1. Linux Filesystem Management
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)
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 PATH variable
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 Owner Other
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