this category contains explanation of all DevOps/CI-CD processes and pipelines


CRONTAB in Linux with commands and examples

vivek bangare

Vivek Bangare


CRONTAB in Linux 

In this article, I will talk about crontab. Crontab is a Linux Utility which allows tasks to be run automatically in the background. In today’s automation era, we usually deploy multiple tasks or runs processes. Sometimes we need to deploy scripts and commands at a specific scheduled time. Manually triggering these scripts and commands is annoying and sometimes uncomfortable also. End of this article you will be able to understand what is CRONTAB and How it is beneficial for day-to-day activity tasks. Let’s begin.

  • Crontab command is mainly used to automate our daily scheduled tasks. 
  • For instance, we can automate processes like data backup,  schedule updates, synchronization of files and many more.
  • Cron is a daemon to run scheduled tasks.
  • Crontab (CRON + TABLE) is a table.
  • Each user has their own crontab to create, modify or delete tasks.
  • By Default cron is enabled for all users, however, we can be restricted from adding an entry in the “/etc/cron.deny” file.
  • Crontab file consists of commands per line.
Crontab Restrictions:
  • username must be define in /usr/lib/cron/cron.allow.
  • if username present in /usr/lib/cron/cron.deny, then that user can’t use crontab.
  • if both cron.deny and cron.allow files are present and empty then all the users can use crontab. If neither file is present then only the root user can perform crontab.

The crontab file has six fields.


Let’s understand some use-cases and commands for crontab

  • To verify list of predefined cron files
ls /etc/cron
  • To write in crontab
crontab -e
  • Suppose you want to delete cronjob without asking
crontab -r
  • Suppose you want to ask before deleting cronjob
crontab -i -r
  • If you want to check whether cronjob is available or not  
crontab -l
  • Suppose you are a root user and want to create a cronjob for specific users
crontab -e -u {user_name}
crontab -l -u {user_name}
Crontab Examples:
  • Suppose we have to remove tmp files from /home/ubuntu/tmp each day at 6:30 PM
30 18 * * * rm /home/ubuntu/tmp/*
  • To run cron every minute
* * * * * rm /home/ubuntu/tmp/*
  • To run a cron of every minute at a specific hour, let’s assume we have run it at the 11th hour.
* 11 * * * rm /home/ubuntu/tmp/*
Bitbucket to AWS EC2 pipeline over SSH via RSYNC

Pipeline – Bitbucket to AWS EC2

vivek bangare

Vivek Bangare


Pipeline - Bitbucket to AWS EC2 over SSH

This Pipeline – Bitbucket to AWS EC2 helps us to transfer and receive data over ssh. Along with that, we will learn how to automate deployment from the bitbucket repository to AWS EC2. Here we will use the Rsync utility. 


Rsync is famous for its delta-transfer algorithm, which reduces the amount of data sent over the network by sending only the differences between the source files and the existing files in the destination. By default, Rsync will upload the whole directory to the target. 

  • You are already done with the setup of the AWS EC2 instance and have the private key as a .pem and public key as a .pub file. Also, you have configured the public key into the ~.ssh/authorized_key file.
  • You have a bitbucket account and one repository for which you have to configure the pipeline.

Firstly, you need administrator access to create the bitbucket pipeline. Else you will not see the Pipelines option in the bitbucket Repository Settings.

Step 1: Create a Pipeline

Pipeline ⇒ Starter pipeline (template) ⇒ Commit

In the left corner there is a section for Pipelines, click bitbucket to AWS.


You will get a below screen, where you can create a bitbucket pipeline, select any of the templates as of now because we will later change it according to our need.


After that, commit that file.


After committing the file,  the directory structure will look like below


Secondly, Open the bitbucket-pipeline.yml file in online edit mode and paste our below continuous deployment pipeline code.

Source ⇒ bitbucket-pipeline.yml

# Push artifacts to ec2 destination using rsync 
image: node:10.15.3

  - step:
        name: Deploy artifacts using rsync to $SERVER instance
          - pipe: atlassian/rsync-deploy:0.3.2
              USER: $USER
              SERVER: $SERVER
              EXTRA_ARGS: "--exclude=.bitbucket/ --exclude=.git/ --exclude=bitbucket-pipelines.yml --exclude=.gitignore"
          - echo "Deployment is done...!"
USER: username by which we ssh to instance
SERVER: IP or public dns of ec2
REMOTE_PATH: remote target directory
LOCAL_PATH: build directory
EXTRA_ARGS: variable is used for exclude files and folder which we don’t want to push to EC2 instance. 

Your pipeline is ready, but what about EC2 access? How do access EC2 instances through this pipeline? For that our most important next step will be to configure the SSH connection.

Step 2. Configuring SSH

Repository Setting ⇒ PipelineSection ⇒ SSH keys


Now you will get the following screen.


Here, you have to paste your public key ( .pub ) content into the public-key section.

Note: The same public key content must be in ~./ssh/authorized_key file in EC2 instance.

Repository Setting ⇒ Pipeline Section ⇒ SSH Keys ⇒ Public key 

And private key( .pem ) content into the private-key section.

Repository Setting ⇒ Pipeline Section ⇒ SSH Keys ⇒ Private key

You need to add the Host address and click on the Fetch button until it gives Fingerprint values. If you don’t configure this step then the pipeline will give an error : Could not resolve hostname key.

Step 3: Configure the env variables

This step is optional, if you configured hardcoded values in bitbucket-pipeline then we don’t need to use these variables, but as a best practice we used Repository variables. Here you will not able to create any repository variable directly unless and until you enabled the pipeline, so first enable pipeline:

Repository Setting ⇒ Pipeline Section ⇒ Setting ⇒ Enable pipeline

Now we can configure variables. 

Repository Setting ⇒ Pipeline Section ⇒ Repository Variable


Add required variables and their values.


That’s it, our pipeline is ready now. For implementing a pipeline, just push any code or add any file to the repository and commit it, Bitbucket will automatically run the pipeline. You can see the result or processing pipeline in the Pipeline section.

Step 4: Final Output

Click on Pipeline Section


This pipeline automatically triggers when any commits happen in your master branch and push all the data from the repository to the EC2 instance. You can configure it for any other branch also.