How to Dockerize a Cron Job

To run a cron job inside a Docker container, you will need to use the cron service and run it in the foreground in your Docker container.

Here’s an example of how you can set this up:

Create Cron File:

Create a file that contains all the cron jobs to be run under the Docker container. For example, let’s call this file `cron`.

cat cron 

Our example file looks like:

cron
* * * * * echo "Current date is `date`" > /var/log/cron

Add all of the crontab jobs in the above file that needs to be run under the Docker container.

Create Dockerfile

Next, create a `Dockerfile` that installs the cron service and copies the script into the container. Here we have provided 3 examples of Dockerfile using the different-2 operating system. The Alpine Linux is very lightweight and is sufficient for running cronjobs. But if you need to set up other environments Linux Ubuntu, Apache, and PHP. choose any of the below given Dockerfile templates based on your requirements.

Dockerfile with Alpine Linux

FROM alpine:3

# Copy cron file to the container
COPY cron /etc/cron.d/cron

# Give the permission
RUN chmod 0644 /etc/cron.d/cron

# Add the cron job
RUN crontab /etc/cron.d/cron

# Link cron log file to stdout
RUN ln -s /dev/stdout /var/log/cron

# Run the cron service in the foreground
CMD [ "crond", "-l", "2", "-f" ]

Dockerfile with Apache and PHP

FROM php:8.0-apache

# Install cron
RUN apt update && \
    apt -y install cron

# Copy cron file to the container
COPY cron /etc/cron.d/cron

# Give the permission
RUN chmod 0644 /etc/cron.d/cron

# Add the cron job
RUN crontab /etc/cron.d/cron

# Link cron log file to stdout
RUN ln -s /dev/stdout /var/log/cron

# Start cron service
RUN sed -i 's/^exec /service cron start\n\nexec /' /usr/local/bin/apache2-foreground

Dockerfile with Ubuntu Linux

FROM ubuntu:latest

# Install cron deamon
RUN apt update && apt install -y cron

# Copy cron file to the container
COPY cron /etc/cron.d/cron

# Give the permission 
RUN chmod 0644 /etc/cron.d/cron

# Add the cron job
RUN crontab /etc/cron.d/cron

# Link cron log file to stdout
RUN ln -s /dev/stdout /var/log/cron

# Run the cron service in the foreground
CMD ["cron", "-f"]

Build and Run Container

You have two files in your current directory. One is `cron` that contains the cronjobs and the second is Dockerfile that have the build instructions for Docker. Run the following command to build the Docker image using the Dockerfile:

docker build -t my_cron . 

Once the image is successfully built, You can launch a container using the image:

docker run -d my_cron  

This will start the cron daemon under the container, which will all the scheduled jobs defined in `cron` file.

Test Setup

The cronjobs should be successfully configured with the docker containers. As we have linked cron log file (/var/log/cron) to the /dev/stdout. Show all the logs generated by the cron service can be seed in `docker logs` command.

First, find the container id or name using `docker ps` command.

docker ps 

Then check the log files of the Docker container.

docker logs container_id

In the cronjobs, I have printed the current date and written them to logs. You can see the below image that shows the current date in logs every minute.



It means the cron jobs are running properly under the Docker container.

Wrapping Up

Cron jobs are a handy way to automate daily tasks on your computer, like backing up files. With Docker, though, things get a little trickier because you can’t just launch a new cron instance from your terminal and expect it to run. When running a cron job in a docker container, you’ll run into a few challenges depending on how and where you want to run the container. The cron job interval is specified in seconds and can range from one minute to just over 100 years. The cron job frequency is specified in terms of the number of times the cron job should be executed every day.

When it comes to scheduling jobs and programs that automatically run at set intervals or can be triggered by another event, you have plenty of options. You can use a general-purpose utility like cron, the built-in scheduler in macOS or Linux, or a specialized tool like AWS Lambda. Cron, though not as powerful as AWS Lambda, is a solid choice if you’re using containers because it’s designed to handle background tasks on Unix systems. With Docker, however, things get a little trickier because you can’t just launch a new cron instance from your terminal and expect it to work.

The post Running a Cronjob Inside Docker: A Beginner’s Guide appeared first on TecAdmin.

