Shell Scripting

 What is Shell?

Shell allows a user to interact with kernel and provide an environment to execute commands.

Types of Shell

• bash
• sh
• ksh
• tsh
• fish
• zsh

How to check your default shell?

poyadav@fedora:~$ echo $0
bash

What is shell scripting?

We can use shell scripting to automate tasks, it contains a set of commands to perform a task which are executed sequencially.

What is shebang?

#!/bin/bash 

It tell our system to use bash for executing the script.

How to execute as bash script?

• ./file.sh    #Make sure file has execute command (chmod +x file.sh)
• /path/file.sh  #Make sure file has execute command
• bash file.sh

Variables

var=hello

var=$(command)

readonly var=value

Arrays

myarray=(1 2 3 "hello" "hii")   #contains space seperated values

echo "${myarray[2]}   #getting 3rd value

echo "${myarray[*]}   #gatiing all values

echo "${myarray[*]:2} #getting specific values

array+=(bye tata)  #adding new values

Key-Value arrays

declare -A myarray

myarray=( [name]=Henry [age]=23 ) 

echo "${myarray[name]}"

Strings Operations

myvar="Hello, how are you?"

length=${#myvar}

upper=${myvar^^}

lower=${myvar,,}

replace=${myvar/Hello/Hey}

slice=${myvar:2:5}

Getting input from user

echo -n "What is your name? "

read name

echo "Your name is $name"

read -p "What is your age? " age

echo "Your age is $age"

Arithmetic Operations

let a++

let a=6*10

((a++))

((a=6*10))

if-else 

if [ condition ];then

echo "PASS"

else

echo "FAIL"

fi

case

echo "where you want to go"

echo "a=Paris"

echo "b=London"

echo "c=USA"

read choice

case $choice in

a) echo "Going to Paris";;

b) echo "Going to London";;

c) echo "Going to USA";;

*) echo "Not going anywhere"

esac

Logical Operators

#Both conditions need to be true/met

if [[ condition1 ]] && [[ condition2 ]];then

echo "PASS"

else

echo "FAIL"

fi

#only one condition need to be true

if [[ condition1 ]] || [[ condition2 ]];then

echo "PASS"

else

echo "FAIL"

for Loop

for i in 1 2 3 4 5

do

echo i

done

#or we can also use

for i in Apple banana grapes

for i in {1..10}

#for loop with file

file=array.sh

for i in $(cat $file);do

echo $i

done

#or we can also use

for ((i=0;i<10;i++))

do

echo "Hello $i"

done

while Loop

#syntaX

count=0

num=10

while [ $count -lt $num ];do

echo "value is $count"

(( count++ ))

done

#reading from file

while read myvar;do

echo $myvar

done < myfile

#while with csv file

while IFS="," read id name age

do

echo "id is $id"

echo "name i $name"

echo "age is $age"

done < file.csv

Infinite Loop

while true;do

echo "Hey"

done

#for infinite loop

for (( ;; ));do

echo "Hey'

done

Functions

fun() {

echo "hello"

}

#another way

function myfun {

echo "Hey"

}

#function with args

myarg() {

echo "Hello $1"

echo "Welcome to $2"

}

fun

myfun

myarg raju manali