eolas/Programming_Languages/Shell_Scripting/Cron.md

---
tags:
  - Programming_Languages
  - shell
---
# Cron


## `cronie` 
In Arch Linux I use `cronie` for cron jobs. (There is no cron service installed by default). Install `cronie` and then enable it in systemd with: 

```bash 
systemctrl enable --now cronie.service
```
## Commands

### List cron jobs
```
crontab -l
```

### Open cron file
```
crontab -e
```
### Check cron log
```bash

journalctl | grep CRON
# Different distros have different loggers
```

## Syntax
````bash
m h d mon dow command
# minute, hour, day of month, day of week, bash script/args
# 0-59, 0-23, 1-31, 1-12, 0-6
````


**Examples**

Run on the hour every hour 

````
0 * * * * mysqlcheck --all-databases --check-only-changed --silent
````

At 01:42 every day: 

````
42 1 * * * mysqlcheck --all-databases --check-only-changed --silent
````

Every half hour: 
```
0,30 * * * * ${HOME}/bash_scripts/automate_commit.sh

```

**Shorthands**

* `@reboot` – Run once, at startup
* `@yearly` – Run once a year, “0 0 1 1 \*”.\</>
* `@annually` – same as @yearly
* `@monthly` – Run once a month, “0 0 1 * \*”
* `@weekly` – Run once a week, “0 0 * * 0”
* `@daily` – Run once a day, “0 0 * * \*”
* `@midnight` – same as @daily
* `@hourly` – Run once an hour, “0 * * * \*”

**Examples**

````
@hourly mysqlcheck --all-databases --check-only-changed --silent

````

**View the logs**

````bash
sudo grep crontab syslog

````