Mastering Linux Filesystem Management: FHS, Permissions & Links

1. Chapter Five Linux Filesystem Management

2. Objectives • Explain the function of the Filesystem Hierarchy Standard • Use standard Linux commands to manage files and directories • Find files and directories on the filesystem • Understand and create linked files • Modify file and directory ownership • Define and change Linux file and directory permissions • Identify the default permissions created on files and directories • Apply special file and directory permissions

3. The Filesystem Hierarchy Standard (FHS) • FHS • Standard of outlining the location of set files and directories on a Linux system • Gives Linux software developers the ability to locate files on a Linux system regardless of the distribution • This allows them to create software that is not distribution specific • A comprehensive understanding of the standard type of directories found on systems is valuable when locating and managing files and directories

4. The Filesystem Hierarchy Standard (FHS) Table 5-1: Linux directories defined by FHS

5. The Filesystem Hierarchy Standard (FHS) Table 5-1 (continued): Linux directories defined by FHS

6. Managing Files and Directories • mkdir command • Takes arguments specifying the absolute or relative pathnames of the directories to create • The mv command requires two arguments at a minimum: • Source file/directory • Target file/directory

7. Managing Files and Directories • cp command • Command used to create copies of files and directories • Recursive • Referring to itself and its own contents • A recursive search includes all subdirectories in a directory and their contents

8. Managing Files and Directories • rm command • Takes a list of arguments specifying the absolute or relative pathname of files to remove • rmdir command • Command used to empty directories

9. Managing Files and Directories Table 5-2: Common Linux file management commands

10. Finding Files • locate command • The fastest method to search for files in the Linux directory tree • A shortcut to the slocate (or secure locate) command • Often returns too much information to display on the screen, as it searches all files on the filesystem

11. Finding Files • find command • Used to find files on the filesystem using various criteria • Does not use a pre-made index of files • Searches the directory tree recursively, starting from a certain directory, for files that meet a certain criteria

12. Finding Files Table 5-3: Common criteria used with find command

13. Finding Files Table 5-3 (continued): Common criteria used with find command

14. Finding Files • which command • Command used to locate files that exist within directories listed in the PATH variable • PATH variable • Stores a list of directories that will be searched in order when commands are executed without an absolute or relative pathname

15. Linking Files • Files may be linked to another in one of two ways: • One file may simply be a pointer or a shortcut to another file (known as a symbolic link or symlink) • The two files may share the same data (known as a hard link)

16. Linking Files • To better understand how files are linked, you must understand how files are stored on a filesystem • On a structural level, a filesystem has three main sections: • The superblock • The inode table • Data blocks

17. Linking Files Figure 5-1: The structure of hard linked files

18. Linking Files • ln (link) command • Command used to create hard and symbolic links • To create hard link, you must use the ln command and specify two arguments: • The existing file to hard-link • The target file that will be created as a hard link to the existing file

19. Linking Files Figure 5-2: The structure of symbolically linked files

20. File and Directory Permissions • Recall that all users must successfully login with a username and password to gain access to a Linux system • Once logged in, users are identified by their username and group memberships • All access to resources depends on whether their username and group membership have the required permission

21. File and Directory Ownership • When a user creates a file or directory, that user’s name and primary group become the owner and group owner of the file, respectively • Primary group • Default group to which a user belongs

22. File and Directory Ownership • touch command • Command used to create new files • Originally used to update the timestamp on a file • chown (change owner) command • Command used to change the owner and group owner of a file or directory

23. File and Directory Ownership • chgrp (change group) command • Command to change the group owner of a file or directory • Takes two arguments at a minimum: • The new group owner • The files or directories to change

24. Managing File and Directory Permissions • Mode • The section of the inode that stores permissions • Divided into three sections based on the user(s) that receive(s) the permission to that file or directory • User (owner) permissions • Group (group owner) permissions • Other (everyone on the Linux system permissions

25. Managing File and Directory Permissions • There are three regular permissions that you may assign to each of the user(s) referenced on the previous slide: • Read • Write • Execute

26. Interpreting the Mode Figure 5-3: The structure of a mode

27. Interpreting the Mode • User • When used in the mode of a certain file or directory, it refers to the owner of that file or directory • Owner • User whose name appears in a long listing of a file or directory and who has the ability to change permissions on that file or directory • Other • When used in the mode of a certain file or directory, it refers to all users on the Linux system

28. Interpreting Permissions Table 5-4: Linux permissions

29. Changing Permissions • chmod (change mode) command • Used to change the mode (permissions) of a file or directory • Takes two arguments at a minimum: • The first argument specifies the criteria used to change the permissions • The remaining arguments indicate the filenames to change

30. Changing Permissions Table 5-5: Criteria used within the chmod command

31. Changing Permissions Figure 5-4: Numeric representation of the mode

32. Changing Permissions Table 5-6: Numeric representations of the permissions in a mode

33. Default Permissions • Umask • Used to alter the permissions on all new files and directories by taking select default file and directory permissions away • Only applies to newly created files and directories • Will never be used to modify the permissions of existing files and directories

34. Default Permissions Figure 5-5: Performing a umask 007 calculation

35. Default Permissions Figure 5-6: Performing a umask 007 calculation

36. Special Permissions • Read, write, and execute are the regular file permissions used to assign security to files • Three more special permissions that you may optionally use on file and directories: • SUID (Set User ID) • SGID (Set Group ID) • Sticky bit

37. Defining Special Permissions • The SUID has no special function when set on a directory • However, if the SUID is set on a file and that file is executed, then the person who executed the file temporarily becomes the owner of the file while it is executing • The SUID can only be applied to binary compiled programs

38. Defining Special Permissions • The SGID has a function when applied to both files and directories • The SGID allows regular users to execute a binary compiled program and become a member of the group that is attached to the file during execution of the program • The sticky bit was used on files in the past to lock them in memory • Today, the sticky bit performs a useful function only on directories

39. Setting Special Permissions • The mode of a file that is displayed using the ls –l command does not have a section for special permissions • Special permissions require execute • They mask the execute permission when displayed using the ls –l command

40. Setting Special Permissions Figure 5-7: Representing special permissions in the mode

41. Setting Special Permissions Figure 5-8: Representing special permissions in the absence of the execute permissions

42. Setting Special Permissions Figure 5-9: Numeric representation of regular and special permissions

43. Chapter Summary • The Linux directory tree obeys the Filesystem Hierarchy Standard • The re are many file management commands that exist to create, change the location of, or remove files and directories • You may find files on the filesystems using a pre-indexed database, or by searching the directories listed in the PATH variable • Files may be created as a pointer to another file or a as a linked duplicate if another file

44. Chapter Summary • Each file and directory has an owner and a group owner • Permissions can be set on the user or owner of a file, members of the group of the file, as well as everyone on the system • There are three regular file and directory permissions and three special file and directory permissions

45. Chapter Summary • Permissions can be changed using the chmod command by specifying symbols or numbers to represent the changed permissions • To ensure security, new files and directories receive default permissions from the system less the value of the umask variable • The root user has all the permissions to all files and directories on the Linux filesystems