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Printing

To print we can use the command echo

Eg:1

#!/bin/bash
echo “Printing First Line” 
echo “Printing Second Line”
echo “Printing Third Line”

To read the input from command line input , user can use the read command

Eg:2

#!/bin/bash
echo "Enter the keyboard input"
read input
echo  “Command line input is : $input”

String Manipulation

Print a file in shell script

Eg:3

#!/bin/bash
cat FileName.txt

Finding the matching line in text with grep , below command will print the all lines in the file that has the word Ubuntu

Eg:4

#!/bin/bash
cat FileName.txt | grep “Ubuntu”

Can Use the option -i with grep command to ignore the case sensitivity. So the below script will print the lines with word Ubuntu ,UBUNTU and any case varinet of word ubuntu

Eg:5

#!/bin/bash
cat FileName.txt | grep -i “Ubuntu”

grep command can be used with option -v to display the non-matching lines. Below command will print the all lines in the file which does not have the word GPL

Eg:6

#!/bin/bash
cat FileName.txt | grep -v  “GPL”

Input Output Operations

Redirect output to a file , > symbol will redirect the output to a file (The > symbol is used for output (STDOUT) redirection.)

Eg:7

#!/bin/bash
cat FileName.txt | grep Ubuntu > Output.txt

Command output of ls -ltr will redrect to Directory_list.txt file

Eg:8

#!/bin/bash
ls -ltr  > Directory_list.txt

The '<' symbol is used for input(STDIN) redirection ,So beneath command wc -l take input as file content of FileName.txt

( wc -l command count the number of lines in a given file)

Eg:9

#!/bin/bash
wc -l < FileName.txt

To Append content to a file , we can use the symbol of >> . For example below commands will append print first sentence and print second sentence sentence to end of the Output.txt file

Eg:10

#!/bin/bash
Echo  “print first sentence” >> Output.txt 
Echo  “print second sentence” >> Output.txt

PIPE

Symbol | is using to pass output of a command to another command as a input

Eg: 11

#!/bin/bash
cat FileName.txt | grep -v  “GPL”

cat FileName.txt will print the contents of the file. In the above by using PIPE | symbol its output is pass to the command grep -v “GPL” for further processing

IF ELSE Statements

If a condition is true, the algorithm performs a certain action; else, it performs something else.

if [ condition ]
then
  trueAction....
else
  falseAction
fi

Below script check whether given Number “A” and “B” are equal to each other

Eg:12

#!/bin/bash
A=5
B=50

if [ $A == $B ]   #Be Mindful about the spaces in between the if , [ , $A , == , $B , ]  characters, which  is a must. Otherwise script will throw a syntax error 
then
   echo "A is equal to B"
else
   echo "A is not equal to B"
fi

Script validates whether number “A” is greater than number “B”

Eg:13

#!/bin/bash
A=5
B=50

if [ $A -gt $B ]
then
   echo "A is greater  than B"
else
   echo "A is less than  B"
fi

Below are the most commonly used operators in IF statements

STRING1 = STRING2   True if STRING1 and STRING2 are equal.
STRING1 != STRING2  True if STRING1 and STRING2 are not equal.
NUMBER1 -eq NUMBER2 True if NUMBER1 and NUMBER2 are equal.
NUMBER1 -gt NUMBER2 True if NUMBER1 is greater than NUMBER2.
NUMBER1 -lt NUMBER2 True if NUMBER1 is less than NUMBER2.
NUMBER1 -ge NUMBER2 True if NUMBER1 is equal or greater than NUMBER2.
NUMBER1 -le NUMBER2 True if NUMBER1 is equal or less than NUMBER2

IF ELIF statements

Format of if elif statements ( Nested if else operations )

If [ conditional expression1 ]
then
	statement1
	statement2
	.
elif [ conditional expression2 ]
then
	statement3
	statement4
elif [ conditional expression3 ]
then 
	statement5
else
	statement6
fi

Eg:14

Refer to the below example which outline how to use if elif conditions .

