Linux Guide to Linux Certification, Second Edition

1
Linux Guide to Linux Certification, Second
Edition

• Chapter 5
• 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

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
• File and subdirectory contents
• Gives Linux software developers ability to locate
  files on any Linux system
• Create non-distributionspecific software

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The Filesystem Hierarchy Standard (continued)
Table 5-1 Linux directories defined by FHS
6
The Filesystem Hierarchy Standard (continued)
Table 5-1 (continued) Linux directories defined
by FHS
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Managing Files and Directories

• mkdir command Creates new directories
• Arguments specify directorys 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

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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

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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

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Managing Files and Directories (continued)
Table 5-2 Common Linux file management commands
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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

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Finding Files (continued)
Table 5-3 Common criteria used with find command
13
Finding Files (continued)
Table 5-3 (continued) Common criteria used with
find command
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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

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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
• 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

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Linking Files (continued)
Figure 5-1 The structure of hard 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
• Hard linked files share two inodes
• Data blocks in symbolically linked files contain
  pathname to target file

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Linking Files (continued)
Figure 5-2 The structure of symbolically linked
files
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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

21
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
• 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
• chgrp (change group) command Change group owner
  of a file or directory

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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 5-3 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
• Shell scripts Text files containing instructions
  for the shell to execute

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Interpreting Permissions
Table 5-4 Linux permissions
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Changing Permissions

• chmod (change mode) command Change mode
  (permissions) of files or directories
• Permissions stored in a files or a directorys
  inode as binary powers of two

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Changing Permissions (continued)
Table 5-5 Criteria used within the chmod command
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Changing Permissions (continued)
Figure 5-4 Numeric representation of the mode
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Changing Permissions (continued)
Table 5-6 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)
Figure 5-5 Performing a umask 022 calculation
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Default Permissions (continued)
Figure 5-6 Performing a umask 007 calculation
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Special Permissions

• 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
• No functionality when set on a directory
• Only applicable to binary compiled programs

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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 directorys group owner is changed to
  match the files group owner

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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 files his/her
  own files

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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 permission
• Via chmod command
• Add an extra digit at front of permissions
  argument

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Setting Special Permissions (continued)
Figure 5-7 Representing special permissions in
the mode
40
Setting Special Permissions (continued)
Figure 5-8 Representing special permissions in
the absence of the execute permissions
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Setting Special Permissions (continued)
Figure 5-9 Numeric representation of regular and
special permissions
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Summary

• The Linux directory tree obeys the FHS
• Many file management commands exist
• 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

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Summary (continued)

• Permissions can be set on the owner of a file,
  members of the group of the file, and everyone on
  the system (other)
• Can find files using locate, which, and find
• 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

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Summary (continued)

• 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