Unix File System Permissions

File/Folder Permissions and Ownership

• File system permissions control the ability of user and group accounts to read, modify, and execute the contents of files and to enter directories.
• Three types of access (permissions): read (r), write (w), execute (x).
• Each file belongs to a specific user and group (ownership).
• Access to files is controlled by user (u), group (g), and other/everyone (o) permission bits, usually set using a numerical value.

Access Levels and Permissions

• Different access levels depending on numerical values:
  • 0 (---) – no access to the file whatsoever
  • 1 (--x) – execute permissions only
  • 2 (-w-) – write permissions only
  • 3 (-wx) – write and execute permissions
  • 4 (r--) – read permissions only
  • 5 (r-x) – read and execute permissions
  • 6 (rw-) – read and write permissions
  • 7 (rwx) – read, write, and execute permissions (full permissions)

Directory Permissions

• Base permission for a directory is 777 (drwxrwxrwx), which grants everyone the permissions to read, write, and execute.
• Example of 644 permissions:
  • Owner/User: Read and Write
  • Group: Read only
  • Other/Everyone: Read only

Setting Permissions and Ownership

• To set permissions, use the chmod command followed by the permission value and the file name.
• Example: chmod 755 file_name sets the permissions to 755 for the file file_name.
• To recursively change permissions, use the -R flag: chmod -R 755.
• To change ownership, use the chown command followed by the user and group names, and the file name.
• Example: chown user:siteground file_name sets the owner to user and the group to siteground.

Understanding Users and Groups

• In Linux, a user is an individual who interacts with the system, with a unique username and user ID (UID).
• User accounts are used to log in, run processes, and access files and directories.
• Groups are collections of users, used to simplify access control and permissions management.
• A group has a unique group ID (GID).

User Management

• Creating users: use the useradd command followed by the username.
• Example: sudo useradd viswa creates a new user named viswa.
• Setting user password: use the passwd command followed by the password.
• Modifying user attributes: use the usermod command.
• Deleting users: use the userdel command with the -r flag to remove the user and their home directory.

Group Management

• Creating groups: use the groupadd command followed by the group name.
• Example: sudo groupadd mygroup creates a new group named mygroup.
• Adding users to groups: use the usermod command with the -aG flag.
• Example: sudo usermod -aG mygroup username adds the user to the mygroup group.
• Changing group ownership of files: use the chown command with the group name.
• Deleting groups: use the groupdel command followed by the group name.

Group Database Files

• /etc/group contains all group details as a list.
• /etc/gshadow contains all group members details as a list.

Password Aging Policy

• To ensure system and network security, various security mechanisms are used, including user password expiration.
• The chage command and /etc/login.defs file are used to manage password policies.
• Password aging policy settings include:
  • The last password change date
  • The password expiration date
  • The password inactive date
  • The account expiration date
  • The minimum number of days between password changes
  • The maximum number of days between password changes
  • The number of days of warning before the password expires

Learn about Unix file system permissions and how they control access to files and directories. Understand read, write, and execute permissions, as well as file ownership.