## Background

Adding jobs to /etc/cron.x keeps things modular and 'clean'. You can add hourly, daily, weekly and monthly (regular) scripts by creating your script in the appropriate folder: `/etc/cron.{hourly,daily,weekly,monthly}`

The cron daemon will run these hourly, daily, etc scripts at intervals defined in `/etc/crontab`

```bash
SHELL=/bin/sh  
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin  
  
# m h dom mon dow user    command  
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 )
```

Observe the `PATH` variable set. This is why you don't need to provide explicit paths (e.g. `/bin/rm`), but it's a good idea to do it anyway.

If you need to run a script akin to adding it to the crontab, you'll need to place it in `/etc/cron.d`

## Adding regular scripts

Let's say you want something to run weekly.

* Create your script
* Move it to `/etc/cron.weekly`

Before you rejoice,

* You *must* add the `#!/bin/sh` line to the top of your scripts! Not
    doing so will result in `Exec format error` messages.
* You *must* set the permissions to 755

## Adding crontab-style scripts

You'd add these to `/etc/cron.d/''scriptname''`. The format's the same, except that you must *explicitly* provide the user executing the script (makes sense.) For example:

```bash
#!/bin/sh  
09,39 * * * *  root  /bin/rm -rf /tmp/junk/*
```

Add that to a file, move it to `/etc/cron.d`. Make sure that:

* The `#!/bin/sh` line exists and is first.
* The permissions to 755.

## `crontab` Syntax

Stolen [from here](https://www.drupal.org/docs/7/setting-up-cron-for-drupal/configuring-cron-jobs-using-the-cron-command) since, even though I've done this for years, I cannot remember this for the life of me.

```bash
# +---------------- minute (0 - 59)
# |  +------------- hour (0 - 23)
# |  |  +---------- day of month (1 - 31)
# |  |  |  +------- month (1 - 12)
# |  |  |  |  +---- day of week (0 - 6) (Sunday=0)
# |  |  |  |  |
  *  *  *  *  *  command to be executed
```