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`cron` is a service used to run jobs on time-basis and is available on most UNIX-like operating system installations.


User `cron` jobs on O2 are run on a single server, `cronserver.rc.hms.harvard.edu`, and can be managed from any O2 login/compute node.


You don't have to do anything special to manage your `crontab` data on `cronserver`.

-   The `crontab` command on all O2 systems (login/compute) will automatically connect to `cronserver` for you.
-   The command is used in exactly the same way as on a standalone system.
-   Run `man crontab` for complete documentation or run `crontab` with no arguments for a brief usage summary.
-   To use a different editor than the default: `env EDITOR=vim crontab -e` (substitute the editor of your choice, e.g. emacs, pico, nano, etc. for "vim" in this example)

**General crontab help**
```
Usage:
  crontab [options]

Options:
  -e or --edit       edit user's crontab
  -l or --list       list user's crontab
  -r or --remove     delete user's crontab
```

The `crontab` command will remind you about `cron` service as follows:


-   For editing operations (`crontab -e`), a brief reminder and reference to this page will be included in the `crontab` editing buffer. This reminder will be removed after you save the file and before it is installed on `cronserver`.
-   For non-editing operations (list, `crontab -l` and remove, `crontab -r`), a brief reminder and reference to this page will be printed on standard error. You can differentiate this text from the actual output of `crontab(1)`, which appears on standard output.


If you need to know the details of where `cron` jobs are run:

-   You can find which O2 application server currently houses `cronserver` by running `ssh cronserver hostname` from any O2 system.
-   However, you should not rely on `cron` administration or execution being housed on any specific system.
-   If you need to refer to the system where `cron` jobs execute, be sure to use the role name `cronserver`.
-   If you need something to execute at a specific time on a specific O2 system, use `ssh(1)` to initiate that command from a `cron` job on `cronserver`.
	-   We recommend that you [contact us](https://rc.hms.harvard.edu/#support) if you do this to make sure you are running your job in an appropriate location and that you are using service role names and application role users when needed.


**Finally, remember that any workloads with significant processing, memory, or storage I/O requirements should be run on an O2 compute node via the SLURM. Please refer to our 
[Using Slurm Basic Guide](https://wiki.rc.hms.harvard.edu/display/O2/Using+Slurm+Basic)**


Please [contact Research Computing](https://it.hms.harvard.edu/our-services/research-computing/contact) if you have any questions about the O2 `cron` service.