benessilfie / bash

This repository contains my notes and scripts learning the bash scripting language

Learning Bash Scripts

Notes

You start every bash script with:

#!/bin/bash

In Bash, #! is called a shebang! 😂

A shebang #! is used to tell the kernel which interpreter to use running the commands present in the file.

This is what happens: When we run a file starting with #!, the kernel opens the file and takes the contents written right after the #! (in the case of a bash script, /bin/bash) and in a way, saves the string /bin/bash after the shebang in a variable say command.

Now when you run or execute your script, we are telling the kernel using a shebang what interpreter it should use to run your commands. So the kernel now knowing that this is a bash script, will precede your file name with the contents of the variable command to execute the file as a bash script using the bash interpreter.

So for example, say you have an executable file called hello.sh with some shell commands and a shebang #!/bin/bash. When you run it, the kernel will execute it as /bin/bash hello.sh.

Okay so let's illustrate the example, let's say hello.sh contains the content:

#!/bin/bash 

echo "Hello, World!"

When you run this script in the terminal, the kernel will notice the shebang in the very first line, get the string after it bin/bash/ and preface the file hello.sh with it to tell the intrepreter that the scripting language you want to use to run the commands in the file should be Bash.

so running hello.sh in the terminal by typing ./hello.sh is the same as typing /bin/bash hello.sh when you use a #! Shebang.

Variables!

So to create a variable in bash, simply type the variable name and use the assignment operator = to give it a value.

Example.

This is an example of a variable that stores the name of the high school I went to.

#!/bin/bash

high_school="St. Augustine's College"

To use a varibale you type the dollar sign $ follow by the variable name like this: $high_school.

#!/bin/bash

high_school="St.Augustine's College"

echo "The best boys school in Ghana is $high_school"

The output of the script above will be: "The best boys school in Ghana is St. Augustine's College"

NOTE: You can also use the read keyword to recieve a user input and store it in a variable.

Example.

#!/bin/bash

echo "What is the name of the best high school in Ghana??"

read high_school

echo "The name of the best high school in Ghana is $high_school"

The output of the script above will be whatever school you type.

Arguments

To make your script take an argument, you first create a parameter in your script and assign it a number. You create a parameter in bash similar to how you create variables but with parameters you assign them a value of $1 incrementing by the number of parameters you want to create.

• To make your script recieve an argument, you create a single parameter like this:

#!/bin/bash

name=$1

echo "Good Morning $name"

sleep 1

echo "You're looking good today $name!!"

sleep 1

echo "You have the most beautiful eyes I've ever seen $name"

• For multiple arguments, you create multpile parameters.

Example.

#!/bin/bash

name=$1
compliment=$2

echo "Good Morning $name"

sleep 2

echo "You're looking good today $name!!"

sleep 2

echo "You have the best $compliment I've ever seen $name"

Variables to store the output of commands

To create a variable to store the ouput of a command, we do it a bit diffrently from how normal variables in bash are created.

It is however similar to how parameters are created but instead of assigning the value of the parameter with the $ and a number, you do this a $ and the name of the command you want to output in brackets.

Example.

#!/bin/bash

user=$(whoami)
date=$(date)
whereami=$(pwd)

# NTS: You are totally imagining Alfred and Bruce having this conversation right now lol

echo "Good day Master $user"

sleep 1

echo "Will you be working in the $whereami directory today?"

sleep 2

echo "What day is it, Alfred?"

sleep 1

echo "Well Sir, Today is $date"

The $RANDOM Variable

$RANDOM is a built in variable that comes with linux. It is used to generate random integers in linux and returns a psuedo-random 16-bit integer between 0 and 32767.

#!/bin/bash

echo $RANDOM

---

NOTE

You can actually create your own system variables like $RANDOM and $PWD.

There are two types of system variables;

• Environment Variables: Variables that are defined for the current shell and can be used by child shells or processes "bash scripts"
• Shell Variables: Variables that are contained exclusively within the shell in which they are created or defined. Shell variables can not be inherited or used by child processes.

Example of a Shell Variable:

#!/bin/bash

PRESIDENT="Addo D"

echo $PRESIDENT

# Output: Addo D

To create an environment variable, create a shell variable and export it. Like this:

#!/bin/bash

PRESIDENT="Addo D"

export PRESIDENT

This env variable is not permanent, when you log out of the system, it will go away. To make your environment variables permanent, add it to the .bashrc file.

#!/bin/bash

nano .bashrc

# When open, add the line below to the .bashrc file

export PRESIDENT="Addo D"

By convention, these types of variables are usually defined using all caps. This helps distinguish them within other contexts.

---

Conditionals Statements - If/Esle

We use if/else statements in shell scripts to make decisions or evaluate a condition. This basically allows us to choose a response or execute a particular command when our condition is met. In bash the syntax for if/else statements are in the eg. below.

#!/bin/bash


echo "You died"

echo "Hey, do you like coffee? (y/n)"

read coffee

if [[ $coffee == "y" ]]; then
        echo "You're awesome"
else
        echo "Get Out!!!!"
fi

Logical Operators in Bash