How to Sepreate Two Commands in Linux

You can separate two or more commands by using semicolons (;), logical AND (&&), or logical OR (||) operators. Which of these operators we use, is totally depends on the requirements. Here is the basic understanding of using these operators.

1. Semicolon (;): is used to separate multiple commands. This executes all the commands without checking the exit status of previous commands.

   command_1;  command_2;  command_n
   

2. Logical AND (&&): is used to separate commands when we want to execute the next command only if the previous command was successfully executed with exit status 0.

   command_1 &&  command_2 &&  command_n
   

3. Logical OR (||): is used to separate commands when we want to execute the next command only if the previous command failed with a non-0 exit status.

   command_1 ||  command_2 ||  command_n
   

How to Schedule a Cron Job

First, switch to the user from which you want to run a cron job. Then open the crontab editor by running the following command.

crontab -e 

Then you add a cron job entry to the file like below:

Running Multiple Commands in Single Cron Job

Let’s discuss the real-life examples of running multiple commands with crontab with different-2 separates.

Using Semicolon (;)

We can separate two or more commands with semicolons, that don’t require checking the exit status of the previous command. For example, you need to change the permission of all files to 777 but need to set 777 for the logs directory.

0 2 * * *   chmod -R 755 /var/www/myapp; chmod -R 777 /var/www/myapp/logs



Using Logical AND (&&)

Use this operator, where you want to run the next command only if the previous is executed successfully (exit status 0). For example, you want to run the backup.sh after successfully changing to /backup directory.

0 2 * * *   cd /backup && bash backup.sh



Using Logical OR (||)

Use the logical OR (||) operator, when you want to run the next command only if the previous is failed (exit status non-0). For example, you want to show a message or send an email if the backup file not found.

0 2 * * *  [ -f /backup/mydb-`date +%F`.sql ] || echo "Today's backup file not found"

Conclusion

In this blog post, you have learned about running multiple commands in a single cron job entry. Also discussed various options to separate commands. The article provides you the basic details about each separator that is used to separate commands.

The post How to Run Multiple Commands in One Cron Job appeared first on TecAdmin.

You can edit the crontab editor by running the crontab -e command and configure your job to run at 02:00 AM daily. For example, to schedule a backup script schedule cron job as below:

0   2   *   *   *    /usr/bin/backup.sh

Once the cron is scheduled, you can list the configured jobs using the crontab -l command. Hoping this tutorial helps you for scheduling the cron job.

The post Running a Cron Job at 2:00 AM Daily appeared first on TecAdmin.

Scheduling a Cronjob for Sunday.

The day of the week is determined by the fifth section of the cron timer. You can specify a day by using numbers such as 0-7, where 0 and 7 both correspond to Sunday, 1 is Monday, 2 is Tuesday, and so on. We may also refer to the first three letters of the day like sun, mon, etc.

The cron daemon can be configured to run shell scripts at 07:30 AM every Sunday using the following line:

• Define a day in Crontab using numbers:

  30  7  *  *  0      script.sh
  

• Define a day in Crontab using numbers:

  30  7  *  *  sun      script.sh
  

You can schedule your weekly cronjob by editing the crontab using “crontab -e” in a text editor.

Scheduing Job with @weekly Keyword

The @weekly crontab keyword provides for executing tasks every Sunday at 12:00 AM. You may replace the time with @weekly, which saves time. The command will execute at precisely 12:00 AM, and we cannot alter it.

@weekly     script.sh

This quick tutorial should help you set up a corn job to run weekly. Please note that you can schedule a job in two-time formats.

The post Running a Cron job every Sunday (Weekly) appeared first on TecAdmin.

In this article, you will learn how to schedule Python using cron and some useful examples of when and how you might use these practices in your organization.

Running Python Script with Crontab

I have created a sample Python application, that required a script to run every 15 minutes. You can use crontab -e to open the crontab editor and add the job as below:

A Python script can be configured using one of the below options depending on the environment:

• Default Python Version: If the application runs with system default Python version, use below crontab settings:

  */15 * * * * python /home/tecadmin/app/cron.py
  

• Non-default Python Version: You can use other Python versions by providing the complete binary path. Some applications required a Python version that is not set as default on the system

  */15 * * * * /usr/bin/python3.10  /home/tecadmin/app/cron.py
  

• Python with Virtual Environment: The applications running with the Python virtual environment can be scheduled as below. Here /home/tecadmin/app/venv is the directory containing virtual environment files.

