# Linux System Management
## Processes
## Networking
### IP Configuration
### Firewall
Uncomplicated Firewall (UFW) is a simple to use tool which allows you to set up firewall rules in less than 5 minutes.
```bash
sudo -s
apt-get install ufw
# --------------------
# This says that it will allow any outgoing connections on port 22 using the TCP protocol
ufw allow out 22/tcp
# This allows all traffic on port 22 TCP/UDP
ufw allow 22
# This will deny ALL outgoing connections
ufw default deny outgoing
```
By default, UFW will create a set of IPv4 and IPv6 - it is recommended that you delete whichever one you don't use.
## Automation
There are tasks that we might do every time we start our system or every so often - like backing up folders, starting an application or multiple applications, or running scripts. Instead of doing it manually, Linux provides a process called **Cron** which we can utilise to automate these boring tasks and do them for us.
### Crontabs
A crontab is a file that's utilised by the cron process to execute each line of instructions.\
Crontabs need 6 values in-order to work:
| Value | Description |
| ----- | ----------------------------------------------- |
| MIN | What minute to execute at |
| HOUR | What hour to execute at |
| DOM | What **D**ay **O**f the **M**onth to execute at |
| MON | What **M**onth **O**f the **Y**ear to execute a |
| DOW | What **D**ay **O**f the **W**eek to execute at |
| CMD | The actual command that'll get executed |
| \* | NULL |
[*TryHackMe - Linux 3*](https://tryhackme.com/room/linuxfundamentalspart3)
For example, if we wanted to start a script called `update_repos.sh` every Friday, we would do:
```bash
* * * * 5 bash /home/username/update_repos.sh >/dev/null 2>&1
```
We can use a tool like [Crontab Generator](https://crontab-generator.org/) to make these for us since memorising the positions is a hassle and we like things easy *right?*\
The asterisks represent NULL inputs if we don't want to include additional values.
#### Creating & Editing a crontab
Enough with the boring stuff, let's look at how to create our own crontabs. Start by opening your terminal and executing `crontab -e`
```bash
no crontab for USER - using an empty one
Select an editor. To change later, run 'select-editor'.
1. /bin/nano <---- easiest
2. /usr/bin/vim.basic
3. /usr/bin/vim.tiny
4. /bin/ed
Choose 1-4 [1]:
```
You might get this on first launch, select 1 if you want to go with `nano`, I prefer Vim so I'll use the second option. Scroll down to the bottom of the list and create a new line by hitting the `return` key (Enter).\
\
Now, go to [Crontab Generator](https://crontab-generator.org/) and create a process to automate!
Once you're done, click generate and paste the output into your crontab editor
Press `Ctrl + X` to save on Nano, or `:wq` on Vim. That's it!\
If you want to remove your saved crontabs, you can run `crontab -r`.
