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
```