1. Linux+ Guide to Linux Certification, Second Edition
Chapter 5
Linux Filesystem Management

2. Linux+ Guide to Linux Certification, 2e
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
Linux+ Guide to Linux Certification, 2e

3. Objectives (continued)
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
Linux+ Guide to Linux Certification, 2e

4. The Filesystem Hierarchy Standard
Filesystem Hierarchy Standard (FHS): Standard set of directories for Linux and UNIX systems
File and subdirectory contents
Gives Linux software developers ability to locate files on any Linux system
Create non-distribution–specific software
Linux+ Guide to Linux Certification, 2e

5. The Filesystem Hierarchy Standard (continued)
Table 5-1: Linux directories defined by FHS
Linux+ Guide to Linux Certification, 2e

6. The Filesystem Hierarchy Standard (continued)
Table 5-1 (continued): Linux directories defined by FHS
Linux+ Guide to Linux Certification, 2e

7. Managing Files and Directories
mkdir command: Creates new directories
Arguments specify directory’s absolute or relative pathname
mv command: Moves files
Two arguments minimum:
Source file/directory (may specify multiple sources)
Target file/directory
Also used to rename files
Linux+ Guide to Linux Certification, 2e

8. Managing Files and Directories (continued)
cp command: Copies files
Same arguments as mv command
Recursive: Referring to itself and its own contents
Recursive search includes all subdirectories in a directory and their contents
–r option
Linux+ Guide to Linux Certification, 2e

9. Managing Files and Directories (continued)
interactive mode: Prompts user before overwriting files
–i option
–f option: Overrides interactive mode
rm command: Removes files
Arguments are a list of files
rmdir command: Removes directories
–r and –f options are helpful
Linux+ Guide to Linux Certification, 2e

10. Managing Files and Directories (continued)
Table 5-2: Common Linux file management commands
Linux+ Guide to Linux Certification, 2e

11. Linux+ Guide to Linux Certification, 2e
Finding Files
locate command: Search for files on system
Shortcut to the slocate command
Information returned may not fit on screen
Use with more or less commands
Uses indexed database of all files on system
Find command: Recursively search for files starting from a specified directory
Linux+ Guide to Linux Certification, 2e

12. Finding Files (continued)
Table 5-3: Common criteria used with find command
Linux+ Guide to Linux Certification, 2e

13. Finding Files (continued)
Table 5-3 (continued): Common criteria used with find command
Linux+ Guide to Linux Certification, 2e

14. Finding Files (continued)
which command: Search for an executable file
Searches the PATH variable
PATH variable: Lists directories on system where executable files are located
Allows executable files to be run without specifying absolute or relative path
Linux+ Guide to Linux Certification, 2e

15. Linux+ Guide to Linux Certification, 2e
Linking Files
Symbolic link: One file is a pointer or shortcut to another
Hard link: Two files share the same data
Linux+ Guide to Linux Certification, 2e

16. Linking Files (continued)
Filesystem has three main structural sections:
Superblock: Contains general information about the filesystem
e.g., number of inodes and data blocks
Inode: Describes a file or directory
Unique inode number, file size, data block locations, last date modified, permissions, and ownership
Inode table: Consists of several inodes
Data blocks: Data making up contents of a file
Linux+ Guide to Linux Certification, 2e

17. Linking Files (continued)
Figure 5-1: The structure of hard linked files
Linux+ Guide to Linux Certification, 2e

18. Linking Files (continued)
ln (link) command: Create hard and symbolic links
Two arguments:
Existing file to link
Target file to create as a link to existing file
Use –s option to create symbolic link
Hard linked files share two inodes
Data blocks in symbolically linked files contain pathname to target file
Linux+ Guide to Linux Certification, 2e

19. Linking Files (continued)
Figure 5-2: The structure of symbolically linked files
Linux+ Guide to Linux Certification, 2e

20. File and Directory Permissions
All users must login with a username and password
Users identified by username and group memberships
Access to resources depends on username and group membership
Must have required permissions
Linux+ Guide to Linux Certification, 2e