#!/bin/bash
number=50

if [ $number -eq 100 ]
then
 echo "number is equal to 100"

elif [ $number -lt 100 ]
then
 echo "number is less than 100"

else
 echo "number is greater than 100"
fi

Loops

In this section of our Bash scripting Tutorial , we'll look at the two different loop formats available to us as well as discuss how to use them in scripting via examples

For Loops

Below script will print the numbers from 1 to 10

Eg:15

#!/bin/sh
for i in 1 2 3 4 5 6 7 8 9 10
do
  echo “$i"
done

Script print the command from 1 to 500

Eg:16

#!/bin/bash
for i in {1..500}
do
   echo " $i "
done

Print the below sentence line by line , and if word matches “API” print WSO2 instead

Eg:17

#!/bin/bash

words=”Deliver enhanced digital experiences and grow your business at speed and scale with WSO2’s industry-leading products for API management and integration and customer identity and access management.”

for value in $words
do

If [ $value == “API” ]
then
Echo “WSO2”
else  
 echo $value
fi 
done

While Loops

While loop which print values from 1 to 10

Eg:18

#!/bin/bash
num=1

while [ $num -le 10 ]
do
        echo $num
        num=`expr $num + 1`
Done

One of major advantages of while loop is that , it can be easily used to read the contents of a FILE by line by line and those lines can be forward for further processing

Script read the continents.txt file line by line and print it

Eg:19

#!/bin/bash
while  read line

do
        echo $line
   
done < continents.txt

Below example will read the continents.txt file line by line and print each line , Additionally it will print Not a continent statement when line matches the word Colombo

Eg:20

#!/bin/bash

while  read line

do
        if [ $line =  "Colombo" ]
        then
        echo "Not a continent"
        else
        echo $line
        fi
done < continents.txt

Retrieving Columns via AWK command

By default, awk command filter the columns by using space as separator

UserStat.txt File

sam   192.168.1.2  https://google.com/mail

Jeff  192.168.4.4  https://facebook.com/profile

Tom   10.10.1.1    https://google.com/gsuite

Simon  172.1.2.1    https://wso2.com/jobs

Below stript will print the first column of UserStat.txt file

Eg:21

#!/bin/bash
cat UserStat.txt | awk '{ print $1 }'

Script will print the first column and third column with space in the middle

Eg:22

#!/bin/bash
cat UserStat.txt | awk '{ print $1 "  " $3 }'

Additionally awk command can be used to filter columns by using custom symbols ,For instace when user need to specify the comma as field separator, he can use option as -F ‘, ‘

UserSalary.csv File

From left to right column are Email, employeeID, FirstName, LastName, Salary

rachel@yourcompany.com,9012,Rachel,Booker,7000

laura@yourcompany.com,2070,Laura,Grey,4500

craig@yourcompany.com,4081,Craig,Johnson,4000

mary@yourcompany.com,9346,Mary,Jenkins,2000

jamie@yourcompany.com,5079,Jamie,Smith,2500

Below commands will print the second column of the file , Please note that COMMA is used as field / column separator

Eg:23

#!/bin/bash
cat EmailStat.csv | awk -F ',' '{ print $2 }'

Script will take the first and third columns and redirect it to the output to file

Eg:24 
#!/bin/bash
cat EmailStat.csv | awk -F ',' '{ print $1" "$3 }'  > output.txt

Mathematical Operations

Below script will run the basic mathematics operations and give the relevant outputs

Eg:25

#!/bin/sh

A=70
B=90

val=`expr $A + $B` #Be Mindful about the spaces in between the "expr" , $A , + , $B  characters, which  is a must. Otherwise script will throw a syntax error 
echo "A + B : $val"

val=`expr $A - $B`
echo "A - B : $val"

val=`expr $A \* $B`
echo "a * b : $val"

val=`expr $B / $A`
echo "B / A : $val"

val=`expr $B % $A`
echo "B % A : $val”

Exercises

  1. Write a shell script to print values from 1 to 20 by using FOR loop .

    • If the number is less than or equal to 5 then print SMALL
    • If the number is larger than or equal to 15 , then print HIGH
    • Else print the number as it is (Answer can be found in exercise_no1.sh)

  2. Write a shell script to print the Names line by line from UserStat.txt file, and print the word WSO2-USER if the username is equal to Tom. Output should be as below format (Answer can be found in exercise_no2.sh)

Sam
Jeff
WSO2-USER
Simon

  1. Read the given UserSalary.txt file and print Salary values larger than 2500 . Output should be as below (Answer can be found in exercise_no3.sh)

7000
4000
4000