A 6502, In The Shell

March 17, 2025 by Jenny List 8 Comments

Shell scripting is an often forgotten programming environment, relegated to simple automation tasks and little else. In fact, it’s possible to achieve much more complex tasks in the shell. As an example, here’s [calebccf] with an emulated 6502 system in a busybox ash shell script.

What’s in the emulator? A simple 6502 system with RAM, ROM, and an emulated serial port on STDIO. It comes with the wozmon Apple 1 monitor and BASIC, making for a very mid-1970s experience. There’s even a built-in monitor and debugger, which from our memories of debugging hand-assembled 8-bit code back in the day, should be extremely useful.

Although the default machine has a generous 32k of RAM and 16k ROM, you can easily adjust these limits by editing machine.sh. In addition, you can get a log of execution via a socket if you like. Don’t expect it to run too fast, and we did have to adjust the #! line to get it to run on our system (we pointed it to bash, but your results may vary).

What you use this for is up to you, but we’re sure you’ll all agree it’s an impressive feat in the shell. It’s not the first time we’ve seen some impressive feats there, though. Our Linux Fu column does a lot with the shell if you want further inspiration.

Linux Fu: Use The Source (Command), Luke

March 13, 2025 by Al Williams 12 Comments

You can argue if bash is a good programming language or not, but you can’t argue that it is a programming language. However, there are a few oddities about it that make it different from most other languages you probably know. For one thing, variables are dynamically scoped. Second, you can easily change variables in an upper scope. This leads to a problem when you want to do something like reset your path:

#!/bin/bash
#: This does NOT work
PATH=/usr/bin:/bin

Well, actually, it does work; it just doesn’t work the way you imagine it might. The key is to realize that when you execute our script (say, resetpath), a new copy of bash runs. It inherits all the variables from your shell. Now the script sets PATH for the new copy of bash. Anything else you run in that script will see your change. But when the script exits, the new copy of bash is gone and the old copy sees the same old PATH it always did.

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Getting Linux Process List Without Forking Using Just A Bash Script

July 29, 2024 by Maya Posch 16 Comments

The ps command is extremely useful when you want to get some quick information on active system processes (hence the name), especially followed by piping it into grep and kin for some filtering. One gotcha is of course that ps doesn’t run in the current shell process, but is forked off into its own process, so what if everything goes wrong and you absolutely need to run ps aux on a system that is completely and utterly out of fresh process IDs to hand out? In that scenario, you fortunately can write a shell script that does the same, but all within the same shell, as [Isabella Bosia] did, with a Bash shell script.

The how and why is mostly covered in the shell script itself, using detailed comments. Initially the hope was to just read out and parse the contents of /proc/<pid>/status, but that doesn’t have details like CPU%. The result is a bit more parsing to get the desired result, as well as a significant amount of cussing in the comments. Even if it’s not entirely practical, as the odds of ending up on a system with zero free PIDs are probably between zero and NaN, but as an ‘entertaining’ job interview question and example of all the fun things one can do with shell scripting it’s definitely highly recommended.

Linux Fu: Mixing Bash And Python

May 4, 2021 by Al Williams 24 Comments

Although bash scripts are regularly maligned, they do have a certain simplicity and ease of creation that makes them hard to resist. But sometimes you really need to do some heavy lifting in another language. I’ll talk about Python, but actually, you can use many different languages with this technique, although you might need a little adaptation, depending on your language of choice.

Of course, you don’t have to do anything special to call another program from a bash script. After all, that’s what it’s mainly used for: calling other programs. However, it isn’t very handy to have your script spread out over multiple files. They can get out of sync and if you want to send it to someone or another machine, you have to remember what to get. It is nicer to have everything in one file.

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Linux-Fu: One At A Time, Please! Critical Sections In Bash Scripts

August 18, 2020 by Al Williams 12 Comments

You normally think of a critical section — that is, a piece of a program that excludes other programs from using a resource — as a pretty advanced technique. You certainly don’t often think of them as part of shell scripting but it turns out they are surprisingly useful for certain scripts. Most often, a critical section is protecting some system resource like a shared memory location, but there are cases where a shell script needs similar protection. Luckily, it is really easy to add critical sections to shell scripts, and I’ll show you how.

Sometimes Scripts Need to Be Selfish

One very common case is where you want a script to run exactly one time. If the same script runs again while the original is active, you want to exit after possibly printing a message. Another common case is when you are updating some file and you need undisturbed access while making the change.

That was actually the case that got me thinking about this. I have a script — may be the subject of a future Linux-Fu — that provides dynamic DNS by altering a configuration file for the DNS server. If two copies of the script run at the same time, it is important that only one of them does modifications. The second copy can run after the first is totally complete.

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Give A Man A Phish, And You Entertain Him For A Day

August 16, 2020 by Sven Gregori 11 Comments

With millions of phishing attempts happening daily, we’ve probably all had our fair share of coming across one. For the trained or naturally suspicious eye, it’s usually easy to spot them — maybe get a good chuckle out of the ridiculously bad ones along the way — and simply ignore them. Unfortunately, they wouldn’t exist if they weren’t successful enough in the big picture, so it might be a good idea to inform the targeted service about the attempt, in hopes they will notify users to act with caution. And then there’s [Christian Haschek], who decided to have some fun and trying to render the phished data useless by simply flooding it with garbage.

After his wife received a text message from “their bank”, [Christian] took a closer look at the URL it was pointing to, and found your typical copy of the real login form at a slightly misspelled address. As the usual goal is to steal the victim’s credentials, he simply wrote a shell script that sends random generated account numbers and PINs for all eternity via cURL, potentially lowering any value the attackers could get from their attempt.

As the form fields limit the input length of the account number and PIN, he eventually wondered if the server side will do the same, or whether it would crash if longer data is sent to it. Sadly, he’ll never know, because after he modified the script, the site itself returned a 404 and had disappeared.

In the quest against phishing attacks, this should count as a success, but as [Christian] seemed to enjoy himself, he yearned for more and decided to take a look at a similar attempt he saw mentioned earlier on Reddit. Despite targeting the same bank, the server-side implementation was more sophisticated, hinting at a different attack, and he definitely got his money worth this time — but we don’t want to give it all away here.

Rest assured, [Christian Haschek] continues the good fight, whether by annoying attackers as he did with ZIP-bombing random WordPress login attempts or battling child pornography with a Raspberry Pi cluster. Well, unless he’s busy hunting down an unidentified device hooked up in his own network.

(Banner image by Tumisu)

Linux Fu: Debugging Bash Scripts

December 11, 2019 by Al Williams 23 Comments

A recent post about debugging constructs surprised me. There were quite a few comments about how you didn’t need a debugger, as long as you had printf. For that matter, we’ve all debugged systems where you had nothing but an LED to flash or otherwise turn on to communicate with the user. However, it is hard to deny that a debugger can help with complex code.

To say you only need printf would be like saying you only need machine language. Technically accurate — you can do anything in machine language. But it sure makes things easier to have an assembler or some language to help you work out your problem. If you write a simple bash script, you can use the equivalent to printf — maybe that’s the echo command, although there is usually a printf command on a typical system, if you want to use it. However, there are other things you can do with bash including a pretty cool debugger if you know how to find it.

I assume you already know how to use echo and printf, but let’s dig into how to use trace execution line by line without the need for echo statements on every other line. Along the way, you’ll learn how to get started with the bash debugger.

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