removing duplicates from the PATH variable. This take on doing it was prompted by a comment from a reader named Shaun on the previous post that asked "if you're willing to use a non-bash solution (AWK) to solve the problem, why not use Perl?" Shaun was kind enough to provide a Perl version of the code, which was good, since I'd have been hard-pressed to come up with one. It's a short piece of code, shorter than the AWK version, so it seemed like it ought to be fairly easy to pick it apart. In the end, I'm not sure I'd call it easy, but it was interesting, and I thought other non-Perl programmers might find it interesting too.

Roman Numerals and Bash

Fun with retro-coding a Roman numeral converter—I head back to my college years and solve me homework anew! I earned a bachelor's degree in computer science back in the dawn of computing. Well, maybe it wasn't quite that long ago, but we did talk about Ada and FORTRAN in class. As a UCSD alumnus, however, it's no surprise that UCSD Pascal was the programming language of choice. Don't worry; no punch cards and no paper tape were involved in my educational endeavors.

What's New in Bash Parameter Expansion

The bash man page is close to 40K words. It's not quite War and Peace, but it could hold its own in a rack of cheap novels. Given the size of bash's documentation, missing a useful feature is easy to do when looking through the man page. For that reason, as well as to look for new features, revisiting the man page occasionally can be a useful thing to do. The sub-section of interest today is Parameter Expansion—that is, $var in its many forms. Don't be confused by the name though, it's really about parameter and variable expansion.

Removing Duplicate PATH Entries

The goal here is to remove duplicate entries from the PATH variable. But before I begin, let's be clear: there's no compelling reason to to do this. The shell will, in essence, ignore duplicates PATH entries; only the first occurrence of any one path is important. Two motivations drive this exercise. The first is to look at an awk one-liner that initially doesn't really appear to do much at all. The second is to feed the needs of those who are annoyed by such things as having duplicate PATH entries.
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