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File/Directory Permissions

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• Unauthorized access to UNIX files and directories is controlled by granting read, write, or execute permission to the owner, group, or other user.

  Owner = The person who Unix recognises as the file's owner.
  Group = users defined as being in a particular group.
  Other = Everyone else in the world, For this reason, the Other category is often called World.

  Read = View the file or directory without making changes.
  Write = Make changes to the file or directory. In general, you never want to allow write access to anyone but yourself and your group.
  Execute = Execute the file or directory. Generally used for directories, CGI programs, and files that include form actions.

• Permissions are shown when listing a file or directory using the -l option. The Screenshot below shows how this would look.

The Octal (Absoloute) Mode

• Octal mode is based on the base eight numbering system (0-7 are the available numbers).

Octal Value	Permissions
4	Read
2	Write
1	Execute

• So basically....

OCTAL	UNIX EQUIVALENT	OWNER	GROUP	OTHER
		read/write/execute	read/write/execute	read/write/execute
777	rwxrwxrwx	yes/yes/yes	yes/yes/yes	yes/yes/yes
755	rwxr-xr-x	yes/yes/yes	yes/no/yes	yes/no/yes
744	rwxr--r--	yes/yes/yes	yes/no/no	yes/no/no
644	rw-r--r--	yes/yes/no	yes/no/no	yes/no/no

• The default permissions on files are 644 and the default permissions on directories are 755, Although this can be changed using the umask Filter.

• The chmod Command can be used to change file and directory permissions using both Unix (symbolic) and Octal mode.
  For Example, to give the owner read/write/execute permissions and the group and world read-only permissions to a file called alan.html using Octal mode you would use the command:

  chmod 744 alan.html

  And to give execute permission to the owner and read permission to group and world using Symbolic mode to a directory called britney you would use the command:

  chmod u+x,go+r britney

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