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Saturday, May 8, 2021

How to add and remove Cronjobs from Linux EC2 instances on AWS using shell scripts

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Manual operations lead to human errors. Adding and removing Cronjobs frequently can be a very time-consuming task. In this article, we will create Shell scripts that automate the addition and deletion of Cronjobs from Ubuntu EC2 instances on AWS. To perform these operations you will need access to the EC2 instance. The user you will use needs to have sudo access so that the user can switch to root and perform addition and deletion of Cronjobs.

Let’s get started.

Pre-requisites

  1. Basic understanding of Shell scripts and Cronjobs.
  2. AWS Account (Create if you don’t have one).
  3. EC2 Instance with the user having sudo access (Click here to learn to create an EC2 instance if you don’t have one or if you want to learn )

What will we do

  1. Create a shell script to add Cronjobs.
  2. Execute the Shell script to add a Cronjob.
  3. Create a shell script to remove Cronjobs.
  4. Execute the Shell script to remove the Cronjob.

Create a shell script to add Cronjobs

Create a file on your local Linux System and add the following code to it. You can also find the code on my Github repo on the following link.

Github Link: https://github.com/shivalkarrahul/DevOps/blob/master/aws/shell-scripts/aws-ec2-add-remove-cron-job/add-cronjob.sh
File: add-cronjob.sh
#!/bin/bash

helpFunction()
{ 
	echo ""
	printf "33[1;32mUsage: $0 -K <internal.pem> -U <internal-user> -I <internal-ip> -a <cron-to-be-added>"
	echo ""
	echo -e "t-K ".pem key of the server on which a cron job has to be added""
	echo -e "t-U UserName of the server on which a cron job has to be added"
	echo -e "t-I IP of the server on which a cron job has to be added"
	echo -e "t-a Name of the cron to be added (in double quotes)"
	echo "Add a new Cron Job"
	echo "e.g."
	echo "./add-cronjob.sh -K /Users/cloudcover/Documents/Rahul/access/rahuls.pem -U ubuntu -I ec2-35-180-234-158.eu-west-3.compute.amazonaws.com -a "0 5 * * 1  testCronJob""

	echo -e "33[0m" #reset color
	exit 1 # Exit script after printing help
}

while getopts "I:K:U:a:" opt
do
	case "$opt" in
		K ) internalServerPemKey="$OPTARG" ;;
		U ) internalServerUser="$OPTARG" ;;	
		I ) internalServerIP="$OPTARG" ;;
		a ) addCron="$OPTARG" ;;
		? ) helpFunction ;; # Print helpFunction in case parameter is non-existent
	esac
done

echo  "******************"
#echo $listCronJobs
# Print helpFunction in case parameters are empty
if [ -z "$internalServerIP" ] || [ -z "$internalServerPemKey" ] || [ -z "$internalServerUser" ] || [ -z "$addCron" ]
then
	printf "33[1;31m"
	echo "Some or all of the parameters are empty";
	helpFunction
fi

# Begin script in case all parameters are correct
printf "33[1;33m------------------------------------------------------------------Before ssh"
echo -e "33[0m" #reset color
echo ".pem key of the server on which a new user has be created		:  	$internalServerPemKey"
echo "UserName of the server on which a new user has be created		: 	$internalServerUser"
echo "IP of the server on which a new user has be created			:	$internalServerIP"
echo "Name of the cron to be added				:	$addCron"


printf "33[1;31mLogging into: "$internalServerPemKey" "$internalServerUser"@"$internalServerIP"33[0mn"

ssh -i "$internalServerPemKey" "$internalServerUser"@"$internalServerIP" << HERE
printf "33[1;33m------------------------------------------------------------------After ssh"
echo -e "33[0m" #reset color
#echo "Executing connect_prod_cron_new.sh"
#sh connect_prod_cron_new.sh
#sleep 2
echo "after ssh"
echo "IP Of the Server:" 
hostname -I
echo "Hostname of the Server:"
hostname
echo "Changing user to root"
	sudo su <><> EOF
	echo "User Switched To;"
	whoami
	printf "33[1;33m------------------------------------------------------------------List of Cron Jobs Before Addition"
	echo -e "33[0m" #reset color
	crontab -l | cat -n
	if [ -n "$addCron" ]
	then
		echo "Inside addCron"
		crontab -l >crontab.tmp
		printf '%sn' "$addCron" >>crontab.tmp
		crontab crontab.tmp && rm -f crontab.tmp
	fi
	printf "33[1;33m------------------------------------------------------------------Updated List of Cron Jobs"
	echo -e "33[0m" #reset color
	crontab -l | cat -n
	printf "33[1;31mExiting from         ---> "$internalServerPemKey" "$internalServerUser"@"$internalServerIP"33[0mn"
	#echo "Existing user	---> $userName"
EOF
HERE

Before you create a new Cronjob, check if the EC2 instance has any existing Cronjobs

Login to the EC2 instance and check existing Cronjobs

ssh -i ~/Downloads/howtoforge-test.pem [email protected]

List the Cronjobs

crontab -l

Execute the Shell script to add a Cronjob

Go to your local Linux machine and add a Cronjob on Ubuntu 18.04 EC2 instance using the following command. This will create a Cronjob that will be triggered every minute and write the current date to a file. You can change the Cronjob as per your requirement.

./add-cronjob.sh -K ~/Downloads/howtoforge-test.pem -U ubuntu -I ec2-15-236-64-128.eu-west-3.compute.amazonaws.com -a "* * * * * /bin/date >> /tmp/cron_output"

Now, you can also go to the EC2 instance to check if the Cronjob has been added or not.

ssh -i ~/Downloads/howtoforge-test.pem [email protected]
sudo -i
crontab -l
cat /tmp/cron_output

In the following screenshot, you can see that the Cronjob has been added and executed every minute.

