Day 4 - Basic Linux Shell Scripting for DevOps Engineer

Day: 4 - Introduction to Linux Shell Scripting

1. What is a shell?

Shell is an interface between the user and the kernel. Even though there can be only one kernel; a system can have many shell running simultaneously. So, whenever a user enters a command through the keyboard, the shell communicates with the kernel to execute it and then displays the output to the user.

1. What is a Shell Scripting?

Shell Scripting is an open-source computer program designed to be run by the Unix/Linux shell. Shell Scripting is a program to write a series of commands for the shell to execute. It can combine lengthy and repetitive sequences of commands into a single and simple script that can be stored and executed anytime which, reduces programming efforts.

1. What is Kernel? Explain its functions.

A kernel is considered the main component of Linux OS. It is simply a resource manager that acts as a bridge between hardware and software. Its main role is to manage hardware resources for users and is generally used to provide an interface for user-level interaction. A kernel is the first program that is loaded whenever a computer system starts. It is also referred to as low-level system software.

The main functions of Kernel:

Memory Management

Process Management

Device Management

Storage Management

1. What is #!/bin/bash?

#!/bin/bash is the first line of the script and is called Shebang. Shebang tells the shell to execute it via bash shell. Shebang is simply an absolute path to the bash interpreter.

There are different shebang lines for each different shell.

#!/bin/bash --> the script should always be run with bash, rather than another shell.

#!/bin/sh --> the interpreter should be a Bourne shell, not Bash.

#!/bin/csh --> interpreter here is a C shell.

#!/bin/ksh --> interpreter is Korn shell.

So we can use any shell type in our script as per the use case and availability of the type of shell in our system.

1. What is swap space?

Swap space, as the name suggests, is a space on a hard disk that is used when the amount of physical memory or RAM is full. It is considered a substitute for physical memory. Its main function is to substitute disk space for RAM when real RAM does not have enough space to hold all programs that are executing, and more space is required. In simple words, it can be used as an extension of RAM by Linux.

1. Write a Shell Script that prints “I will complete #90DaysofDeVops challenge"

/scripts$ vim scripts.sh
#!/bin/bash
echo "I will complete the 90Daysof DevOps challenge"
/scripts$ chmod 777 scripts.sh
/scripts$ ls -l
/scripts$ ./scripts.sh
I will complete the 90Daysof DevOps challenge

1. Write a Shell Script to Print the Addition of two numbers

$ vim add.sh
#!/bin/bash
#Add Two numeric value
((sum=25+35))
#Print the result
echo $sum
$ ./add.sh
60

1. Write a bash script create directories.sh that when the script is executed with three given arguments (one is the directory name and second is the start number of directories and third is the end number of directories ) it creates a specified number of directories with a dynamic directory name.

$ nano createDirectories.sh
#!/bin/bash
dir_name=$1
start=$2
end=$3

for (( i=start; i<=end; i++))
do
 mkdir "$dir_name$i"
done
$ chmod 777 createDirectories.sh
$ ./createDirectories.sh dir 1 90
$ ls
0                     dir18  dir29  dir4   dir50  dir61  dir72  dir83
add.sh                dir19  dir3   dir40  dir51  dir62  dir73  dir84
createDirectories.sh  dir2   dir30  dir41  dir52  dir63  dir74  dir85
dir1                  dir20  dir31  dir42  dir53  dir64  dir75  dir86
dir10                 dir21  dir32  dir43  dir54  dir65  dir76  dir87
dir11                 dir22  dir33  dir44  dir55  dir66  dir77  dir88
dir12                 dir23  dir34  dir45  dir56  dir67  dir78  dir89
dir13                 dir24  dir35  dir46  dir57  dir68  dir79  dir9
dir14                 dir25  dir36  dir47  dir58  dir69  dir8   dir90
dir15                 dir26  dir37  dir48  dir59  dir7   dir80  scripts.sh
dir16                 dir27  dir38  dir49  dir6   dir70  dir81
dir17                 dir28  dir39  dir5   dir60  dir71  dir82

1. Write an Example of If else in Shell Scripting by comparing two numbers

 $ vim compare.sh
 #!/bin/bash
 echo "enter the first number"
 read a
 echo "enter the second number"
 read b
 if [ $a = $b ]
 then
     echo "a is greater then b"
 else
     echo "b is greater then a"
 fi

$ ./compare.sh
Output :- 
enter the first number
15
enter the second number
22
b is greater then a

1. Write an Example of do while loop in shell scripting to print the numbers

 $ vi print.sh
#!/bin/bash
valid=true
count=1
while [ $valid ]
do
echo $count
if [ $count -eq 5 ];
then
break
fi
((count++))
done
$ chmod 777 print.sh
$ ./print.sh
Output :-
1
2
3
4
5

1. Create a Script to back up all your work done till now.

$ vi backup.sh
#!/bin/bash
src_dir=/home/user/Day5
tgt_dir=/home/user/backups
curr_timestamp=$(date "+%Y-%m-%d-%H-%M-%S")
backup_file=$tgt_dir/$curr_timestamp.tgz
echo "Taking backup on $curr_timestamp"

tar -cvzf $backup_file --absolute-names $src_dir
echo "Backup complete"

$ chmod 777 backup.sh
$ ./backup.sh

Thank you for reading! Happy Learning!!

Pooja