Linux File Ownership

3 types of owner:

• User
• Group
• Other

Linux File Permissions

3 permissions for all the 3 owners:

• Read (r)
• Write (w)
• Execute (x)

Example:

We have -rw-rw-r– permission

• The first - implies that we have selected a file. Else, if were a directory, d would have been shown
• The first part is rw-, this suggests that the owner home can:
  • Read the file
  • Write or edit the file
  • He cannot execute the file since the execute bit is set to -
• The second part is rw-, it for the user group home and group members can:
  • Read the file
  • Write or edit the file
• The third part for the world which mean any user. It says r--. This means the user can only
  • Read the file

Changing file/directory permissions

We can use chmod command to set permissions on a file/directory for the owner, group and the world. There are 2 ways to use the command:

• Absolute mode

  In this mode, file permissions are not represented as characters but a three-digit octal number.

  Number	Permission Type	Symbol
  0	No Permission	---
  1	Execute	--x
  2	Write	-w-
  3	Execute + Write	-wx
  4	Read	r--
  5	Read + Execute	r-x
  6	Read + Write	rw-
  7	Read + Write + Execute	rwx

  

  

• Symbolic mode

  We can modify permissions of a specific owner.

  Operator	Description
  +	Adds a permission
  -	Removes the permission
  =	Sets the permission and overrides the permissions set earlier

  User Denotations	Description
  u	user/owner
  g	group
  o	other
  a	all

  

Changing Ownership and Group

For changing ownership, we use chown user filename command. For change the user as well as group, we use chown user:group filename command.

We use chgrp group_name filename command to change group-owner only

Notes:

• The file /etc/group contains all the groups defined in the system

• You can use the command groups to find all the groups you are a member of

  

• You can use the command newgrp to work as a member a group other than your default group

  

• You cannot have 2 groups owning the same file

• You do not have nested groups in Linux. One group cannot be sub-group of other

• x - eXecuting a directory means Being allowed to “enter” a dir and gain possible access to sub-dirs

• There are other permissions that you can set on Files and Directories which will be covered in a later advanced tutorial

Refs:

• https://www.guru99.com/file-permissions.html