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Introduction to Linux

1. Navigating the File System

  • $ pwd : Present working directory.
  • $ cd /path/to/file : Change directory.
  • $ ls -la : List everything in the folder, even hidden files
  • $ mkdir /path/to/file : Make directory.
  • $ rmdir /path/to/file : Remove directory.
  • $ cp /path/to/source /path/to/destination : Copy file.
  • $ rm /path/to/file : Remove file.
  • $ mv /path/to/source /path/to/destination : Move file or rename it.
  • $ locate <filename> : Locate a file.
    \--> Use $ updatedb to refresh the database.
  • $ echo "something" : Displays "something".
  • $ <command> > /path/to/file : Writes the output of the command in the file.
        $ echo "Hello World!" > test.txt
    # Creates a file test.txt in the current directory with "Hello World!" written inside.
  • $ man <command> : Manual page for that command.

2. Users and Privileges

When doing a ls -la we can see the file details.

$ ls -la
total 148
drwxr-xr-x 14 kali    kali     4096 Mar  3 17:16 .
drwxr-xr-x  3 root    root     4096 Jul 27  2020 ..
-rwxr--r--  1 walbert support     0 Oct 31 11:06 test
linux-file-permissions

  • $ cat /etc/passwd to see users.
  • $ passwd : Change password of the current user.
  • $ cat /path/to/file : Displays the file in terminal.
  • $ chmod 777 /path/to/file : Change mode (rwx) of the file.
  • $ adduser <username> : Add a user "username".
  • $ su <user> : Switch user.
  • $ sudo <command> : Super User DO command.

Sudoers file: anyone in there can use the sudo command.

3. Network Commands

  • $ ifconfig : Shows the different network information.
  • $ iwconfig : Wireless network information.
  • $ ping <ip> : Sends ICMP packet (ping) to the given ip.
  • $ arp -a : Shows the IP addresses talking to you and the MAC addresses associated.
  • $ netstat -ano : Shows active connection on your machine (interesting during pentest to see if the machine is talking to another one).
  • $ route : Prints the routing table.

These are becoming deprecated, now the ip command is the goto :

  • $ ip a : IP address, Network, Broadcast
  • $ ip n : ARP table
  • $ ip r : Routing table

4. Installing and Updating tools

$ apt update && apt upgrade
# Updates the system

4.1. Install package

$ apt install pip
# Installs pip

4.2. From Github

$ cd /opt
# Downloads in the opt file : good practice

$ git clone https://github.com/username/repository.git
# Clones the repository on the local machine

4.3. Install gedit

$ sudo apt install gedit

$ gedit test.txt

5. Viewing, Creating and Editing Files

$ echo "test" > test.txt
# Overwrites a test.txt file with "test" inside

$ cat test.txt
test

$ echo "test2" >> test.txt
# Appends the echo standard output to the file test.txt

$ cat test.txt
test
test2
  • $ touch /path/to/file : Creates an empty file with the mentioned name.

6. Scripting with Bash

Let's make a script to grab the IP address after a successful ping

6.1. Command to extract only the IP after the ping

$ ping 192.168.4.29 -c 1 > ip.txt

$ cat ip.txt
PING 192.168.4.29 (192.168.4.29) 56(84) bytes of data.
64 bytes from 192.168.4.29: icmp_seq=1 ttl=64 time=0.028 ms
--- 192.168.4.29 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.026/0.027/0.029/0.001 ms

$ cat ip.txt | grep "64 bytes"
64 bytes from 192.168.4.29: icmp_seq=1 ttl=64 time=0.028 ms

$ cat ip.txt | grep "64 bytes" | cut -d " " -f 4
192.168.4.29:

$ cat ip.txt | grep "64 bytes" | cut -d " " -f 4 | tr -d ":"
192.168.4.29

6.2. The Script

$ vim ipsweep.sh
# Creates the file sweep.sh and opens vim editor

The script should look like this :

#!/bin/bash

if [ "$1" == "" ]
then
echo "You forgot an IP address!"
echo "Syntax : ./ipsweep.sh 192.168.1"

else
for ip in `seq 1 254`; do
ping -c 1 $1.$ip | grep "64 bytes" | cut -d " " -f 4 | tr -d ":" &
done
fi
# When running the script, we also give the beginning of the IP address : $1
# "&" Allows to run multiple commands at the same time

ipsweep.sh

$ chmod +x ./ipsweep.sh
# Allows us to execute the script

$ ./ipsweep.sh 192.168.1
192.168.1.1
192.168.1.22
192.168.1.23
192.168.1.24
192.168.1.53

We can then put all of these IP addresses in a file, in order to help our scanning through nmap : 

$ ./ipsweep.sh 192.168.1 > valid_ipaddress.txt

$ for ip in $(cat valid_ipaddress.txt); do nmap $ip; done