Bash Scripting For Dummies

In this article I’ll go over the very basics of bash scripting so that by the end of it you’ll have an idea what it means and how can you use it for your own needs

UNDERSTANDING BASH

So a shell is a utility that allows us to directly interact with the kernel through issuing commands and it’s considered a very powerful tool and this shell comes in “flavors” that act as environments

Think of an environment this way, MS PowerPoint is an environment to produce and display presentation slides, for example you can’t view those slides inside another software like the calculator

In that sense, a shell is an environment to run commands directly instead of the traditional GUI way of clicking and dragging stuff and like i said that’s a huge advantage for automation and productivity

Other shells include :-

• Python : yes the programming language, it also has an interactive python shell which means it only runs Python commands
• zsh : also another shell environment in many Linux distros
• Powershell : Windows based shell environment

TO BASH OR NOT TO BASH

So a very common question might be “when do i use bash and why”, to answer this let’s consider this real life scenario, you’re an ethical hacker / pentester and you were tasked to scan a network for a specific service running ( let’s say ssh } and try to brute force into that with default credentials, how do you plan on doing that MANUALLY ?????

Here comes the power of shell scripting, with the presence of the powerful built in command line tools in linux, that task could be done very easily with a simple script that could save you a lot of time

Other uses may include :-

• automating sending business emails with ssmtp
• fetching data from an API to send via email or save to a directory
• regularly backup files
• integrate with other powerful tools you have installed
• the list could literally go on further

PREREQUISITES

A basic Linux knowledge is required as i won’t really go over EXACTLY every command so brush up on your command line skills and be familiar with basic commands

EXCITED ENOUGH ? LET’S START BASHING

To begin scripting you first need to locate your bash location, to do that simply run which bash and It’ll return the path to it

In your favorite text editor, in my case it’s just nano since we’re not doing much, type in this very minimal bash script and save it as hello.sh

#! /usr/bin/bash
echo "hello world"

Now to run the script we firstly need to make it executable so chmod +x hello.sh and now we can just type ./hello.sh and should see a ‘hello world’ printed to the screen

VARIABLES IN BASH

To declare a variable, simply name it and assign a value to it like so and to reference it we use dollar sign in front of the variable name

#! /usr/bin/bash
name="mark"
echo $name 

PASSING ARGUMENTS TO OUR SCRIPT

If you’ve programmed long enough and downloaded tools, you’re probably familiar with passing arguments to a script to be used, It’s an essential functionality in most command line tools so let’s learn how to pass those and echo them back for the sake of the demo only

#! /usr/bin/bash
echo $1
echo $2
echo $3
# this is a comment btw
# another syntax is
for i in "$@"
do
  echo "$i"
done 
# more on this stuff later

USER INPUT IN BASH

Best way to take user input it and directly assign it to a variable is the following

#! /usr/bin/bash
read -p "enter your age: " age
echo "you entered $age"

LOGICAL OPERATORS

Bash has a bit tricky way to conduct logical comparisons so please stick to those notes

1. use double or single square brackets [[]] as it enables you to use powerful operators
2. use single or double (( )) in order to use the usual =, !=, >, <, ≤, ≥
3. ALWAYSSSS leave a little space on both ends of square brackets and {}- for example

if [[ "$name" == "adam" ]]
then
   echo "hi adam we missed you"
else
   echo "welcome $name"
fi

Personally that advice saved me tons of time figuring out why is my script not running

4. ALWAYSSSS use double quotes when comparing variables

[[ -e "$file" ]] # True if file exists
[[ -d "$file" ]] # True if file exists and is a directory
[[ -f "$file" ]] # True if file exists and is a regular file
[[ -z "$str" ]]  # True if string is of length zero
[[ -n "$str" ]]  # True is string is not of length zero

# Compare Strings
[[ "$str1" == "$str2" ]]
[[ "$str1" != "$str2" ]]

# Integer Comparisions
[[ "$int1" -eq "$int2" ]] # $int1 == $int2
[[ "$int1" -ne "$int2" ]] # $int1 != $int2
[[ "$int1" -gt "$int2" ]] # $int1 > $int2
[[ "$int1" -lt "$int2" ]] # $int1 < $int2
[[ "$int1" -ge "$int2" ]] # $int1 >= $int2
[[ "$int1" -le "$int2" ]] # $int1 <= $int2

Taken from this amazing source so all credits to him ♥️

CONDITIONALS AND LOOPS

• Simple for loop

for (i=0; i<10; i++)
do
   echo "current number $i"
done

• a for in loop with a clever array syntax 👀

for i in {1..20}
do
  echo "current iteration $i"
done

• while loop

i=10
while [[ "$i" -ge 1 ]]
do
    echo "current iteration: $i"
done

• until loop

i=1
until [[ "$i" -eq 10 ]]
do
  echo "unitl $i"
  ((i++))
done 

ARRAYS IN BASH

The most common data structure in programming is arrays as they allow us to store multiple values at once, unlike a variable

Let’s see how can we declare an array and how to iterate over it

names=(jake mark john bob alice)

Declare an array by using parenthesis and without a comma separating elements or even double / single quotes in case of a string

To access an array element we use the elements index like this “${colors[1]}” and since the @ sign means “all” in bash, we iterate over an array this way

for i in "${colors[@]}"
do
    echo "$i"
done

AND FINALLY FUNCTIONS

A function is a block of code you wish to execute many times so instead of writing it over and over, you create a function and call it by name whenever you need it

Let’s create a quick little function that greets anyone who enters their name

CONCLUSION

Bash scripting is a powerful way to automate tasks on Linux and can be used in endless ways and boosts productivity and having basic knowledge of it is important if you’re planning on becoming an ethical hacker or system administrator or simply want to automate tedious boring tasks and we’ll hopefully explore more in later blogs and build interesting tools with it

Thanks so much for reading ♥️