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Linux Fundamentals.md

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file <filename> to get info on what kind of file it is history for command history for the user

How 2> works:

File descriptor 2 represents standard error. (other special file descriptors include 0 for standard input and 1 for standard output). 2> /dev/null means to redirect standard error to /dev/null. /dev/null is a special device that discards everything that is written to it.

ls -lh to list files in the directory

The first three columns are important in determining characteristics of a file or folder. They show which actions are possible and what user or group has the ability for this; actions such as:

  • read
  • write
  • execute

example above, user2 can read and write the file important however myfolder has a d flag, indicating it's a directory

SSH file transfer (SCP)

Uploading a file (you --> target) scp <local_file> <user>@<ip address>:<path_to_store_the_file> example: scp important.txt ubuntu@192.168.1.30:/home/ubuntu/transferred.txt

Downloading a file (you <-- target) scp <user>@<address>:<remote_file_location> <filename_to_store> example: scp ubuntu@192.168.1.30:/home/ubuntu/documents.txt notes.txt

Serving files from your Host Using Python3 (standard on many linux distro's) we can set up a lightweight HTTPServer Then we can use curl or wget to download these files remotely. simply run python3 -m http.server Then you can simply do something like: `wget http://10.10.134.212:8000/myfile

Processes 101

using the ps command lists processes for our user's session along with a status code, PID, CPU usage etc. and the name of the program

to see processes of other users use the aux switch like: ps aux Another useful command is: top, for real-time statistics of the system's processes (refreshes every 10 seconds or with movements)

If we want to stop a process we can do that with kill <PID>

starting a program on boot: systemctl <option> <service> for example to tell apache to start we'll use systemctl start apache2 there are 4 options for systemctl

  • start
  • stop
  • enable
  • disable enable/disable is to make it a 'startup' task, so it runs on the boot-up

to run a task in the background we can add an & ; example echo "hi" & to push a process away from the foreground we can do Ctrl+Z, to bring it back we use fg in the terminal

Automating tasks:

https://crontab-generator.org/ https://crontab.guru/

An example where we back up our Documents folder every 12 hours 0 */12 * * * cp -R /home/cmnatic/Documents /var/backups/ Cron supports wildcards or asterisk (*) for the fields we dont care about

Crontabs can be edited using crontab -e

System Package Management

Additional repositories can be added using add-apt-repository

An example on adding a new package (In this case for SublimeText) First off, we need to add the GPG key wget -qO - https://download.sublimetext.com/sublimehq-pub.gpg | sudo apt-key add - It is good practice to have separate file for every community/3rd party repository you add. this is done in the /etc/apt/sources.list.d directory

  • Make new file: touch sublime-text.list
  • in that file we add deb https://download.sublimetext.com/ apt/stable/
  • after saving and closing that, we run apt update to include the new entry now we can run apt install sublime-text

removing packages this is done by either add-apt-repository --remove ppa:PPA_Name/ppa or by doing apt remove [software-name-here]

Logs

these are located in the /var/log directory

Some are more interesting than others there are logs such as fail2ban which is used to monitor attempted brute forces the UFW service, which is used as a firewall. `

Common Directories

/etc

this root directory is a commonplace location top store system files used by the operating system. for example the sudoers file, containing a list of users & groups that have permissions to run sudo or a set of commands as root user.

Also home to the passwd and shadow file, that store the passwords for each user in an encrypted format (sha512crypt)

/var

This folder stores data frequently accessed or written by services or applications on the system, such as log files /var/log or other data not linked to a specific user (eg. databases)

/root

unlike the /home directory, this folder is the actual home for our root user

/tmp

this is a volatile directory, used to store data that is only temporary needed, this folder is cleared out on system restarts. This folder is useful for pentesting cause any user can write here, so we can store our enumeration scripts here.