Shell Scripting basics

 

Shell:

Its like a container.

interface between users and kernal / OS.

CLI is a shell

Find your shell:

echo $0: gives the output which shell you are using. cat /etc/shells: gives the output of available shells.

your shell? /etc/passwd

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Shell Scripting:

What is shell Script?

Shell Script is the executable file containing multiple shell commands that are executed sequentially. The file can contain:

1. shell

2. comments

3. commands

4. statements

shell script should have executable permissions.

shell script has to be called absolute path.

if it is called from the current location then, ./script.sh

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Shell Script-Basic Scripts:

Question: Write the script to print "hello world" on the terminal. Answer:

content of script1.sh should be:

#!/bin/bash 

echo Hello World

chmod a+x script1.sh: give the executible permission to all for this script.

/script1.sh: run the script.

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Question: Defining Tasks

#!/bin/bash

# Define small tasks

whoami

echo             #echo command will give the space in the output.

pwd

echo

hostname

echo

Is-ltr

echo

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Question: Defining variables

#!/bin/bash

# Example of defining variables

a=Imran

b=Afzal

c='Linux class'

echo "My first name is $a"

echo "My surname is $b"

echo 'My class is $c'

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Question: Read Input

#!/bin/bash

# Read user input

echo "What is your first name?"

read a

echo

echo "What is your last name?"

read b

echo

echo Hello $a $b

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Question: Scripts to run commands within. 

#!/bin/bash

#Script to run commands within

clear 

echo "Hello `whoami`"

echo

echo "Today is `date`" 

echo

echo "Number of user login: `who | wc -1`"

echo

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Question : rename the filename. 

#!/bin/bash

#This script will rename a file

echo Enter the file name to be renamed

read oldfilename

echo Enter the new file name

read newfilename

mv $oldfilename $newfilename

echo The file has been renamed as $newfilename

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If-then Scripts:

Question: Check the variable

#!/bin/bash 

count=100 

if [ $count -eq 100 ]

then

echo Count is 100

else

echo Count is not 100 

fi

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Question: Check if a file error.txt exist

#!/bin/bash

clear 

if [ -f /home/iafzal/error.txt ] 

then

echo "File exist" 

else

echo "File does not exist "

fi

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Question:Check if a variable value is met

#!/bin/bash

a=`date | awk '{print $1}'`

if [ "$a" == Mon]

then

echo Today is $a

else

echo Today is not Monday

fi

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Question: Check the response and then output

#!/bin/bash

clear

echo

echo "What is your name?"

echo

read a

echo

echo Hello $a sir

echo

echo "Do you like working in IT? (y/n)"

read Like

echo

if [ "$Like" == y ]

then

echo You are cool

elif [ "$Like" == n]

then

echo You should try IT. it's a good field

echo

fi

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For loop Scripts: 

Simple for loop output 

#!/bin/bash

for i in 1 2 3 4 5

do

echo "Welcome $i times"

done

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Simple for loop output

#!/bin/bash

 for i in eat run jump play

do

echo See Imran $i 

done

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Question:

for loop to create 5 files named 1-5

#!/bin/bash 

for i in (1..5)

do

touch $i

done

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Question:

for loop to delete 5 files named 1-5

#!/bin/bash 

for i in (1..5)

do

rm $i

done

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Specify days in for loop

#!/bin/bash

 i=1

for day in Mon Tue Wed Thu Fri

do

echo "Weekday $((i++)): $day"

done

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List all users one by one from /etc/passwd file

#!/bin/bash

i=1

for username in `awk -F: '{print $1)' /etc/passwd

do

echo "Username $((i++)): $username"

done

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