Day 4: Introduction to Shell Scripting

Hello Everyone!

This is Day 4 learning. It is a very Beginner friendly Note. Give it a read :)

Please find below the concepts this article covers on Shell Scripting.

1. Few Basic Commands

2. Checking file and directory exists or not

3. Writing a script

4. Comments

5. Arguments

6. File Creation

7. if, elif, and for loop

---

#To check which flavor of Linux you are using

cat /etc/os-release

#To know which bash you are using

echo $SHELL or echo $BASH

#who are the people who logged in

$who

$who -H (-H displays header)

#Checking if a file exists or not

The ‘-f’ option in brackets tells the command to check whether the specified file exists.

#Checking if a directory exists or not

#$1 in the script represent the command line argument that we will pass while ging execute command.

#!/bin/bash

dir=$1 if [ -d $1 ]

then echo "$1 directory exists"

else

echo "Sorry $1 directory doesnt exists"

fi

---

Writing a Script:

Editors to write the script:

There are different editors where we can write our script. Ex: vim, vi, nano, pico etc

• Vim is a mode-based text editor while Nano is modeless. Mode-based editor means that you need to enter INSERT mode before you can write text to the file.

• Nano is an improvement over the Pico text editor, whereas Vim is an improved version of the Vi editor

We will use vim editor.

#create a file called sample.sh, .sh represents it is a shell script

vim sample.sh

Executing a file:

we can execute in 2 ways

1. bash sample.sh

2. ./ sample.sh

In the second method, we need to assign execute permission to the file

chmod 777 sample.sh

ls -l

this file becomes executable and will be in green color

./sample.sh

Inside vim:

Press i key to insert

Esc :wq to save

Getting Started with Script:

#!/bin/bash

# = sharp

! = bang

#! = shebang

---

Comments:

single line comment - #

Multiline Comment -

<< 'COMMENT'

--

--

COMMENT

---

Arguments:

$1 represents command line argument, when you use $1 in the script it mean you need to give argument after filename. $2 - two arguments, $3 - three arguments.

$# - count of arguments

$$-PID of the script

$*-Represent all the arguments as a single string

$@ - Same as $∗, but differ when enclosed in (")

$?-Represent last return code

Example:

---

Sample file creation :

We will create a directory - (mkdir)

move to that directory - (cd directory name )

create a file in that dir - touch filename

Give data into it - echo "" > filename

---

if statement:

Note: Linux is space sensitive.

There should be space after "if" and after "[[" and before "]]"

n=1

if [ $n -eq 1 ]

then echo "This is true" fi

elif statement:

#!/bin/sh

a=10 b=20

if [ $a == $b ]

then

echo "a is equal to b"

elif [ $a -gt $b ]

then

echo "a is greater than b"

elif [ $a -lt $b ]

then

echo "a is less than b"

else

echo "None of the condition met"

fi

for loop:

#!/bin/bash

echo "Lines from $1"

for (( i=1; i<=$1; i++ ))

do

echo "$i"

done