Bash Scripting

Bash Scripting

Class 30th Jenkins May 24th
What is Script and why it is?
Type of Scripting language
Understand "Shebang(#!)""
Difference Between sh shell.sh,bash shell.sh, &./Shell.sh in Bash scripting.
Working with Bash scripting
Logical operators (AND(&&), OR(||)
File operations
Loops(For,while,Until)
Functions
Working with processes

Shell script: shell script is series of command, Amazon Linux (they combined bash and shell inherited )

so it will support what every give command whether bash/shell

Amazon linux 

[ec2-user@ip-10-0-1-88 ~]$ cd /bin

[ec2-user@ip-10-0-1-88 bin]$ls -lrt 

lrwxrwxrwx 1 root root           4 May 28 02:01 sh -> bash

Debian Linux (separate there)

admin@ip-10-0-0-210:~$ cd /bin

admin@ip-10-0-0-210:/bin$ ls -lrt

-rwxr-xr-x 1 root root 1265648 Mar 29  2024 bash

lrwxrwxrwx 1 root root       4 Jan  5  2023 sh -> dash

Understand shebang(#!)

A shebang(#!)is a special directive at the top of a script that ells the system which interpreter(shell) to use when executing the script.

  #!/bin/bash -->Use Bash shell to execute the script

 #!/bin/sh -->Use sh shell(Could be dash on some systems).

#!/usr/bin/python3-->Use Python 3 interpreter.

Practical 1

Step1:

Amazon Linux 

Example Bash script using shebang

 #!/bin/bash

echo "Hello, Cloud computing!"

[ec2-user@ip-10-0-1-88 ccit]$ sh hello.sh

Hello cloud computing

[ec2-user@ip-10-0-1-88 ccit]$ bash hello.sh

Hello cloud computing

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

-bash: ./hello.sh: Permission denied

Getting error because of it is not execute mode ,give permission using chmod

[ec2-user@ip-10-0-1-88 ccit]$ ls -lrt

-rw-rw-r-- 1 ec2-user ec2-user 41 May 31 09:07 hello.sh

[ec2-user@ip-10-0-1-88 ccit]$ chmod +x hello.sh

[ec2-user@ip-10-0-1-88 ccit]$ ls -lrt

-rwxrwxr-x 1 ec2-user ec2-user 41 May 31 09:07 hello.sh

Now is is allowing because mode was changed 

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Hello cloud computing

 Step2:Changed script in bash to sh ,See here no difference shell , if you are give ./ this and try to execute interpreter will check what shebang used in the script

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/sh

echo "Hello cloud computing"

[ec2-user@ip-10-0-1-88 ccit]$ sh hello.sh

Hello cloud computing

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Hello cloud computing

[ec2-user@ip-10-0-1-88 ccit]$ bash hello.sh

Hello cloud computing

Step3: Array type script executing amazon linux and debian linux

amazon linux 

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/sh

arr=(one two three)

echo ${arr[0]}

[ec2-user@ip-10-0-1-88 ccit]$ sh hello.sh

one

Debian Linux

Getting error 

admin@ip-10-0-0-210:~/ccit$ cat hello.sh

#!/bin/sh

arr=(one two three)

echo ${arr[0]}

admin@ip-10-0-0-210:~/ccit$ sh hello.sh

hello.sh: 2: Syntax error: "(" unexpected

admin@ip-10-0-0-210:~/ccit$ bash hello.sh

one

admin@ip-10-0-0-210:~/ccit$ ./hello.sh

-bash: ./hello.sh: Permission denied

 After given  permission, after changed script sh to bash , shell no arrey concept only has bash

admin@ip-10-0-0-210:~/ccit$ chmod +x hello.sh

admin@ip-10-0-0-210:~/ccit$ ls -lrt

total 4

-rwxr-xr-x 1 admin admin 48 May 31 09:34 hello.sh

admin@ip-10-0-0-210:~/ccit$ cat hello.sh

#!/bin/bash

arr=(one two three)

echo ${arr[0]}

 See below with bash both working ,if you explicitly run shell sh it will through error  

admin@ip-10-0-0-210:~/ccit$ ./hello.sh

one

admin@ip-10-0-0-210:~/ccit$ bash hello.sh

one

admin@ip-10-0-0-210:~/ccit$ sh hello.sh

hello.sh: 2: Syntax error: "(" unexpected

I have changed back to sh ,see below sh interpreter trying run shell for the array it will not support getting ,previously same script run  with bash.

