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Week 4Linux File System Management
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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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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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The Filesystem Hierarchy Standard Filesystem Hierarchy Standard (FHS):
standard set of directories for Linux and UNIX systemsStandard file and subdirectory contentsSimplifies the task of finding specific filesGives Linux software developers ability to
locate files on any Linux system○ Create non-distribution–specific software
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The Filesystem Hierarchy Standard (continued)
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Table 4-1: Linux directories defined by the Filesystem Hierarchy Standard
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The Filesystem Hierarchy Standard (continued)
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Table 4-1 (continued): Linux directories defined by the Filesystem Hierarchy Standard
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Managing Files and Directories mkdir command: creates new directories
Arguments specify directory’s absolute or relative pathname
mv command: moves filesMinimum of two arguments:
○ Source file/directory (may specify multiple sources)
○ Target file/directoryPathnames can be absolute or relative
○ For multiple files, can use wildcards in pathnameAlso used to rename files or directories
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Managing Files and Directories (continued) cp command: copies files
Same arguments as the mv commandAlso used to make copies of files
Recursive: referring to itself and its own contentsRecursive copy command copies the
directory and all subdirectories and contentsRecursive search includes all subdirectories
in a directory and their contentsUse –r option
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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 filesCan use wildcardsInteractive mode by default
○ Use -f option to override
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Managing Files and Directories (continued) rmdir command: removes directories
Arguments are a list of filesCan use wildcardsInteractive mode by default
○ Use -f option to overrideCannot 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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Managing Files and Directories (continued)
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Table 4-2: Common Linux file management commands
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Finding Files
locate command: Search for files on systemReceives full or partial filename as argumentUses premade indexed database of all files
on system○ To update the database use updatedb
commandInformation returned may not fit on screen
○ Use with more or less commands
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Finding Files (continued) find command: recursively search for files
starting from a specified directorySlower than locate command, but more versatileFormat: 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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Finding Files (continued)
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Table 4-3: Common criteria used with the find command
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Finding Files (continued)
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Table 4-3 (continued): Common criteria used with the find command
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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 fileSearches the PATH variableIf the file is not found, lists the directories
that were searched
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What is an INODE? Inodes store information about files and folders, such as file
ownership, access mode (read, write, execute permissions), and file type.
Fixed number of inodes per file system Inodes do not contain file names, only file metadata. Use df -i to see inode usage Use ls -i to determine a filenames inode number
WARNING: You can use up all of a filesystems inodes without using all of the storage space on the disk it resides.
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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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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 blockThe 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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Linking Files (continued)
Hard linked files share the same inode and inode numberMust reside on the same filesystem
To remove hard linked files, delete one of the linked filesReduces the link count for the file
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Linking Files (continued)
21Linux+ Guide to Linux Certification, 3e
Figure 4-1: The structure of hard linked files
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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 fileData 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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Linking Files (continued)
Linux+ Guide to Linux Certification, 3e 23
Figure 4-2: The structure of symbolically linked files
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Linking Files (continued)
ln (link) command: Create hard and symbolic linksTwo arguments:
○ Existing file to link○ Target file to create as a link to existing file
Use –s option to create symbolic linkArguments can be relative or absolute
pathnames
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File and Directory Permissions All users must login with a username
and password Users identified by username and group
membershipsAccess to resources depends on username
and group membershipMust have required permissions
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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 groupSame 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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File and Directory Ownership (continued) chown (change owner) command:
change ownership of a file or directoryTwo 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 directorySame arguments and options as for chown
command
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Managing File and Directory Permissions Mode: inode section that stores
permissions Three sections, based on the user(s) that
receive the permission:User permissions: ownerGroup permissions: group ownerOther permissions: everyone on system
Three regular permissions may be assigned to each user:ReadWriteExecute
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Interpreting the Mode
29Linux+ Guide to Linux Certification, 3e
Figure 4-3: The structure of a mode
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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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Interpreting Permissions
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Table 4-4: Linux permissions
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Changing Permissions
chmod (change mode) command: change mode (permissions) of files or directoriesTwo 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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Changing Permissions (continued)
33Linux+ Guide to Linux Certification, 3e
Table 4-5: Criteria used within the chmod command
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Changing Permissions (continued)
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Figure 4-4: Numeric representation of the mode
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Changing Permissions (continued)
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Table 4-6: Numeric representations of the permissions in a mode
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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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Default Permissions (continued)
37Linux+ Guide to Linux Certification, 3e
Figure 4-5: Performing a umask 022 calculation
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Default Permissions (continued)
38Linux+ Guide to Linux Certification, 3e
Figure 4-6: Performing a umask 007 calculation
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Special Permissions
Three more optional special permissions for files and directoriesSUID (Set User ID)SGID (Set Group ID)Sticky bit
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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 directoryOnly applicable to binary compiled programs
○ Cannot be used on shell scripts
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Defining Special Permissions (continued) SGID
Applicable to files and directoriesIf 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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Defining Special Permissions (continued) Sticky bit
Previously used to lock files in memoryCurrently only applicable to directoriesEnsures that a user can only delete his/her
own files when given write permissions in a directory
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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 permissionIndicating letter in the mode will be capitalized
Add special permissions via chmod commandAdd an extra digit at front of permissions
argument
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Setting Special Permissions (continued)
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Figure 4-7: Representing special permissions in the mode
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Setting Special Permissions (continued)
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Figure 4-8: Representing special permissions in the absence of the execute permissions
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Setting Special Permissions (continued)
46Linux+ Guide to Linux Certification, 3e
Figure 4-9: Numeric representation of regular and special permissions
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Summary
The Linux directory tree obeys the FHSAllows system files to be located in standard
directories
Many file management commands exist Can find files using different commands
locate: search preindexed databasewhich: search PATH variablefind: search for file based on criteria
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Summary (continued) Files can be created as pointers to another
file or as a linked duplicate of another fileCalled symbolic and hard links, respectively
Each file and directory has an owner and a group ownerOwner 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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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 filesystemRoot user can change the ownership of any file
or directory on the Linux filesystem
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