Allow / Block ssh / sftp for specific Users / Group

Allow ssh / sftp to selected users
If you want to give ssh / sftp privileges to certain users, here is the solution.
Add following line in the file /etc/sshd_config

AllowUsers username1 username2 username3

If you want to allow certain groups only, add following line in /etc/sshd_config

AllowGroups username1 username2 username3

Restart service ssh.

Block ssh / sftp for selected users
If you want to restrict certain users to access ssh / sftp, here is the solution.
Add following line in the file /etc/sshd_config

DenyUsers username1 username2 username3

If you want to block certain groups only, add following line in /etc/sshd_config

DenyGroups username1 username2 username3

Restart service ssh.

Extract an iso file in Linux using command line

Extract an iso file on terminal in Linux

Run following command

sudo mount -o loop "/path/of/iso/file.iso" /mnt
Example :
sudo mount -o loop "/home/user/Downloadsubuntu-12.04.3-desktop-i386.iso" /mnt

Now the iso file contents will be mounted in /mnt
You can copy them anywhere.

sudo cp -r /mnt /your/desired/destination/directory
Example :
sudo cp -r /mnt /home/user/Documents/files

If you do not want to use command line, you can extract it using Archive Manager.
Right click on the iso file and select "Extract here'.
If you have Archive manager, you can mount it using Archive Mounter and files will be opened as separate drive.

Postfix - Change Sender's email address from root

When you send mail using Postfix through command line or similar way, it shows sender's e-mail address root@domain-name. You do not want this and you want your defined email should be visible in recipient's mailbox. Follow the steps.

Add following line in the file /etc/postfix/main.cf 

smtp_generic_maps = hash:/etc/postfix/generic

Now edit the file /etc/postfix/generic, if it is not there, create it.
Add your 'from address' and 'the address you want to be seen in place of from address' in this file.
Example :

root@domain.com linuxamination@gmail.com
user1@domain.com linuxamination@gmail.com

You can define different mail addresses for each user of Linux. 
Now Run following commands in terminal.

postmap /etc/postfix/generic

and

service postfix restart

Now it displays linuxamination@gmail.com as sender, when I send mail as a root or user1.

echo "content-of-the-mail" | mail -s "subject-of-the-mail" "recipient@mail-address.com"