--> shell is useful to automate our regular tasks,

monitoring several server resources like cpu utilization and memory utilization

Q. How to know how many shell types are supporting by your linux server?

Ans: check /etc->shells file

--> To know which shell we are using

[root@ip-172-31-35-179 ~]# echo $SHELL

/bin/bash

[root@ip-172-31-35-179 ~]# echo $0

-bash

--> Switch to another shell

serverresourcesmonitor.sh

THRESHOLD=80

->df

->free

->top

-->mail

-->hostname

-->ifconfig

step-1) use any editor like vi to write shell script

syntax: vi <script_name>

step-2) after writing shell script set execute permission for your script as follow

syntax: chmod u+x <scrpitFileNAME>

step-3) execute your script as

syntax: sh <script-name>

./<script-name>

. <script-name>

#!/bin/bash

#!-->called as shebang line

------------------------------------------------------------------------CLASS-1 END------------------------------------------------------------------------

# sh -x hello.sh --> running the shell script in debug mode.

->If we want run a particular code in debug mode add set -x and set +x in shell script.

Example:

-------

#!/bin/bash

echo "hello guys"

set -x

echo "gd mrng"

echo "welocme to shell script"

set +x

echo "Today date is"

date

COMMENTS:

---------

# ->Single line comment

<<MLC

......

....... ->Multi-line comment (first and last same name should be there)

.......

.......

MLC

variables;

----------

a=10 (no space in between)

name=venki (no space in between)

# printenv or # env (to list the system defined variables)

Q. is is possible to change system defined variable values?

Ans: yes

Example:

--------

[ec2-user@ip-172-31-35-179 ~]$ echo $HISTSIZE

1000

[ec2-user@ip-172-31-35-179 ~]$ export HISTSIZE=1100 (this is temporary change)

[ec2-user@ip-172-31-35-179 ~]$ echo $HISTSIZE

1100

--> If we want change those values permanantely add those values in .bash_profile for user specifc or add in cd /etc->profile file for all the users.

COMMAND LINE ARGUMENTS:

-----------------------

-->while executing the shell script the values are passed through the command prompt

Example:

-------

[ec2-user@ip-172-31-35-179 ~]$ cat file1.sh

echo "file-name" $0

echo "a value is" $1

echo "b value is" $2

echo "c value is" $3

[ec2-user@ip-172-31-35-179 ~]$ sh file1.sh 11 12 13

file-name file1.sh

a value is 11

b value is 12

c value is 13

$0-->gives file name

$1-->first argument

$2-->second, $3-->third....,,,

$#-->Number of arguments

$*-->All the args (as a one string)

$@-->All the args(Each arg as a one string)

# echo $$ -->Displays process id

# $?-->previous command execution status(0=success)

0--> success

127-->standard error code for command not found

[ec2-user@ip-172-31-35-179 ~]$ cat file1.sh

echo "file-name: " $0

echo "a value is: " $1

echo "b value is: " $2

echo "c value is: " $3

echo "number of args: " $#

echo "all the args using *: " $*

echo "all the args using @: " $@

echo "process ID: " $$

date

echo "previous command execution status: " $?

[ec2-user@ip-172-31-35-179 ~]$ sh file1.sh 1 2 3

file-name: file1.sh

a value is: 1

b value is: 2

c value is: 3

number of args: 3

all the args using *: 1 2 3

all the args using @: 1 2 3

process ID: 4470

Tue Aug 2 12:08:21 UTC 2022

previous command execution status: 0

[ec2-user@ip-172-31-35-179 ~]$ cat file1.sh

echo "file-name: " $0

echo "a value is: " $1

echo "b value is: " $2

echo "c value is: " $3

echo "number of args: " $#

echo "all the args using *: " $*

echo "all the args using @: " $@

echo "process ID: " $$

Date

echo "previous command execution status: " $?

[ec2-user@ip-172-31-35-179 ~]$ sh file1.sh 1 2

file-name: file1.sh

a value is: 1

b value is: 2

c value is:

number of args: 2

all the args using *: 1 2

all the args using @: 1 2

process ID: 4532

file1.sh: line 9: Date: command not found

previous command execution status: 127

Needs to pass 3 args to execute shell script:

if [ $# -eq 3 ]

then

echo "file-name: " $0

echo "a value is: " $1

echo "b value is: " $2

echo "c value is: " $3

echo "number of args: " $#

echo "all the args using *: " $*

echo "all the args using @: " $@

echo "process ID: " $$

Date

echo "previous command execution status: " $?

else

echo "usage sh $0 arg1 arg2 arg3"

fi

----------------------------------------------------------------CLASS-2 END--------------------------------------------------------------------------------

String:

-------

->string is nothing but one or more characters enclosed in a single or double quotes.

