1. Linux+ Guide to Linux Certification, Third Edition
Chapter 4
Linux Filesystem Management

2. Objectives Find files and directories on the filesystem
Understand and create linked files
Explain the function of the Filesystem Hierarchy Standard
Use standard Linux commands to manage files and directories
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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
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4. The Filesystem Hierarchy Standard
Filesystem Hierarchy Standard (FHS): standard set of directories for Linux and UNIX systems
Standard file and subdirectory contents
Simplifies the task of finding specific files
Gives Linux software developers ability to locate files on any Linux system
Create non-distribution–specific software
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5. The Filesystem Hierarchy Standard (continued)
Table 4-1: Linux directories defined by the Filesystem Hierarchy Standard
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6. The Filesystem Hierarchy Standard (continued)
Table 4-1 (continued): Linux directories defined by the Filesystem Hierarchy Standard
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7. Managing Files and Directories
mkdir command: creates new directories
Arguments specify directory’s absolute or relative pathname
mv command: moves files
Minimum of two arguments:
Source file/directory (may specify multiple sources)
Target file/directory
Pathnames can be absolute or relative
For multiple files, can use wildcards in pathname
Also used to rename files or directories
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8. Managing Files and Directories (continued)
cp command: copies files
Same arguments as the mv command
Also used to make copies of files
Recursive: referring to itself and its own contents
Recursive copy command copies the directory and all subdirectories and contents
Recursive search includes all subdirectories in a directory and their contents
Use –r option
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9. Managing Files and Directories (continued)
Interactive mode: Prompts user before overwriting files
–i option
–f option (force): Overrides interactive mode
rm command: Removes files
Arguments are a list of files
Can use wildcards
Interactive mode by default
Use -f option to override
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10. Managing Files and Directories (continued)
rmdir command: removes directories
Arguments are a list of files
Can use wildcards
Interactive mode by default
Use -f option to override
Cannot be used to remove directory full of files
To delete directory and all its contents (subdirectories and files), use rm –r command
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11. Managing Files and Directories (continued)
Table 4-2: Common Linux file management commands
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12. Finding Files locate command: Search for files on system
Receives full or partial filename as argument
Uses premade indexed database of all files on system
To update the database use updatedb command
Information returned may not fit on screen
Use with more or less commands
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13. Finding Files (continued)
find command: recursively search for files starting from a specified directory
Slower than locate command, but more versatile
Format: find <start directory> -criteria <what to find>
e.g., find /root –name project
If using wildcard metacharacters, ensure that they are interpreted by the find command
Place wildcards in quotation marks
To reduce search time, specify subdirectory to be searched
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14. Finding Files (continued)
Table 4-3: Common criteria used with the find command
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15. Finding Files (continued)
Table 4-3 (continued): Common criteria used with the find command
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16. Finding Files (continued)
PATH variable: lists directories on system where executable files are located
Allows executable files to be run without specifying absolute or relative path
which command: search for an executable file
Searches the PATH variable
If the file is not found, lists the directories that were searched
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17. Linking Files
Symbolic link: one file is a pointer or shortcut to another
Hard link: two files share the same data
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18. Linking Files (continued)
Filesystem has three main structural sections:
Superblock: Contains general information about the filesystem
e.g., number of inodes and data blocks, size of each data block
The inode table: consists of several inodes, each of which describes a file or directory
Unique inode number, file size, data block locations, last date modified, permissions, and ownership
Data blocks: Data making up contents of a file
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19. Linking Files (continued)
Hard linked files share the same inode and inode number
Must reside on the same filesystem
To remove hard linked files, delete one of the linked files
Reduces the link count for the file
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20. Linking Files (continued)
Figure 4-1: The structure of hard linked files
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21. Linking Files (continued)
Symbolic linked files do not share the same inode and inode number with their target file
Symbolic linked file is a pointer to the target file
Data blocks in the linked file contain only a pathname for the target file
Linked file and target file have different sizes
Editing symbolic linked file actually edits the target file
If the target file is deleted, symbolic link serves no function
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22. Linking Files (continued)
Figure 4-2: The structure of symbolically linked files
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23. 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
Arguments can be relative or absolute pathnames
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24. 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
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25. 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
whoami command: view current user name
groups command: view group memberships and primary group
touch command: create an empty file
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26. File and Directory Ownership (continued)
chown (change owner) command: change ownership of a file or directory
Two arguments:
New owner
File to change
Can use –R option for contents of directory
chgrp (change group) command: change group owner of a file or directory
Same arguments and options as for chown command
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27. 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
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28. Interpreting the Mode Figure 4-3: The structure of a mode
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29. 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
Permissions are not additive
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30. Interpreting Permissions
Table 4-4: Linux permissions
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31. Changing Permissions
chmod (change mode) command: change mode (permissions) of files or directories
Two arguments at minimum
Criteria used to change permissions
Filenames to change
Permissions stored in a file’s or a directory’s inode as binary powers of two
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32. Changing Permissions (continued)
Table 4-5: Criteria used within the chmod command
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33. Changing Permissions (continued)
Figure 4-4: Numeric representation of the mode
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34. Changing Permissions (continued)
Table 4-6: Numeric representations of the permissions in a mode
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35. 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
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36. Default Permissions (continued)
Figure 4-5: Performing a umask 022 calculation
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37. Default Permissions (continued)
Figure 4-6: Performing a umask 007 calculation
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38. Special Permissions
Three more optional special permissions for files and directories
SUID (Set User ID)
SGID (Set Group ID)
Sticky bit
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39. Defining Special Permissions
SUID
If set on a file, user who executes the file becomes owner of the file during execution
e.g., ping command
No functionality when set on a directory
Only applicable to binary compiled programs
Cannot be used on shell scripts
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40. 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 file’s group owner is set to be the directory’s group owner and not the user’s primary group
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41. 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 his/her own files when given write permissions in a directory
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42. 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
Indicating letter in the mode will be capitalized
Add special permissions via chmod command
Add an extra digit at front of permissions argument
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43. Setting Special Permissions (continued)
Figure 4-7: Representing special permissions in the mode
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44. Setting Special Permissions (continued)
Figure 4-8: Representing special permissions in the absence of the execute permissions
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45. Setting Special Permissions (continued)
Figure 4-9: Numeric representation of regular and special permissions
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46. Summary The Linux directory tree obeys the FHS
Allows system files to be located in standard directories
Many file management commands exist
Can find files using different commands
locate: search preindexed database
which: search PATH variable
find: search for file based on criteria
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47. Summary (continued)
Files can be created as pointers to another file or as a linked duplicate of another file
Called symbolic and hard links, respectively
Each file and directory has an owner and a group owner
Owner can change permissions and grant ownership
Permissions can be set on the owner of a file, members of the group of the file, and everyone on the system (other)
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48. Summary (continued)
Three regular file and directory permissions (read, write, execute) and three special file and directory permissions (SUID, SGID, sticky bit)
Permissions can be changed using chmod
New files and directories receive default permissions from the system
The root user has all permissions to all files and directories on the Linux filesystem
Root user can change the ownership of any file or directory on the Linux filesystem
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