linux_commands.txt

Commands

To check active ssh connections took from Machine
$ ss -t -a 

Extract Zip file Commands:  

tar -tvf <tar_file> => To view the contents of tar file
tar -xvf <tar_archive> => To extract all content
sed -i 'nd' <file name> => To delete particular line[n here is line number]
sed -n '<start>,<end>p' <file name> => To view the specific(start to end) lines.

sed -i 's/old-text/new-text/g' input.txt => To replace using sed

df -h path => Will give you used space in that path

df -Ph . | awk 'NR==2 {print $4}' => Will give available space in current directory

open . => To open the current folder in files

lsb_release -a => To know about distribution information (lsb stands for linux standard base)
press ctrl+r to search commands you have already key in

$ echo $? => To find if the last command is executed successfully (correct if returns 0)

$ ip route => To find the default gateway
$ file <filename> => To know the type of the file
$ cd /var/log => There you find the syslog file to get the log of all system activities

$ nproc => To list the processors in your pc

$ uname -m => To find the architecture of your system(bit)

VI command
  <ctrl+d> => To scroll down page
  <ctrl+f> => Page forward
  <ctrl+b> => page backward
  <ctrl+u> => To scroll page up

  To search a word , esc mode /<word> 

$ PS1='<text_toshow_in_prompt>' => To change the prompt text

cd /etc -> open hostname file to edit hostname and reboot

$ dmesg => To get the log of system start up 

PUBLIC KEY 
$ To add Public key of websites => $ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys FCEF32E745F2C3D5
$ gpg --keyserver keyserver.ubuntu.com --search-keys jeyendran785@gmail.com => To search for someone's public key using gmail id , will give their pb key
     add that key using previous command
    
Tr command :

$ cat <file_name> | tr <'txt1'> <'TXT2'> => To replace all characters in txt 1 like 't' 'x' 't' to 'T' 'X' 'T'

$ cat <file_name> | tr -d <txt> => To remove characters not replace

$ echo -n <word_tobe_encoded> | base64 => To encode a word(secure data) into base64 format to sent across networks
  (eg. $ echo -n "password" | base64)

$ echo <encoded_txt> | base64 -d => To decode a base64 encoded text

nmap -Pn <ipaddress> => to find active running services of remote machine
pgrep <processName> => will return the process id (eg. $pgrep ssh)
ss => this command will give the socket statistics for local sock address and remote socket address of active running processes
mpstat => To see cpu performance
lsof -i :<portnumber> => To find process id of a specific port
kill -9 <pID> => To kill the process recursively

LISTEN FOR ACTIVE PORTS:

Run any one of the following command on Linux to see open ports:
sudo lsof -i -P -n | grep LISTEN
sudo netstat -tulpn | grep LISTEN
sudo ss -tulpn | grep LISTEN
sudo lsof -i:22 ## see a specific port such as 22 ##
sudo nmap -sTU -O IP-address-Here