# Cron and Crontab

Cron is a tool for configuring scheduled tasks on Unix systems, used to schedule commands or scripts to run periodically and at fixed intervals; tasks range from backing up the users' home folders every day at midnight, to logging CPU information every hour.

The command `crontab` (cron table) is used to edit the list of scheduled tasks in operation, and is done on a per-user basis; each user (including `root`) has their own `crontab`.

## Editing crontab

Run `crontab` with the `-e` flag to edit the cron table:

```
crontab -e
```

### Select an editor

The first time you run `crontab` you'll be prompted to select an editor; if you are not sure which to use, choose `nano` by hitting `Enter`.

The layout for a cron entry is made up of six components: Minute, hour, day of month, month of year, day of week, and the command to be executed.

```
# m h  dom mon dow   command
```

```
# * * * * *  command to execute
# ┬ ┬ ┬ ┬ ┬
# │ │ │ │ │
# │ │ │ │ │
# │ │ │ │ └───── day of week (0 - 7) (0 to 6 are Sunday to Saturday, or use names; 7 is Sunday, the same as 0)
# │ │ │ └────────── month (1 - 12)
# │ │ └─────────────── day of month (1 - 31)
# │ └──────────────────── hour (0 - 23)
# └───────────────────────── min (0 - 59)
```

For example:

```
0 0 * * *  /home/pi/backup.sh
```

This cron entry would run the `backup.sh` script every day at midnight.