Chmod explained

Hi folks

Many guys here are well vexed with file permissions and how to chmod files, but I had always wondered what it was until I did a little read up on Google, and this is what I got
For those who don’t understand why it’s always been said chmod 777/755/644….etc.
May be you will appreciate them after reading this tutorial

Let’s start

In Unix/Linux whenever you check the property of any file, under the permissions tab, there are 3 categories of users of the file

• Owner (the owner of the file and by default has read and write permissions)




• Group (Any group the owner might belong and by default has read permissions)




• Others (Other users on the operating system, and by default have read only permissions)

PERMISSIONS EXPLAINED

[COLOR='#FF0000']Read permission[/COLOR]: The user can only open the file to see the content without being able to modify the content of the file.




[COLOR='#FFA500']Write permissio[/COLOR]n: The user can open the file with a text editor and change/modify the contents by deleting, adding and saving the new settings.




[COLOR='#00FF00']Execute permission[/COLOR]: which gives the user the right to run the file as a program. Execute the file as a program if it’s a binary file or a shell script

All the file permissions read, write, execute have certain numeric values associated with them

Read has a numeric value of [COLOR='#FF0000']4[/COLOR]




Write has a numeric value of [COLOR='#FFA500']2[/COLOR]




Execute has a numeric value of [COLOR='#00FF00']1[/COLOR]

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Which means that at best a file can have a numeric value of 7, which is 4+2+1=7

So a file created in Unix/Linux by default has

Owner -read and write permission which is 4+2=6




Group -read permission which is 4




Others -read permission which is 4

Quote

So by default any file created in Linux has 644 permission

UMASK

Umask is a system variable in Linux, when you type umask in the terminal you get a value 0022
But we consider the value 022.Now when a file is created what Linux does is that it subtracts the value of umask from 666 for a file and 777 for a directory, so by default a file has 644 permissions and a directory has 755 permissions.

HOW TO CHMOD IN TERMINAL

Change directory or cd to the directory where the file is located
e.g.

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cd /usr/bin/

Now let’s assume you file is called kingjj
So we can change permissions to 644 by typing

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chmod 644 kingjj

Or simply type

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chmod 644 /usr/bin/kingjj