$ cat /var/etc/shadow         [you cant see access denied]
$ sudo cat /var/etc/shadow        [noew you can see]
$ sudo cat /var/etc/passwd
$ sudo cat /var/etc/group

$ sudo useradd userName

$ sudo userdel userName

$ sudo passwd userName

$ cat ~/.ssh/id_rsa.pub

$ ssh -i "key-pair_fileName_PATH ubuntu@ip_address    [ubuntu or any os]

$ scp -i <keypair_PATH> < keypair_PATH> <user@ip_address>: filePath 

$ ls -l

$ chmod +x fileName            [make executable file]
$ chmod +rwx fileName          [ make permission read,write, executable]
$ chmod -rwx fileName          [ remove all permission]
$ chmod -x fileName            [remove executable permission]

Description: for read - 4, write - 2, execute -1. all together 7. for 777 means 1st 7 for user, 2nd for group and 3rd for all user

$ chmod 777 fileName    [all permission for user,group and all]
$ chmod 742 fileName    [all permission for user, read for group, write for all]
$ chmod 124 fileName    [execute permission for user, write for group, read for all]
$ chmod 711 fileName    [ all permission for user, execute for groups and all]

$ sudo chown userName fileName    [it will change files user ownership]
$ sudo chgrp groupName fileName    [it will change files group ownership]
$ sudo chmod userName:groupName fileName     [it will change both user & group ownership]

$ passwd

$ uname

$ uname -a
$ uname -r     [to check only version numaric ]

$ sudo usermod -a -g groupName userName

$ sudo deluser userName groupName

$ finger

$ finger userName