admin@ip-10-0-0-210:~/ccit$ cat hello.sh

#!/bin/sh

arr=(one two three)

echo ${arr[0]}

admin@ip-10-0-0-210:~/ccit$ ./hello.sh

./hello.sh: 2: Syntax error: "(" unexpected

Practical 2

Inputs reading from the script

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/bash

echo "Please enter your name"

read uname

echo "The supplied user name is:$uname"

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Please enter your name

Raja

The supplied user name is:Raja

 We can given directly using below script 

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/bash

read -p "Please enter your name:" $uname

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Please enter your name:Suresh

Practical 3 : Athematic Operations

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/bash

num1=10

num2=5

echo "Sum of the numbers:"$((num1+$num2))

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Sum of the numbers:15

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/bash

num1=10

num2=5

echo "Sum of the numbers:"$((num1+$num2))

echo "Substraction of the numbers:"$((num1-$num2))

echo "Multiplication of the numbers:"$((num1*$num2))

echo "Division numbers:"$((num1/$num2))

echo "Sum of the numbers:"$((num1%$num2))

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Sum of the numbers:15

Substraction of the numbers:5

Multiplication of the numbers:50

Division numbers:2

Sum of the numbers:0

Practical 4 : 

Conditional operations

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/bash

string1="ccit"

string2="ccit"

if [[ $string1==$string2 ]]; then

echo "both are same "

else

echo "both are not same"

fi

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

both are same

Practical 4: 

if you are passing any value to variable that variable will change automatically

Number 

Equal

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/bash

string1=5

string2=10

if [[ $string1 -eq $string2 ]]; then

echo "both are same "

else

echo "both are not same"

fi

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

both are not same

Not Equal

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/bash

string1=5

string2=10

if [[ $string1 -ne $string2 ]]; then

echo "both are not equal "

else

echo "both are same"

fi

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

both are not equal

 Note: 

String ==,!=

num -eq , -ne

less-than -  -lt -le 

Greater than  -gt -ge

and & 

or ||

And operator and OR Operator 

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/bash

echo "Enter username"

read uname

echo "Enter password"

read pass

if [[ $uname == "ccit" && $pass == "123" ]]; then

        echo "Valid user"

else

        echo "Invalid user"

fi

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Enter username

ccit

Enter password

123

Valid user

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Enter username

ccit

Enter password

345

Invalid user

OR Operator 

if [[ $uname == "ccit" || $pass == "123" ]]; then

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Enter username

ccit

Enter password

345

Valid user

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Enter username

raja

Enter password

123

Valid user

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

Enter username

raja

Enter password

34

Invalid user

File exist or not current directory 

[ec2-user@ip-10-0-1-88 ccit]$ cat ./hello.sh

#!/bin/bash

filepath="./file.txt"

if [[ -e $filepath ]]; then

        echo "file exists"

else

        echo "file does not exists"

fi

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

file does not exists

[ec2-user@ip-10-0-1-88 ccit]$ touch file.txt

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

file exists

Looping statements 

For Loop

for i (1..23);

done

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/bash

for i in {1..5}; do

        echo "the value is :"$i

done

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

the value is :1

the value is :2

the value is :3

the value is :4

the value is :5

while Loop

[ec2-user@ip-10-0-1-88 ccit]$ cat hello.sh

#!/bin/bash

count=0

while [[ $count -lt 10 ]]; do

        echo "the values is :"$count

        count=$((count+1))

done

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

the values is :0

the values is :1

the values is :2

the values is :3

the values is :4

the values is :5

the values is :6

the values is :7

the values is :8

the values is :9

For -le lessthan equal to 

while [[ $count -le10 ]]; do

[ec2-user@ip-10-0-1-88 ccit]$ ./hello.sh

the values is :0

the values is :1

the values is :2

the values is :3

the values is :4

the values is :5

the values is :6

the values is :7

the values is :8

the values is :9

the values is :10

--Thanks