Check the Cronjob

Create a shell script to remove Cronjobs

Now, if you think you need to remove the Cronjob you added, you can easily do it using the shell script available on my Github. 

Create a new file on your local system with the following code.

Github Link: https://github.com/shivalkarrahul/DevOps/blob/master/aws/shell-scripts/aws-ec2-add-remove-cron-job/remove-cronjob.sh
File: remove-cronjob.sh
#!/bin/bash

helpFunction()
{ 
	echo ""
	printf "33[1;32mUsage: $0 -K <internal.pem> -U <internal-user> -I <internal-ip> -l <yes/no>"
	echo ""
	echo -e "t-K ".pem key of the server on which a cron job has to be removed""
	echo -e "t-U UserName of the server on which a cron job has to be removed"
	echo -e "t-I IP of the server on which a cron job has to be removed"
	echo -e "t-l List the existing Cron Jobs, provide "yes" as a parameter. Get a list first and then specify job no which needs to be removed"
	echo -e  "e.g."
	echo "Remove a new Cron Job"
	echo "./remove-cronjob.sh -K /Users/cloudcover/Documents/Rahul/access/rahuls.pem -U ubuntu -I ec2-52-47-90-247.eu-west-3.compute.amazonaws.com -l yes"
	echo -e "33[0m" #reset color
	exit 1 # Exit script after printing help
}

while getopts "I:K:U:l:" opt
do
	case "$opt" in
	K ) internalServerPemKey="$OPTARG" ;;
	U ) internalServerUser="$OPTARG" ;;	
	I ) internalServerIP="$OPTARG" ;;
	l ) showListOfJobs="$OPTARG" ;;
	? ) helpFunction ;; # Print helpFunction in case parameter is non-existent
	esac
done

echo  "******************"
echo $listCronJobs

# Print helpFunction in case parameters are empty
if [ -z "$internalServerIP" ] || [ -z "$internalServerPemKey" ] || [ -z "$internalServerUser" ] || [ -z "$showListOfJobs" ]
then
	printf "33[1;31m"

	echo "Some or all of the parameters are empty";
	helpFunction
fi

# Begin script in case all parameters are correct
printf "33[1;33m------------------------------------------------------------------Before ssh"
echo -e "33[0m" #reset color
echo ".pem key of the server on which a new user has be created		:  	$internalServerPemKey"
echo "UserName of the server on which a new user has be created		: 	$internalServerUser"
echo "IP of the server on which a new user has be created			:	$internalServerIP"

if [ $showListOfJobs == "yes" ]
then

	printf "33[1;31mLogging into: "$internalServerPemKey" "$internalServerUser"@"$internalServerIP"33[0mn"
	ssh -i "$internalServerPemKey" "$internalServerUser"@"$internalServerIP" << HERE
		printf "33[1;33m------------------------------------------------------------------After ssh"
		echo -e "33[0m" #reset color
		echo "after ssh"
		hostname -I
		hostname
		echo "Changing user to root"
		sudo su << EOF
			echo "User Switched To;"
			whoami
			printf "33[1;33m------------------------------------------------------------------List of Cron Jobs Before Deletion"
			echo -e "33[0m" #reset color
			crontab -l | cat -n
			printf "33[1;31mExiting from         ---> "$internalServerPemKey" "$internalServerUser"@"$internalServerIP"33[0mn"
EOF
HERE
fi


echo "Enter Cron Job Line Number to be removed"
read lineNumber
printf "33[1;31mLogging into: "$internalServerPemKey" "$internalServerUser"@"$internalServerIP"33[0mn"
ssh -i "$internalServerPemKey" "$internalServerUser"@"$internalServerIP" << HERE
	printf "33[1;33m------------------------------------------------------------------After ssh"
	echo -e "33[0m" #reset color
	echo "after ssh"
	hostname -I
	hostname
	#sleep 2
	echo "Changing user to root"
	sudo su << EOF
		echo "User Switched To;"
		whoami
		printf "33[1;33m------------------------------------------------------------------List of Cron Jobs Before Deletion"
		echo -e "33[0m" #reset color
		crontab -l | cat -n
		crontab -l | sed -e "$lineNumber"d >crontab.tmp
		crontab crontab.tmp && rm -f crontab.tmp
		printf "33[1;33m------------------------------------------------------------------Updated List of Cron Jobs"
		echo -e "33[0m" #reset color
		crontab -l | cat -n
		printf "33[1;31mExiting from         ---> "$internalServerPemKey" "$internalServerUser"@"$internalServerIP"33[0mn"
EOF
HERE

Execute the Shell script to remove the Cronjob

To remove the Cronjobs, execute the shell script. It will list all the Cronjob available on your Ubuntu 18.04 EC2 instance. You can then select the job to be deleted, the script will do the deletion for you.

./remove-cronjob.sh -K ~/Downloads/howtoforge-test.pem -U ubuntu -I ec2-15-236-64-128.eu-west-3.compute.amazonaws.com -l yes

Remove the Cronjob

Now, you can execute the same script again to list the Cronjob in the EC2 instance.

./remove-cronjob.sh -K ~/Downloads/howtoforge-test.pem -U ubuntu -I ec2-15-236-64-128.eu-west-3.compute.amazonaws.com -l yes

List the Cronjobs using the script

You can also check the Cronjob from the EC2 instance itself.

ssh -i ~/Downloads/howtoforge-test.pem [email protected]
sudo -i
crontab -l

List the Cronjob from the instance

Conclusion

In this article, we saw Shell scripts to add and remove Cronjobs from Ubuntu EC2 instance. This will help to automate the manual task of adding or removing Cronjobs and also avoid potential human errors that can come due to manual operations.

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