Basic and Advance Linux Scripting

Shell scripting session 1

What is shell

- Shell is an interpreter between kernal and user

Types of shell

1. Sh shell

2. BASH shell

3. Cshell

4. kshell

For out identitiy will use .sh extension

#!/bin/bash -- Shebang (It is always the first line of script)

chmod a+x abc.sh   --- adding execute permission on abc.sh

There are 2 ways to run bash script

1- ./abc.sh  (x permission required)

2- bash abc.sh

Assigning variable

var_name=value

name=Aaftab

echo "My name is $name"

- here $ name is replace with Aaftab

#!/bin/bash

IP=$(ip a s | tail -4 | head -1 | tr -s " " | cut -d " " -f3 | cut -d "/" -f1)

echo "My IP address is -----> $IP"

note - if we want to assign the value of executed command in variable the  always used $()

Bash script

#!/bin/bash

IP=$(ip a s | tail -4 | head -1 | tr -s " " | cut -d " " -f3 | cut -d "/" -f1)

MEMORY=$(free -h | head -2 | tail -1 | tr -s " " | cut -d " " -f2)

FMEM=$(free -h | head -2 | tail -1 | tr -s " " | cut -d " " -f4)

MAC=$(ip a s | head -8 | tail -1 | tr -s " " | cut -d " " -f3)

DISK=$(lsblk | head -2| tail -1 | tr -s " " | cut -d " " -f4)

echo "My IP address is -----> $IP"

echo "My IP Total memory is -----> $MEMORY"

echo "My Free memory is -----> $FMEM"

echo "My MAC is -----> $MAC"

echo "My Disk size  is -----> $DISK"

output -->

My IP address is -----> 172.31.47.231

My IP Total memory is -----> 761Mi

My Free memory is -----> 319Mi

My MAC is -----> 02:5e:b5:ba:41:26

My Disk size  is -----> 10G

Scripting Session 2

read -- when we want to take input from user then we use read

ex-

#!/bin/bash

read -p "Enter your name -->" NAME

echo "My name is $NAME"

expr -- use to perform addition, substraction , multiplication adn division

#!/bin/bash

read -p "Enter first number -->" N1

read -p "Enter second number -->" N2

SUM=$(expr $N1 + $N2)

echo "Addition of two number is $SUM"

command line argument

It is use to pass the argument at run time

ex-

#!/bin/bash

SUM=$(expr $1 + $2)

echo "Addition of two number is $SUM"

$1 - first argument

$2 - second

$3- third

etc....

#!/bin/bash

MEM=$(free -h | head -2 | tail -1 | tr -s " " | cut -d " " -f2 | cut -d "M" -f1)

UMEM=$(free -h | head -2 | tail -1 | tr -s " " | cut -d " " -f3 | cut -d "M" -f1)

UTI=$(expr 100 \* $UMEM / $MEM)

echo "Memory utilization is $UTI%"

output -- Memory utlization is 33%

Task - 

Free memory %

Addition,multiplication, deivision, substraction

Buffer cache in %

Scripting Session 3

Condition - It is used to  execute as per the situation

if [];then

Syntax

if [ condition ];then

statement1

statement2

else

Statement 3

fi

Syntx - if with elif

if [ condition ];then

statement1

statement2

elif [ condition ];then

statement3

else

Statement 4

fi

Syntx with multiple conditions using operator

if [ condition1 ] || [ condition2 ];then

statement1

else

statement2

fi

operators

||  (or) -- it is used when either first or second condition should be match

&& (and) -- it is used when both condition should be match

#!/bin/bash

read -p "Please enter valid name-->" NAME

if [ "$NAME" == "aaftab" ];then

echo "Welcome to Pune"

else

echo "Invalid user"

fi

#!/bin/bash

read -p "Please enter valid name-->" NAME

if [ "$NAME" == "aaftab" ];then

echo "Welcome to Pune"

elif [ "$NAME" == "moin" ];then

echo "welcome to Pune"

else

echo "Invalid user"

fi

#!/bin/bash

read -p "Please enter valid name-->" NAME

if [ "$NAME" == "aaftab" ] || [ "$NAME" == "moin" ];then

echo "Welcome to Pune"

else

echo "Invalid user"

fi

Write script to verify if input value is more than 80 then print  first class between 60 to 80 second class and less than 60 third class

