NC Bash Scripting.md

#Bash #Linux #Networkchuck

Learn [[NC Linux|Linux Basics]] to understand bash scripting.

Bash

It stands for Bourne Again Shell.

Running Bash Script

bash example.sh

If you want to run it without using using bash, give the file suitable [[NC Linux#^1927f0|permissions]].

sudo chmod +x example.sh
./example.sh

. refers to the current directory.

Writing Bash Script

#! is known as shebang. which $SHELL - Displays the type of shell we are in now.

#!/bin/bash

# This is a comment
echo "This is a bash script. Write your linux commands in this file."
echo "Sleeping for 5 sec"
sleep 5
echo "End of program"

Variables

#!/bin/bash

name="kathir"

echo "Hello $name"
sleep 1
echo "How are you $name?"

Getting inputs from User

#!/bin/bash

echo "What's your name:"
read name

echo "Hello $name"
sleep 1
echo "How are you $name?"

Command line Arguments

#!/bin/bash

name=$1

echo "Hello $name"
sleep 1
echo "How are you $name?"

We obtain inputs form user through Positional arguments.

./example.sh kathir

Output of a command as an argument

#!/bin/bash

user=$(whoami)
date=$(date)

echo "You are logged in as $user"
echo "Today's date is $date"

Environment Variables

They are built-in variables. $RANDOM - Returns a random integer between 0 and 32767 $SHELL - Returns the current shell being used $USER - Name of the logged in user $PWD - Returns the present working directory $OLDPWD - Returns the previous working directory $_ - Returns the previously executed command

You can view these variables by using echo and can even use in your bash script.

Making our own variables

export name="kathir"

By using the export command, the variable you created can be accessed by any script.

These variables are forgot once the system is rebooted. To make these variables permanent, add the previous command to the .bashrc file in your home directory and reboot to see the changes.

Arithmetic Operations

echo $((2+3)) $((2-3)) $((2*3)) $((10/3)) $((10%3))

Conditional Statements

#!/bin/bash

sysnum=$(($RANDOM % 2))
echo "Pick 0 or 1:"
read usrnum
echo $sysnum

if [[ $sysnum == $usrnum && $USER != "root" ]]; then
	echo "Same"
else
	echo "Different"
	exit 1
fi

echo "Enter any integer:"
read num

if [[ $num > 0 ]]; then
	echo "Positive"
elif [[ $num < 0 ]]; then
	echo "Negative"
else
	echo "Zero"
fi

Tab is not a necessary syntax but is included for better readability.

|| - or && - and

Looping Statements

There are also break and continue statements.

While loop

Runs while the condition is true

#!/bin/bash
x=1
# -le is 'lesser than or equal to'
while [[ $x -le 10 ]]
do
	echo $x
	(( x++ ))
done

while true
do
	echo "Infinite Looping..."
	((x--))
	if [[ $x == 0 ]]; then
		break
	fi
done

Until Loop

Runs until the condition is true.

#!/bin/bash

until [[ $num == 0 ]]
do
	echo "Do not enter 0:"
	read num
done

For Loop

#!/bin/bash
for i in {1..21..3};
do
	echo $i
done

for name in a b c d e f g h i j k;
do
	echo $name
done

for city in $(cat cities.txt);
do
	echo $city
done

Functions

#!/bin/bash

function hello() {
    echo "Hello World Function!"
}

hello

Passing arguments to a function work in the same way as passing arguments to a script:

#!/bin/bash

function hello() {
    echo "Hello $1!"
}

hello kathir