1 Overview

As system administrators, we always encounter the need to schedule tasks. We can cronachieve this by serving in the Linux system . In addition, we can in the container systemcron

Now, this tutorial will clarify that in the Docker container cronin the first method, we embed the cronservice in the docker image, and the other method will explain how to install the scheduling service in the container.

2. Cron service - use Dockerfilemethod

DockerfileBuilding an image is one of the easiest ways to create a container image. So, what should we do? Basically, it Dockerfileis a simple text file that contains a set of instructions for building an image. We need to provide scheduling tasks, crondetailed information, and slave Dockerfile cronservices.

2.1. WritingDockerfile

Let's take a quick look at an example:

$ tree
 .
 ├── Dockerfile
 └── get_date.sh
 0 directories, 2 files

DockerfileFROMStart with the first line of command, which will get the requested image from the configured registry. In our example, the default registry configuration is DockerHub. Then MAINTAINER, it is metadata used to capture author information. ADDThe instruction get_date.shcopies the script from the image build path of the host to the target path of the image.

After copying the script to the build image, the RUNinstructions will grant executable permissions. Not only that- RUNinstructions help to execute any shell command as a new image layer above the current layer and submit the result. RUNUpdate the aptrepository and cronit is still crontab cronscheduling in the image .

Finally, we will use the CMDcommandcron

$ cat Dockerfile

 # Dockerfile to create image with cron services
 FROM ubuntu:latest
 MAINTAINER baeldung.com

 # Add the script to the Docker Image
 ADD get_date.sh /root/get_date.sh

 # Give execution rights on the cron scripts
 RUN chmod 0644 /root/get_date.sh

 #Install Cron
 RUN apt-get update
 RUN apt-get -y install cron

 # Add the cron job
 RUN crontab -l | { cat; echo "* * * * * bash /root/get_date.sh"; } | crontab -

 # Run the command on container startup
 CMD cron

2.2. Build and run cron image

Once Dockerfileready, we can use docker buildcommands to build the image. The dot (.) instructs the Docker engine to Dockerfile Dockerfilecreate a docker layer from each instruction given on the current path to form the final build image . The typical build output looks like this:

$ docker build .
 Sending build context to Docker daemon 3.072kB
 Step 1/8 : FROM ubuntu:latest
 ---> ba6acccedd29
 Step 2/8 : MAINTAINER baeldung.com
 ---> Using cache
 ---> e6b3946b2382
 Step 3/8 : ADD get_date.sh /root/get_date.sh
 ---> 4976f058d428
 Step 4/8 : RUN chmod 0644 /root/get_date.sh
 ---> Running in 423a4e9adbab
 Removing intermediate container 423a4e9adbab
 ---> 76d972a082ba
 Step 5/8 : RUN apt-get update
 ---> Running in badc0d84f6ff
 Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
 ...
 ... output truncated ...
 ...
 Removing intermediate container badc0d84f6ff
 ---> edb8a19b891c
 Step 6/8 : RUN apt-get -y install cron
 ---> Running in efd9b8a67d98
 Reading package lists...
 Building dependency tree...
 ...
 ... output truncated ...
 ...
 Done.
 invoke-rc.d: could not determine current runlevel
 invoke-rc.d: policy-rc.d denied execution of start.
 Removing intermediate container efd9b8a67d98
 ---> 2b80000d32a1
 Step 7/8 : RUN crontab -l | { cat; echo "* * * * * bash /root/get_date.sh"; } | crontab -
 ---> Running in 1bdd3e0cc877
 no crontab for root
 Removing intermediate container 1bdd3e0cc877
 ---> aa7c82aa7c11
 Step 8/8 : CMD cron
 ---> Running in cf2d44873b36
 Removing intermediate container cf2d44873b36
 ---> 8cee091ca87d
 Successfully built 8cee091ca87d

Since we have cronpre-installed the service into the image and embedded the task crontab, when we run the container cronor, we can use the docker runcommand to start the container. Subsequently, the docker run -it"option helps to enter the container through the bash prompt.

The following figure shows the script cronin the container get_date.sh:

$ docker run -it 8cee091ca87d /bin/bash
 [email protected]:/#
 [email protected]:/# date
 Mon Nov 15 14:30:21 UTC 2021

 [email protected]:/# ls -ltrh ~/date.out
 ls: cannot access '/root/date.out': No such file or directory

 [email protected]:/# ls -ltrh /root/get_date.sh
 -rw-r--r-- 1 root root 18 Nov 15 14:20 /root/get_date.sh

 [email protected]:/# crontab -l
 * * * * * bash /root/get_date.sh

 [email protected]:/# ls -ltrh ~/date.out
 -rw-r--r-- 1 root root 29 Nov 15 14:31 /root/date.out

 [email protected]:/# cat /root/date.out
 Mon Nov 15 14:31:01 UTC 2021

3. Cron service-real-time container method

Or, we can work in a Docker container cron cron. So, what is the modus operandi?

Let's docker runquickly run a command using commands. ubuntuUsually, the container is a lightweight operating system and does not include cronservices as its default package.

Therefore, we need to ** enter the container's interactive shell and use the aptrepository command cron** service:

$ docker run -it ubuntu:latest /bin/bash
 Unable to find image 'ubuntu:latest' locally
 latest: Pulling from library/ubuntu
 7b1a6ab2e44d: Pull complete
 Digest: sha256:626ffe58f6e7566e00254b638eb7e0f3b11d4da9675088f4781a50ae288f3322
 Status: Downloaded newer image for ubuntu:latest
 root:/#
 root:/# which cron
 root:/# apt update -y
 Get:1 http://archive.ubuntu.com/ubuntu focal InRelease [265 kB]
 ...
 ... output truncated ...
 ...
 All packages are up to date.
 root:/# apt upgrade -y
 ...
 ... output truncated ...
 ...
 root:/# apt install cron vim -y
 Reading package lists... Done
 ...
 ... output truncated ...
 ...
 Done.
 invoke-rc.d: could not determine current runlevel
 invoke-rc.d: policy-rc.d denied execution of start.
 [email protected]:/# which cron
 /usr/sbin/cron

 $ docker cp get_data.sh 77483fc20fc9: /root/get_date.sh

We can use the get_date.sh docker cpcommand to copy get_date.sh from the host to the container. crontab -eUse the vieditor to edit cronthe cronconfiguration below to run the script every minute. In addition, output a timestamp indicating the execution of the script:

root:/# export EDITOR=vi
 [email protected]:/# crontab -e
 * * * * * bash /root/get_date.sh

 root:/# date
 Mon Nov 17 11:15:21 UTC 2021

 root:/# ls -ltrh ~/date.out
 ls: cannot access '/root/date.out': No such file or directory

 root:/# ls -ltrh /root/get_date.sh
 -rw-r--r-- 1 root root 18 Nov 17 11:09 /root/get_date.sh

 root:/# ls -ltrh ~/date.out
 -rw-r--r-- 1 root root 29 Nov 17 11:16 /root/date.out

 root:/# cat /root/date.out
 Mon Nov 17 11:16:01 UTC 2021

4 Conclusion

All in all, we have explored the cronspecific details of the assignment. The method ofcron using Dockerfile is tocron automatically execute scripts for service and task scheduling configuration.

Although it croncan be installed and configured in a running container, it is only a runtime construct unless we use a docker commitconstruct image.

In addition, depending on our usage, both options have their advantages.