Cron is a time-based program used explicitly to initiated other programs at particular times on a Linux system. AWS Linux AMI comes with cron pre-installed and configured, like every other modern Linux installation. The base configuration allows for set up of a task that should be run hourly, daily, weekly or monthly as well as any other time period.
Quick Job Setup
Setting up a job to run hourly, daily, weekly or monthly is very quick. These instructions assume you have already setup an AWS instance and have an SSH client (like PuTTY) available.
- Log in to your instance via the SSH client. Transfer to the root user.
- Go to the '/etc' directory
- Open the appropraite 'cron.XXXXX' directory. For example, if you want to add an hourly task, open the 'cron.hourly' directory.
- Create a new file (shift+F4)
- Add a single line for each job you want run. For example if you want to run a PHP script, type "/usr/bin/php -q /path/to/script/script.php", substituting for the correct path to PHP and the script file.
- Save the file. The name doesn't really matter so long as it is unique. Cron will call every file in the directory at the appropriate time and run the commands in each file.
Custom Job Setup
Setting up a job to run at custom time requires you to understand the crontab syntax. The syntax is not terribly complex (it is similar than Regular Expressions) but is complex enough that you don't want to deal with it if you do not need to.
Crontab Syntax
Crontab is the configuration structure for cron jobs. Simply, the files are composed of two parts: the settings followed by the jobs.
Crontab settings
The settings section states what should be run, where it is located, who should run it (as in Linux User) and a few other special commands. In the below example, the first 4 lines are settings and the last line is a job.
Example crontab file
- SHELL=/bin/bash
- PATH=/sbin:/bin:/usr/sbin:/usr/bin
- MAILTO=root
- HOME=/
- 01 * * * * root run-parts /etc/cron.hourly
- "SHELL" identifies which shell state you want to run the scripts under. If it is not included, most systems will default the shell indicated in '/etc/passwd' or just fail to run.
- "PATH" is location of the cron initiated scripts. If you are regularly running scripts in '/usr/bob/scripts' you could add the path here to avoid having to type '/usr/bob/scripts' for every script. In the above example, the PATH line would become "PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/bob/scripts"
- "MAILTO" is the address of the webmaster who will receive an email every time these scripts are run.
- "HOME" is the home directory for cron. It is basically prefixed to any relative path name you use in any script. It is only actually useful if you are going to be running many scripts from the same crontab file.
Crontab jobs
The settings section is the easy part of crontab files. Each job is composed of seven fields each separated by a space. Any field that is not set identified with an asterisk (*). minute hour day month weekday user cmd
- minute is the minute the job is to be run.
- hour is the hour (on a 24 hour clock) when the job is to be run.
- day is the numerical representation of the day of the month. The values range from 1-31.
- month is the month's numerical representation. The values range from 1-12.
- weekday is the day of the week you want the job to be run on. The values range from 0-7, with Sunday being 0 & 7. If the day and weekday are both specified, the command runs when EITHER is true.
- user is the Linux user you want to run the command.
- cmd is the command to be run. This is no different than the commands in the quick job setup section above.
With the exception of day and weekday, all of the other time commands must be true in order for the script to run. Crontab also supports step function (non-whole number inputs) and ranges. Ranges work just like page ranges do in most word processors with comma (,) separating individual values and dashes (-) representing all values from the first number to the second number. For example, "3-5,7" would fire on 3, 4, 5, and 7. Step functions use a forward slash to fire when the value becomes a whole number. For example a minute value of "*/15" would fire every 15 minutes, when the current minute divided by 15 is a whole number.
Creating a new Crontab File
These instructions assume you have already setup an AWS instance and have an SSH client (like PuTTY) available.
- Log in to your instance via the SSH client. Transfer to the root user.
- Go to the '/etc/cron.d' directory
- Create a new file (shift+F4)
- Add each setting value as its own line at the beginning of the file. The above example file's values should work on the AWS Linux AMI installation.
- Add crontab job lines for each job you want to run.
- Save the file. The name doesn't really matter so long as it is unique. Cron will call every file in the directory at the appropriate time and run the commands in each file.