What is the meaning of "chmod 666"?

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

chmod command change attributes from a file/folder:

• chmod 666 file/folder means that all users can read and write but cannot execute the file/folder;
• chmod 777 file/folder allows all actions for all users;

• chmod 744 file/folder allows only user (owner) to do all actions; group and other users are allowed only to read.

  permission to:  user(u)   group(g)   other(o)     
                  /¯¯¯\      /¯¯¯\      /¯¯¯\
  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  - user: the owner that create the file/folder
                   group: the users from group that owner is member
                   other: all other users
  

Alternatively, you can execute the command with a more intuitive syntax, without needing to think in binary or octal (but the knowledge of numeric syntax is so important): chmod u=rw, g=rw, o=rw file/folder

Remember that the permission changes with chmod command requires at least 3 arguments, so chmod 666 does nothing without explicit file/folder to change permissions.

Also be sure to criticize if it does not produce insecure issues or simply if it is an useless permission change, because chmod 666 will allow file/folder write to all and the execution to none.

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.

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.

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.