Category: linux

# ssh localhost – connect to localhost
accept keyFingerprint. This is because the key cannot be verified.
# exit – exit from ssh connection
# ssh -p 2022 user@server.com
Key fingerprint is stored into:
# ~/.ssh/known_hosts
If the key fingerprint is wrong, there will be: Add correct host key in …… this is happening because the server has been installed on same IP address and key fingerprint from computer don’t match the key fingerprint from server. In order to restore the error, delete the key fingerprint from client. Identify the line from
# ~/.ssh/known_hosts
and delete it using dd from vim.
On server check the settings from:
# vim /etc/ssh/sshd_config

Create public/private key pair.
# ssh-keygen
The private key /home/user/.ssh/id_rsa will be created. Enter the passphrase in order to use this key.
The public key is created:
/home/user/.ssh/id_rsa.pub.
Copy public key to server:
# ssh-copy-id localhost
Use the password for remote user.
In order to copy to a remote host:
# ssh-copy-id -p 9822 user@domain.com
The option -i is useful when the file.pub is not located and have to manually insert the path to that file.
Test it:
# ssh -p 9822 user@domain.com
scp is for secure copy. Is used for copy files:
# scp -P 9822 /local/filename nameOfServer:/path/to/file
Is using “P” for port and not “p” because “p” is used for preserve the files attributes

test it:
1. Open a console and login as root.
ctrl+alt+F2 and login as root.
ctrl+alt+F1 – return to console.
2. from console login to localhost.
# ssh localhost
3. exit to close the session
# exit
4. create a ssh key pair.
# ssh-keygen > file for private > password (could be none)
5. use the key pair in order to login.
# ssh-copy-id localhost

# cut -f 1 filename – filter a file.
Cut is using a separator for fields.
# cut -f 1 -d : /etc/passwd

# tr – translate is replace characters. It works with pipe | in order to work with the output from other command.
# echo hello | tr a-z A-Z
This command is show all uppercase.
This is working ok if there are not different letters.
# echo hello | tr [:lower:] [:upper:]
# man tr
and awk was for file processing. Now there are not used anymore.
# sed 2q /etc/passwd
show the first 2 lines from file
# sed -n /^root/p /etc/passwd
Sed is not using anymore because there are other commands which are nice: grep.
# cp /etc/passwd ~
Replace an user with something else:
# sed -i ‘s/linda/julia/g’ passwd
awk is doing many things which can be done with other tools. Cutting information:
# ps aux | grep apache
# ps aux | grep apache | cut -f 2
or
# ps aux | grep apache | awk ‘{ print $2 }’

1. Use head and tail to display the 5th line of file /etc/passwd
# head -n 5 /etc/passwd | tail -n 1
2. use sed to do the same.
# sed -n ‘5p’ /etc/passwd
3. use awk to filter the first column out of the results of command ps aux
# ps aux | awk ‘{ print $1 }’
4. use grep to show the names of all files in /etc that have lines starting with the text ‘root’
# cd /etc
# grep ^root * 2> /dev/null

tar is tape archive
c- create
v – verbose
f – name of file
# tar cvf /home/user.tar /directoryToBeCreated
# file fileName (show shich kind of file is)
z is for compression to tgz
# tar czf /home/user.tar /directoryToBeCreated
Before check what is in side:
# tar tvf filename
The directory names are relative.
Extract:
# tar xvf
Ordering is important !!!!
-C switch to specific directory:
# tar xvf archiveName.tar -C /tmp
I’m backup the configuration files and after that If I’m removing a configuration file, I can restore it like this:
# tar xvf /root/etc.tar -C / etc/wgetrc
(I’m using etc/wgetrc in order to extract only a file from archive).

# ls -il /etc/hosts
-l long listing and -i is inode
# ln sourceFile destinationFile
# ln /etc/hosts computers
symbolic:
# ls -s sourceFile destinationFile
# ls -s /etc/hosts computers
On soft links the permissions are differents and in pointing to another inode.
Normal user cannot create hard links for /etc/shadow.
Can link to files where you have at least READ permissions.

Update and install dependencies
yum -y update kernel*
reboot -r now
yum install -y gcc kernel-devel kernel-headers dkms make bzip2 perl
export KERN_DIR=/usr/src/kernels/`uname -r`

or Install dependencies
# sudo yum groupinstall “Development Tools”
# sudo yum install kernel-devel

Mount the Virtualbox Additions CD ISO

# su –
# mkdir /media/cdrom/
# mount /dev/cdrom /media/cdrom/
# cd /media/cdrom/
# ./VBoxLinuxAdditions.run

# yum install VirtualBox-4.1

https://wiki.centos.org/HowTos/Virtualization/VirtualBox
https://www.gaggl.com/2014/07/install-virtualbox-additions-on-centos-7-guest-vm/

CentOS 7 VirtualBox Guest Additions Installation

Those to links are useful:
http://www.howtoforge.com/how-to-configure-ssh-keys-authentication-with-putty-and-linux-server-in-5-quick-steps
http://www.tonido.com/blog/index.php/2009/02/20/ssh-without-password-using-putty/#.VcMLSvmqpn8