COMMAND LINE FUN

• Introduction to Kali Linux most popular and common commands

1 - THE BASH ENVIRONMENT

• Bash is a shell to run complex commands and preform different tasks

1.1 - ENVIRONMENT VARIABLES

• A new environment is initialized when opening new bash treminal
• environment data is stored on ENVIRONMENT VARIABLES

$PATH : directories which bash searches in for commands
echo $PATH

$USER : the current user
$HOME : the current users home directory
export b=1.1.1.1 # make new global environment variable
b=1.1.1.1 # make new environment variable but only accessible in current shell
env # Show all environment varialbes

1.2 - TAB COMPLETION

• Bash auto complete commands with TAB Key

ls D<tab>
Downloads/ Desktop/ Documents/

ls De<tab>
Desktop/

1.3 - BASH HISTORY TRICKS

• Bash store used commands in history file named .bash_history in user’s home directory
• 2 env variables contains history size
  • $HISTSIZE which is number of commands stored in memory for the current session
  • HISTFILESIZE which is how many commands kept in history file
  • These can be saved in bash history file
• arrow key up moves backward in history
• arrow ket down moves forward in history
• CTRL + R searchs in commands and fins suitable one

# Show history commands
history

# Run command with the number associated
!<number> 

# Execute last command in our terminal session (like arrow key up and enter)
!!

2 - PIPING AND REDIRECTION

• Every program has 3 data streams connected to it
  • stdin : 0
  • stdout : 1
  • stderr : 2
• We can connect streams between programs by piping using pipe operation |

2.1 - REDIRECTING TO A NEW FILE

# Replace <file name> content with abcd, If the file does not exist it will be created
echo abcd > "file name"

2.2 -  REDIRECTING TO AN EXISTING FILE

# Append content to the existing file
echo abcd >> "file name"

2.3 -  REDIRECTING FROM A FILE

# Counts characters in a file
wc -m < filename.txt

2.4 - REDIRECTING STDERR

# Redirect standard error of command to error.txt
ls ./non_existing_file 2>error.txt

2.5 - PIPING

# Read file and pipes it to wc which counts characters of the file
cat filename.txt | wc -c
cat filename.txt | wc -c > count.txt 

3 - TEXT SEARCHING AND MANIPULATION

• File and Text Handling with (grep, sed, awk, cut) advanced usage requires good understanding of regex

3.1 - GREP

• Search for text i output and print matches

ls -la /usr/bin | grep zip # Print lines which hash zip in it

# Case insensitive
grep -i

# Search Recursively
grep -r

3.2 - SED

• Text editing on a stream of text

# Replace harder with harder s means replace
echo "Try hard" | sed 's/hard/harder'
"Try harder"

3.3 - CUT

• Extract section of text and print it

# Split text by ',' and prints second element
echo 'a,b,c,d' | cut -d ',' -f 2 
b

cut -d ":" -f 1 /etc/passwd
"usernames in /etc/passwd"

3.4 - AWK

• A programming language designed for text proccessing and data extraction
• cut only can accept one character field seperator but awk is more flexible

echo "aaaa::bbbb::cccc" | awk -F "::" '{print $1 $3}'
aaaa cccc
# -F is field seperator and print $1 $3 extract first and third field

3.5 - PRACTICAL EXAMPLES

# Prints IP Addresses
cat access.log | cut -d ' ' -f 1 | sort -u

cat access.log | cut -d ' ' -f 1 | sort | uniq -c | sort -urn # Print IP Add

cat access.log | grep <IP> | cut -d '"' -f 2 | uniq -c

cat access.log | grep <IP> | grep '/admin' | sort -u

4 - EDITING FILES FROM THE COMMAND LINE

• It is important to have text editing skill inside terminal without any GUI-based editors like ‘gedit’/leafpad
• We will cover 2 of the most important terminal text editors VI and NANO

4.1 - NANO

• Simple and fast learning text editor

nano <file name>

CTRL + O # Save changes
CTRL + K # Cut current like
CTRL + U # Paste cutted line at cursor location
CTRL + W # Search in the file
CTRL + x # Exit the file

4.2 - VI

• Complex but powerful text editor

vi <file name>

i in command mode # Enter insert mode
esc in insert mode # Exit insert mode and enter command mode
dd in command mode # Delete and cut current line
yy in command mode # Copy current line
p in command mode # Paste clipboard content
x in command mode # Delete current character
:w in command mode # Save file
:q! in command mode # Quit without saving
:wq in command mode # Save file and quit