Length of string:

----------------

[ec2-user@ip-172-31-35-179 ~]$ cat string.sh

string_var=venki

echo "length of string is :" ${#string_var}

[ec2-user@ip-172-31-35-179 ~]$ sh string.sh

length of string is : 5

[ec2-user@ip-172-31-35-179 ~]$ cat string.sh

string_var='venki potla'

echo "length of string is :" ${#string_var}

[ec2-user@ip-172-31-35-179 ~]$ sh string.sh

length of string is : 11

SubString:

---------

Example:

--------

[ec2-user@ip-172-31-35-179 ~]$ cat string.sh

string_var='venki potla is very good boy'

echo "sub string is :" ${string_var:4} (ignores first 4 characters)

[ec2-user@ip-172-31-35-179 ~]$ sh string.sh

sub string is : i potla is very good boy

echo ${string_var:n1:n2} n1->from starting how many characters ignore

n2-> from there how many characters needs to display

Example:

--------

[ec2-user@ip-172-31-35-179 ~]$ cat string.sh

string_var='venki potla is very good boy'

echo "sub string is :" ${string_var:6:5}

[ec2-user@ip-172-31-35-179 ~]$ sh string.sh

sub string is : potla

Example:

--------

[ec2-user@ip-172-31-35-179 ~]$ cat string.sh

string_var='venki potla is very good boy'

echo "sub string is :" ${string_var:(-3)}

[ec2-user@ip-172-31-35-179 ~]$ sh string.sh

sub string is : boy

Arthimatic operations:

----------------------

Example:

---------

[ec2-user@ip-172-31-35-179 ~]$ cat arthmetic.sh

echo "arithmetic operations"

expr 2 + 3

expr 2 - 3

expr 2 \* 3 (space is mandatory)

expr 10 / 2

expr 20 % 3

[ec2-user@ip-172-31-35-179 ~]$ sh arthmetic.sh

arithmetic operations

5

-1

6

5

2

-> echo "the addition is" `expr 2 + 3`

[ec2-user@ip-172-31-35-179 ~]$ cat aa.sh

expr $1 + $2

[ec2-user@ip-172-31-35-179 ~]$ sh aa.sh 12 13

25

user interaction using read:

----------------------------

[ec2-user@ip-172-31-35-179 ~]$ cat arthmetic.sh

echo "enter a"

read a

echo "enter b"

read b

expr $a + $b

expr $a - $b

expr $a \* $b

expr $a / $b

expr $a % $b

[ec2-user@ip-172-31-35-179 ~]$ sh arthmetic.sh

enter a

2

enter b

3

5

-1

6

0

[ec2-user@ip-172-31-35-179 ~]$ cat aa.sh

echo "please enter your name"

read name

echo "the name you entered is:" $name

[ec2-user@ip-172-31-35-179 ~]$ sh aa.sh

please enter your name

venky

the name you entered is: venky

---------------------------------------------------------------CLASS-3 END----------------------------------------------------------------------------------

ARRAY:

-----

[ec2-user@ip-172-31-35-179 ~]$ cat readex.sh

echo "please enter devops tools"

read -a devopsTools

echo "the tools are:" ${devopsTools[*]}

echo "the 4th value is" ${devopsTools[3]}

[ec2-user@ip-172-31-35-179 ~]$ sh readex.sh

please enter devops tools

git maven nexus sonarqube tomcat docker k8s

the tools are: git maven nexus sonarqube tomcat docker k8s

the 4th value is sonarqube

[ec2-user@ip-172-31-35-179 ~]$ cat read4.sh

echo "favourite color"

read (not passing anything with read so use $REPLY to get that value)

echo "favorite color is :" $REPLY

[ec2-user@ip-172-31-35-179 ~]$ sh read4.sh

favourite color

red

favorite color is : red

[ec2-user@ip-172-31-35-179 ~]$ cat example.sg

read -p "please enter your name: " username

read -p "please enter password: " password

echo "username is: " $username

echo "password is:" $password

[ec2-user@ip-172-31-35-179 ~]$ sh example.sg

please enter your name: venki

please enter password: password@123

username is: venki

password is: password@123

[ec2-user@ip-172-31-35-179 ~]$ cat example.sg

read -p "please enter your name: " username

read -sp "please enter password: " password

echo "username is: " $username

echo "password is:" $password

[ec2-user@ip-172-31-35-179 ~]$ sh example.sg

please enter your name: venki

please enter password: username is: venki

password is: venki@123

input and output redirection:

------------------------------

> -->redirect standard output

>> -->append the standard output

< -->standard input

[ec2-user@ip-172-31-35-179 ~]$ cat example.sh

read -p "please enter your name: " username

read -p "please enter password: " password

echo "username is: " $username

echo "password is:" $password

date

[ec2-user@ip-172-31-35-179 ~]$ sh example.sh > output.log

please enter your name: venki

please enter password: venki123

[ec2-user@ip-172-31-35-179 ~]$ cat output.log

username is: venki

password is: venki123

Wed Aug 3 05:39:19 UTC 2022 (only output is going to redireted not error)

