Introduction

Code would quickly become an ugly and unruly mess if there wasn’t a way to easily repeat and reuse it. You’ve already seen some of that with loops. They’re great for repeating the same task multiple times right away. What if you wanted to reuse a block of code whenever you wanted? Well, that’s where functions come in.

Here’s another trip back to math class. If you remember, functions took in a number, did something to it, then outputted the new value. They were often represented in tables with the input on one side and the output on the other. Functions in programming are similar. Sometimes they take input. Sometimes they don’t. Much of the time they return a value as well, but they don’t always have to. In every case, they are used to repeat an operation whenever they are used, and that’s the greatest similarity with the math concept.

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Introduction

Do you remember variables from math class in school? Variables in programming are actually very similar. Variables are just symbols that that represent a value and that value can be changed; thus the name variable. Unlike in math, variables in programming can be much more free form. Variables don’t just have to be a letter. Variables can be a single character, but they are more commonly a word or a short descriptive phrase in lower case with words separated by underscores. It’s actually best to name variables something descriptive so the you and anyone else that you’re working with knows exactly what that variable is, even much later on in the code.

Types of Variables

Python is a dynamic duck typed language. Don’t worry too much about the terminology, but that means that Python doesn’t force you to specify which types variables are when you create them. Oh yeah, there are types of variables. Even though you don’t necessarily have to specify their type when you create them, it’s a good idea to know what type you want them to be. Later on, having the wrong type of variable will invariably get you into big trouble.

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The following is an example on how to pass and access command line arguments which a Python script. Save the following python script to a file eg. python-arguments.py

from sys import argv

name, first, second, third, fourth = argv

print "Script name is:", name
print "Your first argument is:", first
print "Your second argument is:", second
print "Your third argument is:", third
print "Your fourth argument is:", fourth


# Alternatively we can access "argv" argument list directly using range. For exmaple:

# Print all arguments except script name
print argv[1:]

# Print second argument
print argv[2]

# Print second and third argument
print argv[2:4]

# Print last argument
print argv[-1]

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The following link will extract all URL’s for a given web page.

#!/usr/bin/env python3

# Python Version:  3.4.2
# bs4 version: 4.3.2-2

from urllib.request import urlopen
from bs4 import BeautifulSoup

html = urlopen("http://gnu.org") # Insert your URL to extract
bsObj = BeautifulSoup(html.read());

for link in bsObj.find_all('a'):
    print(link.get('href'))

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The following bash script can be used to fill empty cells within a CSV file. We assume that your file is evenly distributed so that it contains same number of columns in each row separated by comma. If your file is TABseparated use a following linux command to convert it to comma separated value file before you proceed.
Example:

$ cat test 
1       2       4       4
2       3       3       3
$ sed 's/\t/,/g' test 
1,2,4,4
2,3,3,3
OR
$ cat test | tr '\t' ','
1,2,4,4
2,3,3,3

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If you are stubborn C programmer and wish to get your way when using BASH you will be happy to know that BASH offers C style syntax for writing for loops. Below you can find two examples of C style bash for loop:

Simple c-style bash for look with three iterations:

#!/bin/bash
MAX=3
for ((i=1; i <= MAX ; i++)) ; do
	echo "$i"
done

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Symptoms

This error message appears when you try to remove, move or copy a long list of files. When using your shell a command can only accept a limited number of arguments. When the number of arguments supplied to the command exceeds the permitted number of arguments an error message will appear:

-bash: /bin/rm: Argument list too long

linux command to find your limit for maximum arguments:

# getconf ARG_MAX
2097152

Example:

# rm *
-bash: /bin/rm: Argument list too long

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When using a bash shell all your entered commands are remembered by history library. The history library will keep track of every command you have entered. This is a default for most if not all Linux systems. However, the commands you enter are first temporarily stored into an internal memory and are written to your ~/.bash_history  only after you properly exit your shell session.

Depending on your shell usage this may cause some undesired results. For example, if your connection to a remote host gets disconnected, your history file will not get updated and thus you will lose all you previously entered commands. Furthermore, while your commands for one session are temporarily stored within system’s internal memory you would not be able to access it from another shell session.

Use the following linux command to force your shell to append every command entered during a current shell session into ~/.bash_history file:

shell 1: $ history -a

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The Bash “Shellshock” bug is used to spread malware using botnets. To make sure that you can prevent your system against Shellshock exploit make sure that your system is up to date. Once you update your system use below vulnerability test to help you find out whether your system is vulnerable to Bash Shellshock attacks.

Simply open a terminal on your Linux system and execute the following linux command

$ env i='() { :;}; echo Your system is Shellshock vulnerable' bash -c "echo Shellshock Linux system vulnerability test"

If your system is vulnerable to the Bash “Shellshock” bug the above command will produce a following output:

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Let’s start by a simple execution example of GNU R Rscript front-end script. Use which command to locate Rscript interpreter:

$ which Rscript
/usr/bin/Rscript

alternatively define your interpreter as /usr/bin/env Rscript

#!/usr/bin/Rscript

print("Hello R")

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Introduction

For most of us WEP encryption has become a joke. WPA is quickly going the same way thanks to many tools such as Aircrack-ng. On top of this, wired networks are no strangers to unwanted guests as well. Anyone serious about security should have a good Intrusion Detection system in their toolbox.

There are already some very good IDS’s (Intrusion Detection Systems) available. Why would anyone want to re-invent the wheel in Bash??? There are a couple of reasons for this. Obviously Bash scripts can be very light weight. Especially compared to some of the GUI programs that are out there. While programs like Etherape suck us in with pretty colors, they require constant monitoring to know when the network has changed. If you are like most of us, you only use the computer for two things, work and play. By using the system bell to alert for new clients online you can leave this script running and not have to have a constant watch. If you do decide you want to inspect what a suspicious client is doing more closely, you can always open up etherape, wireshark, or your tool of choice. But until you have a problem you can play or work on other things.

Another bonus to this program is that it will only show ip addresses on the networks connected to your computer. If you were hosting a busy server or perhaps downloading the latest Linux distro though a torrent client, an IDS may be flooded with connections. Looking for a new malicious client can be like looking for a needle in a hay stack. While this script may seem simple compared to other IDS’s, simplicity can have its perks too.

What you will need

Nmap is required for this script to work. We will not be doing any port scanning. However, to make this script fast we needed something better than a regular ping. Nmap’s -sP parameter will only use a ping scan to check if a clients up. There were some variations in how Nmap outputs information between versions. So far this script has only been tested using Nmap 5.00 (Debian Squeeze) and 5.21 (Debian Sid). You may have luck with other distros and versions of Nmap. However, with all the possibilities I could only support a couple at this time.

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Here is a simple GNU R script print a single line:

#!/usr/bin/Rscript

print("hello R")

where or execution output is:

$ ./script.R
[1] "hello R"

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