Understanding Linux File Permissions
Richard A. Blum, Christine Bresnahan · 2015
The Linux system follows the Unix method of file permissions, allowing individual users and groups access to files based on a set of security settings for each file and directory. This chapter discusses how to use the Linux file security system to protect data when necessary and share data when desired. It describes the files and utilities required for user accounts in order to explain how to use them when working with file permissions. The chapter explains how to decipher the permissions and where they come from. It talks about the concept of "Sharing Files", and further explains how to change the existing permissions, the default owner, and the default group settings for a file or directory. The chown and chgrp commands change the default owner and group of the file. The chapter concludes with a discussion on how to use the set GID bit to create a shared directory.