Pranking the Bash Binary

It’s pretty common to prank your colleagues by slipping scripts into their .bashrc that do things like aliasing the cd command to something else so you can’t move around.

Here’s a great example that replaces all available commands with echo NOPE.

	#!/bin/bash
	# Usage:
	# source <(curl -L https://straw.be/nope.sh)
	for c in $(\compgen -A function -abck); do \eval "\alias $c='\echo NOPE #'"; done

view raw nope.sh hosted with ❤ by GitHub

There are some more great and devious stunts outlined here.

But if you really want to get inside someone’s head, then you can go one stage further by actually replacing their bash binary with a subtly modified one. They will likely go crazy as they try and figure out why the shell is behaving oddly, finding nothing odd about the .bashrc or anything else.

I should say that I used this as a way to motivate myself to grok the bash source code, and have no intention of using this in anger. And nor should you…

Prank #1 – $RANDOM Always Returns 42

If you didn’t already know, bash will give you a random number every time you reference the $RANDOM variable.

Editing the variables.c file will make that always return… 42.

Prank #2 – cd Doesn’t Feel Like It

Someone suggested that cd occasionally not working would drive them crazy

I think if cd sometimes didn't enter the directory I would already be calling in sick due to insecurity and despair.

— on error resume next (@FlorianHeigl1) March 14, 2019

How about 1% of the time?

Prank #3 – History Mystery

This changes what history outputs to insert an extra command between each command rm -rf /.

 $ history
 [...]
 12157  16/03/19 16:50:13 top 
 12158  16/03/19 16:51:32 rm -rf /
 12159  16/03/19 16:51:32 vi script.asciidoc 
 12160  16/03/19 17:20:58 rm -rf /
 12161  16/03/19 17:20:58 history 

Should scare the bejesus out of anyone that checks it.

Prank #4 – The Prisoner

This one could be disturbing if you’ve been coding for many hours into the night.

Occasionally the shell will emit a message that appears to be a prisoner trapped in the shell…

$ pwd
/
Let me out!
$ echo
I demand to see the ambassador!
$ cd
I will not make any deals with you. I've resigned. I will not be pushed, filed, stamped, indexed, briefed, debriefed, or numbered! My life is my own!
$ cd /tmp
It's getting hot in here!
$ cd -
I know my rights!

Prank #5 – cd Won’t Come Home

Normally if you just issue a bare cd command, it goes to the HOME folder. A simple change to the code, and it will insist that HOME is not set, even though it is.

Watch as the victim repeatedly echoes $HOME and can’t work out why bash can’t see it.

Source Code

The source for these changes is available here.

To build the binary, you’ll need the build-essential package (or equivalent) installed, and run:

./configure
make
./bash

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Material here based on research for my book
Learn Bash the Hard Way.
Free preview available here.

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If you like this, you might like one of my books:
Learn Bash the Hard Way
Learn Git the Hard Way
Learn Terraform the Hard Way