5 - COMPARING FILES

• Many technically oriented proffesionals rely on this

5.1 - COMM

• Compares two files and print uniq and common lines

# Print unique lines in file1 in first collumn, uniqe lines inf file2 in second column and common files in third column
comm file1 file2

# Print common lines since we suppressed columns 1,2
comm -12 file1 file2

5.2 - DIFF

• like COMM prints different lines wut is more complex
• Two most popular formats, context and unified

diff -c file1 file2 # Display result in context format 
# - indicates lines in first file not in the second
# + inficates lines in second file not in the first

diff -u file1 file2 # Display result in unified format 

5.3 - VIMDIFF

• combinations of vim and diif (opens two files and shows differences visually)

vimdiff file1 file2
CTRL + W + arrow key # Switch between windows
] + c # Jump to the next change
[ + c # Jump to the previous change
d + o # Get change from other windows and put it in current one
d + p # Get change from current windows and put it in other one
:q! # Quit vimdiff

6 - MANAGING PROCESSES

• Linux kernel manages multi-tasking through use of processes
• Each process is assgined a number called process ID (PID)
• Linux also introduce concept of jobs

cat file | wc -c # Two processes but one job

6.1 - BACKGROUNDING PROCESSES (BG)

• Sometimes it is needed to send processes to background and gain control later
  1. We can do it by putting & at the end of our command

ping -c 400 localhost > ping_results.txt &

1. We can suspend and stop execution of current foreground command with CTRL + Z

ping -c 400 localhost > ping_results.txt
CTRL+Z

1. We can resume previous suspended command with bg command

bg # Runs and resume job in background

6.2 - JOBS CONTROL: JOBS AND FG

• builtin jobs utility list jobs in current terminal session
• Specific jobs can be resumed by their PID or command name

jobs # List jobs in current terminal session
fg %1 # Resume job number 1 and returns it in foreground
fg # Resume only one job and returns it in foreground

6.3 - PROCESS CONTROL: PS AND KILL

• ps which stands for process status
• ps list processes wide not for curernt terminal

ps -ef
# -e displays all processes -f List all format listing

ps -fC <command name>
# -C command name

• We can stop a process by kill command and require PID

kill <PID>

7 - FILE AND COMMAND MONITORING

• Monitor files and commands in real time with tail and watch

7.1 - TAIL

• Monitor log files entry as they are being written

sudo tail -f /var/log/apache2/access.log
# -f continuously update the tail output in real time

tail -nx filename # Prints last x lines of the file

7.2 - WATCH

• Watch command is used to run specific commands repeatedly in specific period of time
• By default it’s preiod is 2 seconds but we can change it

watch -n <seconds> <command> # Runthe specified <command> in every <n> seconds
# Use CTRL + C to exit it and return to the terminal

8 - DOWNLOADING FILES

• Study tools to download files from command line

8.1 - WGET

• Downloads files using HTTP and FTP protocol

# Downloads file from URL and saves it in different name by -O (capital case o) option
wget -O filename <URL LINK>

8.2 - CURL

• A tools to transfer data to or from a server using a host of protocols
• Download or Upload files

# Downloads file from URL and saves it in different name by -o (lower case o) option
curl -o filename <URL LINK>

8.3 - AXEL

• A download accelerator which transfer file through http or ftp protocol with multiple connections

axel -a -n 20 -o filename <LINK>
# -a shows all progress during download
# -n 20 number of connections which is 20 here
# -o filename to save

9 - CUSTOMIZING THE BASH ENVIRONMENT

9.1 - BASH HISTORY CUSTOMIZATION

• HISTCONTROL environment variable defines whether or not removes duplicate commands

export HISTCONTROL=ignoredups # Remove duplicate variables
export HISTCONTROL=ignorespace # Remove commands start with space
# by default oth are enabled

export HISTIGNORE="&:ls:[bf]g:exit:history" # Ignore specific commands

export HISTTIMEFORMAT='%F %T ' # Show date and 24 hour format clock
# Other formats can be found in strftime man page

9.2 - ALIAS

• An string defined to use as specific command with shorter name

alias lsa = 'ls -la' # When we enter lsa it will execute 'ls -la'

9.2 - PERSISTENT BASH CUSTOMIZATION

• The behaviour of interactive shell bash is in /etc/bash.bashrc
• system wide bash settings can be changed by editing .bashrc located in every user’s home directory
• We can put alias in .bashrc or other commands to be executed every time a bash promt

10 - WRAPPING UP