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This post will explain how we can run cronjobs within Docker image based on Alpine Linux distribution, so am going to use my newest PHP 7.2 docker image, which you can find at zaherg/php-7.2-xdebug-alpine .
First, we will have to pull the docker image using the command:
docker pull zaherg/php-7.2-xdebug-alpine
Now that we have downloaded the image, we will have to familiarize our self with the structure for the
directory which we can find under
/etc/periodic, this directory should contain the following folders:
Your scripts should be put within the folder that you think is the best for you, but you need to make sure that your scripts follow these two rules:
- start with
- that your script is executable
Let's start playing with my docker image, but since it runs with the user `www-data` who do not have write permission on those directories, so we will have to run the image with the root user using the following command
docker run --rm -it --user root zaherg/php-7.2-xdebug-alpine:latest sh
once we are inside the docker image we can go to one of the directories listed above, or even create our own if we like, and create a simple script which will just `echo hello world`
cd /etc/periodic /etc/periodic/ # mkdir everymin /etc/periodic/ # cd everymin /etc/periodic/everymin # cat > hello << EOF #!/bin/sh echo "Hello world" >> /var/log/cron.log 2>&1 EOF /etc/periodic/everymin # chmod a+x hello /etc/periodic/everymin # ls -al total 12 drwxr-xr-x 2 root root 4096 Dec 3 12:20 . drwxr-xr-x 1 root root 4096 Dec 3 12:00 .. -rwxr-xr-x 1 root root 57 Dec 3 12:01 hello
As we can see I created a directory called
everymin , and then created a file within the
everymin directory and
hello (no suffix or extension), and remember to make sure that your script is executable.
Now we will see how to add our script to the cronjob list, as we have mentioned before alpine use
busybox so a small search you will see that the file which
crond reads is located at
cd /etc/crontabs/ /etc/crontabs # ls -al total 12 drwxr-xr-x 1 root root 4096 Dec 1 16:31 . drwxr-xr-x 1 root root 4096 Dec 3 12:00 .. -rw------- 1 root root 355 Dec 3 12:01 root
we need to add our
everyday directory to the
root file so we can sure it will be executed
min hour day month weekday command */15 * * * * run-parts /etc/periodic/15min 0 * * * * run-parts /etc/periodic/hourly 0 2 * * * run-parts /etc/periodic/daily 0 3 * * 6 run-parts /etc/periodic/weekly 0 5 1 * * run-parts /etc/periodic/monthly * * * * * run-parts /etc/periodic/everymin
and remember you will have to use
To run the scheduler all we need to do is to execute the command
crond -f as from the help
means in the foreground
/ # crond --help BusyBox v1.24.2 (2016-06-22 17:51:28 GMT) multi-call binary. Usage: crond -fbS -l N -d N -L LOGFILE -c DIR -f Foreground -b Background (default) -S Log to syslog (default) -l N Set log level. Most verbose:0, default:8 -d N Set log level, log to stderr -L FILE Log to FILE -c DIR Cron dir. Default:/var/spool/cron/crontabs
now you will get something like the following image
Now that you know how to deal with
crond in Alpine Linux, you can build your own Docker image that use this
information to run some scheduled tasks.
PS: if you know a better way please share your idea, so we can all benefit from it.