Linux File Permission Mechanism

Linux File Permissions :

Understanding Linux File & Directory Permissions :

Linux File and directory Permission mechanism
	Read	Write	Execute
Owner	400	200	100
Group	40	20	10
Others	4	2	1

If you want to give Read, Write & Execute permission to Owner, Groups & Others, it means full permission.

400+200+100+40+20+10+4+2+1 = 777

If you do not want to give Write permission to Groups & Others, it means 

400+200+100+40+10+4+1 = 755

The best thing of this calculation, you can not make sum of a number in two different ways .

It means 644 can be made in this way only that is 400+200+40+4

The meaning of 644 is Read & Write permission to Owner and Read permission to Group and Others only.

To give 777 to a directory :

# chmod -R 777 /path/of/the/directory

where -R means Recursive, it means give this permission to all sub-directories and files.

To give 777 to a file :

# chmod 777 /path/of/the/file

Now you can create & give permissions by your own.

Unable to boot : please use a kernel appropriate for your cpu

Virtual Box Error :
Unable to boot : please use a kernel appropriate for your cpu

Go on
Settings -> System -> Processor 

&

Make the Check-box PAE/NX Enable.

Make sure your virtual machine is not started otherwise it shows options disabled.

Copy Directory Structure without Files in Linux

If you want to copy the directory structure without any files, you can use following command.

rsync -av --include '*/' --exclude '*' /path/source/dir /path/destination/dir

'rsync' gives you utility to copy directory with files or without files.

If you want to copy the directory structure without any files from one host to another. You can use

rsync -av --include '*/' --exclude '*' /path/source/dir user@host:/path/destination/dir

Examples:
Suppose I want to copy the directory structure of /etc in /root/tmp on same host.

rsync -av --include '*/' --exclude '*' /etc /root/tmp

It will copy all directories of /etc in the /root/tmp without any files.
It will not create directory etc in the /root/tmp. It will start from sub-directories of etc.