  */15 * * * * /home/tecadmin/app/venv/bin/python  /home/tecadmin/app/cron.py
  

Wrap Up

In this quick how-to tutorial, you have learned to schedule Python scripts with crontab in Linux and macOS systems.

The post Scheduling a Python Script with Crontab appeared first on TecAdmin.

For example:

0    */12   *    *    *      script.sh

The above script will run at 12 AM (00:00:00) and 12 PM (12:00:00) every day.

As you can see that using */12 executes cron exactly at 12AM and 12PM. But, sometimes you may need to run crontab at different times like 10 AM and 10 PM. In that case, you need to set hours like 10,22. See the below example:

0    10,22   *    *    *      script.sh

You can change hours of your choice like 09,17 to execute cron at 09 AM and 05 PM.

Hope this tutorial helps you with crontab configuration and schedule jobs twice a day.

The post Running cron job every 12 hours (twice a day) appeared first on TecAdmin.

In this article, we will see how we can run a job cron job every 10 minutes, 20 minutes, or 30 minutes. Use the following timer options to schedule a job in crontab to run on specified intervals.

• Running a corn every 10 minutes
  

  */10  *  *  *  *  /opt/script.sh

• Running a corn every 20 minutes
  

  */20  *  *  *  *  /opt/script.sh

• Running a corn every 30 minutes
  

  */30  *  *  *  *  /opt/script.sh

The above jobs run at specific intervals.

The post Running a Cron Every 10, 20 or 30 Minutes appeared first on TecAdmin.

Syntax:

Use the following syntax to schedule a command or script to run every 5 minutes using crontab.

*/5   *   *   *   *    command

Example

To schedule a PHP script to run every 5 minutes. Run the command crontab -e to edit crontab in an editor and schedule your job. You can use crontab -e command to edit the crontab editor:

*/5 * * * * /usr/bin/php -f /var/www/html/cron.php

The cronjob will run the above command at 5-minute intervals.

The post Running a Cron Every 5 Minutes appeared first on TecAdmin.

Problem

While using the wget with cron job saved the downloaded files under home directory. Due to this a large number of junk files get created in your system.

Solution

Use -O option with wget command to write the result file (data) to specific file and location. Next select /dev/null device file as target file. This will discard anything written to it. In result, no junk files will be created in your home directory.

For example, the original cron job command is:

wget https://www.example.com/cron.php

Update above command to:

 wget -q -O /dev/null https://www.example.com/cron.php

Here:

• -q Turn off wget command output
• -O /dev/null Write downloaded content (file) to /dev/null device.

That’s it. Hope this tutorial helps you to avoid unwanted files on root generated by wget cron jobs.

The post [SOLVED] Cron job wget writing files to root directory appeared first on TecAdmin.

This tutorial will show you the several options to list all scheduled cron jobs for users on Linux systems.

How to List Cron Jobs of Current User

The default crontab command works for the current logged in user. You can list all the scheduled cron jobs for the current user, execute:

crontab –l 

Output:

All the user cron jobs are generally located under /var/spool/cron/crontabs directory. A separate file is created for all the user accounts with thier name.

List Cron jobs of Other User

A root or sudo priviledged user can also view scheduled cronjobs of other users. Use -u followed by the username to list all jobs that belong to a specific user.

For example:

sudo crontab –u username –l 

Replace username with the actual username you want to view cron jobs.

List Cron Jobs Running by System

The root user can access and modify the crontab’s of the operating system. You can view the system’s cronjobs by running the following command as root or sudo privileged account.

less /etc/crontab 

Output:

# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name command to be executed
17 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )

How to List Hourly Cron Jobs

You can view the /ettc/cron.hourly directory to find all the cron jobs scheduled to run on every hour.

ls -la /etc/cron.hourly 

Output:

total 20
drwxr-xr-x   2 root root  4096 Apr 23  2020 .
drwxr-xr-x 142 root root 12288 Jan 19 15:21 ..
-rw-r--r--   1 root root   102 Feb 14  2020 .placeholder

The above output shows, that there is no cron job schedule to run hourly. You can see a file .placeholder in each directory, which is created to avoid accidental deletion of directory by package manager. When no other file exists in directory.

