Linux Guide to Linux Certification, Third Edition

1
Linux Guide to Linux Certification, Third Edition

• Week 4
• Linux File System 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

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

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-distributionspecific software

5
The Filesystem Hierarchy Standard (continued)

Table 4-1 Linux directories defined by the
Filesystem Hierarchy Standard
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The Filesystem Hierarchy Standard (continued)

Table 4-1 (continued) Linux directories defined
by the Filesystem Hierarchy Standard
7
Managing Files and Directories

• mkdir command creates new directories
• Arguments specify directorys 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

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

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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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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Managing Files and Directories (continued)

Table 4-2 Common Linux file management commands
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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

13
Finding Files (continued)

• find command recursively search for files
  starting from a specified directory
• Slower than locate command, but more versatile
• Format find ltstart directorygt -criteria ltwhat to
  findgt
• 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)

Table 4-3 Common criteria used with the find
command
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Finding Files (continued)
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 file
• Searches the PATH variable
• If 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 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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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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Linking Files (continued)
Figure 4-1 The structure of hard linked files
22
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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Linking Files (continued)
Figure 4-2 The structure of symbolically linked
files
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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

25
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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File and Directory Ownership

• Primary group users default group
• During file creation, files owner and group
  owner set to users 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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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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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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Interpreting the Mode
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

Table 4-4 Linux permissions
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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 files or a directorys
  inode as binary powers of two

33
Changing Permissions (continued)
Table 4-5 Criteria used within the chmod command
34
Changing Permissions (continued)
Figure 4-4 Numeric representation of the mode
35
Changing Permissions (continued)
Table 4-6 Numeric representations of the
permissions in a mode
36
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)
Figure 4-5 Performing a umask 022 calculation
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Default Permissions (continued)
Figure 4-6 Performing a umask 007 calculation
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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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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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Defining Special Permissions (continued)

• SGID
• Applicable to files and directories
• If set on a file, user who executes the file
  becomes member of the files group during
  execution
• If a user creates a file in a directory with SGID
  set, the files group owner is set to be the
  directorys group owner and not the users
  primary group

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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

43
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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Setting Special Permissions (continued)
Figure 4-7 Representing special permissions in
the mode
45
Setting Special Permissions (continued)
Figure 4-8 Representing special permissions in
the absence of the execute permissions
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Setting Special Permissions (continued)
Figure 4-9 Numeric representation of regular and
special permissions
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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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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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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