#!/bin/bash

read -p "Please enter Percentage-->" PER

if [ $PER -gt 80 ];then

echo "First class"

elif [ $PER -gt 60 ] && [ $PER -lt 80 ];then

echo "Second class"

else

echo "Third class"

fi

Write script to verify if input value is more than 80 then print  memroy alert between 60 to 80 memory warning and less than 60 normal memory utlization

#!/bin/bash

MEM=$(free -h | head -2 | tail -1 | tr -s " " | cut -d " " -f2 | cut -d "M" -f1)

UMEM=$(free -h | head -2 | tail -1 | tr -s " " | cut -d " " -f3 | cut -d "M" -f1)

UTI=$(expr 100 \* $UMEM / $MEM)

if [ $UTI -gt 80 ];then

echo "Memory Alert and utilization is $UTI%"

elif [ $UTI -gt 60 ] && [ $UTI -lt 80 ];then

echo "Memory warning and utilization is $UTI%"

else

echo "Memory is normal and utlization is $UTI%"

fi

lt -- less than

gt -- greater than

le -- less than equal to

ge -- greater than equal to

! $var  -- not equal to

Scripting Session 4

Use if condition to check wether file is exist or not

#!/bin/bash

if [ -f $1 ];then

echo "File is exist"

else

echo "File is not exist"

fi

Use if condition to check wether file is exist or not

#!/bin/bash

if [ -d $1 ];then

echo "Directory is exist"

else

echo "Directory is not exist"

fi

Control the input statement

#!/bin/bash

if [ $# -eq 2 ];then

add=$(expr $1 + $2)

echo "Addition is $add"

else

echo "Please enter two numbers"

fi

$# - number of argument is passed

$? - exit state of previous command (0- sucess, non-zero - fail)

$0 - name of shell script we have written

$1 - first argument

$2 - second argument

#!/bin/bash

tar -cvf /home/ec2-user/abc.tar /home/ec2-user &>/dev/null

if [ $? -eq 0 ];then

echo "Backup sucessfull"

else

echo "Backup fail"

fi

#!/bin/bash

if [ $# -eq 1 ];then

        if [ -f $1 ];then

        echo "File is exist"

        else

        echo "File is not exist"

        fi

else

echo "Please enter file name"

fi

Scripting Session 5

note-  While loop is used to read line by line and for loop is used to read word by word

Loops - It is used to write stetemt multiple times until condition is matched

Types of loops

While

for

Until

1. While loop - It will execute condition line by line until given condition in bracket is not wrong

while [ condition ]

do

statement

done

#!/bin/bash

NUM=0

read -p "Enter number to print line-->" count

while [ $NUM -ne $count ]

do

echo "My name is Aaftab"

NUM=$(expr $NUM + 1)

done

2. Until loop - It will execute condition line by line until given condition in bracket is  wrong

until [ condition ]

do

statement

done

#!/bin/bash

NUM=0

read -p "Enter number to print line-->" count

until [ $NUM -eq $count ]

do

echo "My name is Aaftab"

NUM=$(expr $NUM + 1)

done

Combination of while loop with if condition using infinity loop

#!/bin/bash

while [ 1 == 1 ]

do

read -p "Enter your name -->" NAME

        if [ "$NAME" == "exit" ];then

        exit;

        else

        echo "Your name is $NAME"

        fi

done

To take the file input using while loop always used read

syntax

while read var

do

statement

done < file.txt

example

#!/bin/bash

while read file

do

echo "My name is -->" $file

done < name.txt