How to List Daily Cron Jobs

Similarly, you can list all the scheduled job to run on daily basis. Most of the application jobs can be find in this directory.

ls -la /etc/cron.daily 

Output:

total 72
drwxr-xr-x   2 root root  4096 Dec 28 15:28 .
drwxr-xr-x 142 root root 12288 Jan 19 15:21 ..
-rwxr-xr-x   1 root root   311 Jul 16  2019 0anacron
-rwxr-xr-x   1 root root   539 Apr 13  2020 apache2
-rwxr-xr-x   1 root root   376 Dec  5  2019 apport
-rwxr-xr-x   1 root root  1478 Apr  9  2020 apt-compat
-rwxr-xr-x   1 root root   355 Dec 29  2017 bsdmainutils
-rwxr-xr-x   1 root root   384 Nov 19  2019 cracklib-runtime
-rwxr-xr-x   1 root root  1187 Sep  6  2019 dpkg
-rwxr-xr-x   1 root root   377 Jan 21  2019 logrotate
-rwxr-xr-x   1 root root  1123 Feb 25  2020 man-db
-rw-r--r--   1 root root   102 Feb 14  2020 .placeholder
-rwxr-xr-x   1 root root  4574 Jul 18  2019 popularity-contest
-rwxr-xr-x   1 root root   383 Jan  6  2020 samba
-rwxr-xr-x   1 root root   214 Apr  2  2020 update-notifier-common

How to List Weekly Cron Jobs

The weekly cron jobs are scheduled under /etc/cron.weekly directory.

ls -la /etc/cron.weekly 

Output:

total 32
drwxr-xr-x   2 root root  4096 Apr 23  2020 .
drwxr-xr-x 142 root root 12288 Jan 19 15:21 ..
-rwxr-xr-x   1 root root   312 Jul 16  2019 0anacron
-rwxr-xr-x   1 root root   813 Feb 25  2020 man-db
-rw-r--r--   1 root root   102 Feb 14  2020 .placeholder
-rwxr-xr-x   1 root root   211 Apr  2  2020 update-notifier-common

How to List Montly Cron Jobs

All the monthly cron jobs are scheduled under /etc/cron.monthly directory.

ls -la /etc/cron.monthly 

Output:

total 24
drwxr-xr-x   2 root root  4096 Apr 23  2020 .
drwxr-xr-x 142 root root 12288 Jan 19 15:21 ..
-rwxr-xr-x   1 root root   313 Jul 16  2019 0anacron
-rw-r--r--   1 root root   102 Feb 14  2020 .placeholder

How to View Application Specific Cron Jobs

May of applications scheduled cron jobs for regular works. These jobs can be found under hourly, daily, weekly, or monthly cron jobs.

For example, Apache web server created cron job file under /etc/cron.daily. It means the job is executed on daily basis. You can see the cron job content by accessing file content as below:

cat /etc/cron.daily/apache2 

Output:

#!/bin/sh

# run htcacheclean if set to 'cron' mode

set -e
set -u

type htcacheclean > /dev/null 2>&1 || exit 0
[ -e /etc/default/apache-htcacheclean ] || exit 0


# edit /etc/default/apache-htcacheclean to change this
HTCACHECLEAN_MODE=daemon
HTCACHECLEAN_RUN=auto
HTCACHECLEAN_SIZE=300M
HTCACHECLEAN_PATH=/var/cache/apache2/mod_cache_disk
HTCACHECLEAN_OPTIONS=""

. /etc/default/apache-htcacheclean

[ "$HTCACHECLEAN_MODE" = "cron" ] || exit 0

htcacheclean ${HTCACHECLEAN_OPTIONS}    \
                -p${HTCACHECLEAN_PATH}  \
                -l${HTCACHECLEAN_SIZE}

Conclusion

In this tutorial, you have learned to view, list or display cron jobs on a Linux system. Additinaly, you found details about cron jobs executed on hourly, daily, weekly or monthly basis.

The post How to View or List Cron Jobs in Linux appeared first on TecAdmin.