I am using Linux. What is the meaning of chmod 666?

Chmod change attributes from a file/folder

• chmod 666 means that all users can read and write but cannot execute


• chmod 777 allows all actions for all users


• 
  

  chmod 744 allows only owner to do all actions; group and other users are allowed only to read

  
  
  

  permission to:  owner      group      other     
  
                  /¯¯¯      /¯¯¯      /¯¯¯
  
  octal:            6          6          6
  
  binary:         1 1 0      1 1 0      1 1 0
  
  what to permit: r w x      r w x      r w x
  
  
  
  binary         - 1: enabled, 0: disabled
  
  
  
  what to permit - r: read, w: write, x: execute
  
  
  
  permission to  - owner: the user that create the file/folder
  
                   group: the users from group that owner is member
  
                   other: all other users
  
  

  
  

In really plain speak: it makes a file read- and write-able by the file owner, the file owner's group and every one else using the machine (all). Applied against a directory it lets everyone read (get file contents lists) of a directory and write (create, edit files in the directory) but not execute files from the directory.

For more detailed information how chmod works check out this handy tutorial.

As mentioned in other answers, chmod means change mode. It affects the read, write and executable permissions for the owner, group and other categories of users. The numbers that follow the command (in this case 666), indicate how those permissions are modified for the file the command is run on (for 666, it means that owner, group and other have read and write permissions, but no executable permissions).

By changing the numbers to different values you effectively change the permissions for the file. The link I've referenced above has a little tool for figuring out what values you need to put in to get the permissions scheme you're after. It also goes over the switch options available for the command and some examples to help you understand better how it works.

The chmod command (abbreviated from change mode) is a Unix command that lets an operator tell the system how much (or little) access it should permit to a file. Command chmod 666 means that all users will have read and write permissions.

If your questions is more about the 666 part than the chmod part, I would refer you to The Linux Documentation Project where is a decent explanation of how file permissions work in Linux.

chmod

chmod is change mode. It's application is changing permissions of files and it's for Unix.
Alternatively, There's utlity called attrib which does the same and it's for Windows.

The 666 is an octal representation of 438 decimal.

Explanation

Syntax: [user][group][other]



Permission:

        User:

            (1) Can execute

            (2) Can write

            (4) Can read



        Group:

            (1) Can execute

            (2) Can write

            (4) Can read



        Other:

            (1) Can execute

            (2) Can execute

            (4) Can read





Join: [6][6][6]

      /   |   

   User Group Other



Add permission numbers then place them on each box.

Mixing permission will add the permission properties together.

Example

# (Read, Write, Execute) permission to all groups

chmod 777 sample.dat

That said, This permission bit is for allowing Read, Write permission of file for all groups.