21. File and Directory Ownership
Primary group: User’s default group
During file creation, file’s owner and group owner set to user’s username and primary group
Same for directory creation
touch command: Create an empty file
Linux+ Guide to Linux Certification, 2e

22. File and Directory Ownership (continued)
chown (change owner) command: Change ownership of a file or directory
chgrp (change group) command: Change group owner of a file or directory
Linux+ Guide to Linux Certification, 2e

23. Managing File and Directory Permissions
Mode: Inode Section that stores permissions
Three sections, based on the user(s) that receive the permission:
User permissions: Owner
Group permissions: Group owner
Other permissions: Everyone on system
Three regular permissions may be assigned to each user:
Read
Write
Execute
Linux+ Guide to Linux Certification, 2e

24. Linux+ Guide to Linux Certification, 2e
Interpreting the Mode
Figure 5-3: The structure of a mode
Linux+ Guide to Linux Certification, 2e

25. Interpreting the Mode (continued)
User: Refers to owner of a file or directory
Owner: Refers to users with ability to change permissions on a file or directory
Other: Refers to all users on system
Shell scripts: Text files containing instructions for the shell to execute
Linux+ Guide to Linux Certification, 2e

26. Interpreting Permissions
Table 5-4: Linux permissions
Linux+ Guide to Linux Certification, 2e

27. Linux+ Guide to Linux Certification, 2e
Changing Permissions
chmod (change mode) command: Change mode (permissions) of files or directories
Permissions stored in a file’s or a directory’s inode as binary powers of two
Linux+ Guide to Linux Certification, 2e

28. Changing Permissions (continued)
Table 5-5: Criteria used within the chmod command
Linux+ Guide to Linux Certification, 2e

29. Changing Permissions (continued)
Figure 5-4: Numeric representation of the mode
Linux+ Guide to Linux Certification, 2e

30. Changing Permissions (continued)
Table 5-6: Numeric representations of the permissions in a mode
Linux+ Guide to Linux Certification, 2e

31. Linux+ Guide to Linux Certification, 2e
Default Permissions
New files given rw-rw-rw- permissions by default
umask: Takes away permissions on new files and directories
umask command: Displays the umask
Changing the umask
Use a new umask as an argument to the umask command
Linux+ Guide to Linux Certification, 2e

32. Default Permissions (continued)
Figure 5-5: Performing a umask 022 calculation
Linux+ Guide to Linux Certification, 2e

33. Default Permissions (continued)
Figure 5-6: Performing a umask 007 calculation
Linux+ Guide to Linux Certification, 2e

34. Linux+ Guide to Linux Certification, 2e
Special Permissions
SUID (Set User ID)
SGID (Set Group ID)
Sticky bit
Linux+ Guide to Linux Certification, 2e

35. Defining Special Permissions
SUID
If set on a file, user who executes the file becomes owner of the file during execution
No functionality when set on a directory
Only applicable to binary compiled programs
Linux+ Guide to Linux Certification, 2e

36. Defining Special Permissions (continued)
SGID
Applicable to files and directories
If set on a file, user who executes the file becomes member of the file’s group during execution
If a user creates a file in a directory with SGID set, the directory’s group owner is changed to match the file’s group owner
Linux+ Guide to Linux Certification, 2e

37. Defining Special Permissions (continued)
Sticky bit
Previously used to lock files in memory
Currently only applicable to directories
Ensures that a user can only delete files his/her own files
Linux+ Guide to Linux Certification, 2e

38. Setting Special Permissions
Special permissions require execute
Mask the execute permission when displayed by the ls –l command
May be set even if file or directory does not have execute permission
Via chmod command
Add an extra digit at front of permissions argument
Linux+ Guide to Linux Certification, 2e

39. Setting Special Permissions (continued)
Figure 5-7: Representing special permissions in the mode
Linux+ Guide to Linux Certification, 2e

40. Setting Special Permissions (continued)
Figure 5-8: Representing special permissions in the absence of the execute permissions
Linux+ Guide to Linux Certification, 2e

41. Setting Special Permissions (continued)
Figure 5-9: Numeric representation of regular and special permissions
Linux+ Guide to Linux Certification, 2e