[ec2-user@ip-172-31-35-179 ~]$ cat example.sh

read -p "please enter your name: " username

read -p "please enter password: " password

echo "username is: " $username

echo "password is:" $password

Date

[ec2-user@ip-172-31-35-179 ~]$ sh example.sh > output.log

please enter your name: venki

please enter password: venki123

example.sh: line 5: Date: command not found

[ec2-user@ip-172-31-35-179 ~]$ cat output.log

username is: venki

password is: venki123

FILE DECRIPTORS:

---------------

0 -> standard input

1 -> standard output

2 -> standard error

[ec2-user@ip-172-31-35-179 ~]$ cat example.sh

read -p "please enter your name: " username

read -p "please enter password: " password

echo "username is: " $username

echo "password is:" $password

Date

[ec2-user@ip-172-31-35-179 ~]$ sh example.sh > out.log 2>&1

venki

venki123

[ec2-user@ip-172-31-35-179 ~]$ cat out.log

please enter your name: please enter password: username is: venki

password is: venki123 (along with o/p error also saved)

example.sh: line 5: Date: command not found

[ec2-user@ip-172-31-35-179 ~]$ cat example.sh

read -p "please enter your name: " username

read -p "please enter password: " password

echo "username is: " $username

echo "password is:" $password

Date

[ec2-user@ip-172-31-35-179 ~]$ sh example.sh 2> err.log 1> output.log

venki

venki123

[ec2-user@ip-172-31-35-179 ~]$ cat output.log

username is: venki

password is: venki123

[ec2-user@ip-172-31-35-179 ~]$ cat err.log (both error and output in different files)

please enter your name: please enter password: example.sh: line 5: Date: command not found

CONDITIONAL STATEMENT:

-----------------------

if syntax:

if [ condition ]

then

.......

.......

.......

.......

else

......

......

......

......

fi

example:

--------

[ec2-user@ip-172-31-35-179 ~]$ cat if.sh

read -p "enter a :" a

read -p "enter b :" b

if [ $a -gt $b ]

then

echo "$a is bigger"

else

echo "$b is bigger"

fi

[ec2-user@ip-172-31-35-179 ~]$ sh if.sh

enter a :21

enter b :78

78 is bigger

Q.write a one shell script that accepts the file name from the user and check the file is existed in current directory or not.?

Ans:

[ec2-user@ip-172-31-35-179 ~]$ cat shell.sh

read -p "enter file name :" filename

if [ -f /home/ec2-user/$filename ]

then

echo "file is existed"

else

echo "file does not existed"

fi

[ec2-user@ip-172-31-35-179 ~]$ pwd

/home/ec2-user

[ec2-user@ip-172-31-35-179 ~]$ ls

err.log example.sh if.sh shell.sh

[ec2-user@ip-172-31-35-179 ~]$ sh shell.sh

enter file name :err.log

file is existed

[ec2-user@ip-172-31-35-179 ~]$ sh shell.sh

enter file name :file1.txt

file does not existed

[ec2-user@ip-172-31-35-179 ~]$ ls

err.log example.sh if.sh shell.sh

[ec2-user@ip-172-31-35-179 ~]$ cat shell.sh

read -p "enter file name :" filename

if [ -f /home/ec2-user/$filename ]

then

echo "file is existed"

echo "file content displaying if exists"

cat $filename

else

echo "file does not existed"

echo "creating a new file with that name"

touch $filename

fi

[ec2-user@ip-172-31-35-179 ~]$ sh shell.sh

enter file name :example.sh

file is existed

file content displaying if exists

read -p "please enter your name: " username

read -p "please enter password: " password

echo "username is: " $username

echo "password is:" $password

Date

[ec2-user@ip-172-31-35-179 ~]$ sh shell.sh

enter file name :file.txt

file does not existed

creating a new file with that name

[ec2-user@ip-172-31-35-179 ~]$ ls

err.log example.sh file.txt if.sh shell.sh

read -p "enter file name :" filename

if [ -f /home/ec2-user/$filename ]

then

echo "file is existed"

echo "the path of file is :" `find -name $filename`

echo "file content displaying if exists"

cat $filename

else

echo "file does not existed"

echo "creating a new file with that name"

touch $filename

fi

-f -->To check file is existed or not

-x -->To execute permission

-r -->read permission

-w -->write permission

----------------------------------------------------------------------------CLASS-4 END---------------------------------------------------------------------

FOR LOOP:

---------

Example:

-------

[ec2-user@ip-172-31-35-179 ~]$ cat for.sh

echo "for loop demo..."

for (( a=1; a<=5; a++ ))

do

echo $a

done

echo "for loop demo is over..."

[ec2-user@ip-172-31-35-179 ~]$ sh for.sh

for loop demo...

1

2

3

4

5

for loop demo is over...

WHILE LOOP:

----------

Example:

-------

[ec2-user@ip-172-31-35-179 ~]$ cat while.sh

echo "while loop demo starts.."

a=1

while [ $a -le 5 ]

do

echo $a

a=`expr $a + 1`

done

[ec2-user@ip-172-31-35-179 ~]$ sh while.sh

while loop demo starts..

1

2

3

4

5

SWITCH:

------