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.

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 Day Of the Month to execute at
MON	What Month Of the Year to execute a
DOW	What Day Of the Week to execute at
CMD	The actual command that'll get executed
*	NULL

TryHackMe - Linux 3

For example, if we wanted to start a script called update_repos.sh every Friday, we would do:

* * * * 5 bash /home/username/update_repos.sh >/dev/null 2>&1

We can use a tool like Crontab Generator 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

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 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.