We start by creating the bash script first and continue with the cron job after the bash script has been tested to work as intended. All commands in the following paragraphs are executed from the command line.

vi backup.sh

This will create an empty text file in the vi editor. This will be the script that will be executed by the cron job on a daily basis.

bin/sh
/usr/bin/mysqldump -A -u [Username] -p[Password] | gzip > /backups/mysqldump`date +%m_%d_%y`.gz


Press i to enter insert mode and copy and paste the two lines above in the editor. What the script does is to use the mysqldump function to create a mysql backup and store it gzipped in the backups directory using mysqldumpDate.gz as the name with data being the day the backup was created.

The two variables [Username] and [Password] have to be edited and the username and password of a user with sufficient rights entered. It might also be necessary to change the location of the backup folder.

Press ESC to enter command mode and :x to save the file.

./backup.sh

Execute the script to see if it is working correctly. It should create the dump of the MySQL database in the defined directory. It might be necessary to change the permission level of the backup.sh file if you get a permission error during execution.

chmod 755 backup.sh

If the script is working as intended it can be added as a cron job. The following command will open the user crontab list.

crontab -e

If you are in the vi editor you press i to enter insert mode again and add the following line

0 7 * * * /path/to/backup.sh

Please refer to the tutorial for a detailed instruction on how to set the cron job up to suite your needs. This one will execute the script every day on 7am.

It is a good idea to check back after the next execution to verify that everything is working as intended.

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The cron system works by reading various crontab files either found in a users’ directory or within the /etc directory. These cron files can be edited with either the crontab command or by editing the various files within /etc. Only the root user can edit the /etc files because those files (found in the subdirectories /etc/cron.d, /etc/cron.daily, /etc/cron.hourly, /etc/cron.weekly, /etc/cron.monthly) are used for system services.  Instead of focusing on system services, I will illustrate how to automate the execution of a user-created bash script.

This imaginary bash script will be called myscript.sh. This script will be housed in /home/jlwallen/ and will have executable permission (by way of chmod u+x myscript.sh).  What we want to do is automate the execution of this script so that it executes once per day at the same time every day (we’ll say 7am).

Before we get into editing with crontab we first need to chat about how cron views time.The cron system looks at time like this:

• Minute(0-59)
• Hour (0-23)
• Day of the month (1-31)
• Month (1-12)
• Day of the week (0-6 with Sunday being 0)

The structure of cron time looks like this:

Minute Hour Day of month Month Day of the week

The trick for cron is that when you do not have an entry for a section you add an “*”. So the time entry for a cron job that should run at 7am every day of the week would look like:

0 7 * * *

That is not the complete cron entry, just the time portion. Now, on to editing with crontab.

Crontab

Users edit their cron entries with the crontab command. The crontab command has a few switches:

• e – Edit your crontab
• l – List your crontab
• r – Delete all crontab entries

To add a new crontab entry enter the command crontab -e which will place you in the vi text editor with your user crontab file opened. If you’re not familiar with vi I’ll give you the crash course as we edit the crontab.

First click the “i” key to go into the insert mode. You can now add text. Enter the cron entry to run the myscript.sh every day at 7am. This entry will look like:

* 7 * * * ~/myscript.sh

Now hit the Esc key to get out of the insert mode. Once out of insert mode you need to write the file and quite crontab. To do this hit the “:” key followed by “wq” (for write and quite). Finish the job by hitting the Enter key.

You should see “crontab: installing new crontab” at your bash prompt.

Congratulations, you’ve just created your first cron job!

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