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.

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