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

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Linux+ Guide to Linux Certification, Third Edition Chapter 4 Linux Filesystem Management Slide 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 Linux+ Guide to Linux Certification, 3e2 Slide 3 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 Slide 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 Linux+ Guide to Linux Certification, 3e4 Slide 5 5 The Filesystem Hierarchy Standard (continued) Table 4-1: Linux directories defined by the Filesystem Hierarchy Standard Slide 6 Linux+ Guide to Linux Certification, 3e6 The Filesystem Hierarchy Standard (continued) Table 4-1 (continued): Linux directories defined by the Filesystem Hierarchy Standard Slide 7 Linux+ Guide to Linux Certification, 3e7 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 Slide 8 Linux+ Guide to Linux Certification, 3e8 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 Slide 9 Linux+ Guide to Linux Certification, 3e9 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 Slide 10 Linux+ Guide to Linux Certification, 3e10 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 Slide 11 Linux+ Guide to Linux Certification, 3e11 Managing Files and Directories (continued) Table 4-2: Common Linux file management commands Slide 12 Linux+ Guide to Linux Certification, 3e12 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 Slide 13 Linux+ Guide to Linux Certification, 3e13 Finding Files (continued) find command: recursively search for files starting from a specified directory Slower than locate command, but more versatile Format: find -criteria 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 Slide 14 Linux+ Guide to Linux Certification, 3e14 Finding Files (continued) Table 4-3: Common criteria used with the find command Slide 15 Linux+ Guide to Linux Certification, 3e15 Finding Files (continued) Table 4-3 (continued): Common criteria used with the find command Slide 16 Linux+ Guide to Linux Certification, 3e16 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 Slide 17 Linux+ Guide to Linux Certification, 3e17 Linking Files Symbolic link: one file is a pointer or shortcut to another Hard link: two files share the same data Slide 18 Linux+ Guide to Linux Certification, 3e18 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 Slide 19 Linux+ Guide to Linux Certification, 3e19 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 Slide 20 Linux+ Guide to Linux Certification, 3e20 Linking Files (continued) Figure 4-1: The structure of hard linked files Slide 21 Linux+ Guide to Linux Certification, 3e21 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 Slide 22 Linux+ Guide to Linux Certification, 3e22 Linking Files (continued) Figure 4-2: The structure of symbolically linked files Slide 23 Linux+ Guide to Linux Certification, 3e23 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 Slide 24 Linux+ Guide to Linux Certification, 3e24 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 Slide 25 Linux+ Guide to Linux Certification, 3e25 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 Slide 26 Linux+ Guide to Linux Certification, 3e26 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 Slide 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 Linux+ Guide to Linux Certification, 3e27 Slide 28 Linux+ Guide to Linux Certification, 3e28 Interpreting the Mode Figure 4-3: The structure of a mode Slide 29 Linux+ Guide to Linux Certification, 3e29 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 Slide 30 Linux+ Guide to Linux Certification, 3e30 Interpreting Permissions Table 4-4: Linux permissions Slide 31 Linux+ Guide to Linux Certification, 3e31 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 Slide 32 Linux+ Guide to Linux Certification, 3e32 Changing Permissions (continued) Table 4-5: Criteria used within the chmod command Slide 33 Linux+ Guide to Linux Certification, 3e33 Changing Permissions (continued) Figure 4-4: Numeric representation of the mode Slide 34 Linux+ Guide to Linux Certification, 3e34 Changing Permissions (continued) Table 4-6: Numeric representations of the permissions in a mode Slide 35 Linux+ Guide to Linux Certification, 3e35 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 Slide 36 Linux+ Guide to Linux Certification, 3e36 Default Permissions (continued) Figure 4-5: Performing a umask 022 calculation Slide 37 Linux+ Guide to Linux Certification, 3e37 Default Permissions (continued) Figure 4-6: Performing a umask 007 calculation Slide 38 Linux+ Guide to Linux Certification, 3e38 Special Permissions Three more optional special permissions for files and directories SUID (Set User ID) SGID (Set Group ID) Sticky bit Slide 39 Linux+ Guide to Linux Certification, 3e39 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 Slide 40 Linux+ Guide to Linux Certification, 3e40 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 Slide 41 Linux+ Guide to Linux Certification, 3e41 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 Slide 42 Linux+ Guide to Linux Certification, 3e42 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 Slide 43 Linux+ Guide to Linux Certification, 3e43 Setting Special Permissions (continued) Figure 4-7: Representing special permissions in the mode Slide 44 Linux+ Guide to Linux Certification, 3e44 Setting Special Permissions (continued) Figure 4-8: Representing special permissions in the absence of the execute permissions Slide 45 Linux+ Guide to Linux Certification, 3e45 Setting Special Permissions (continued) Figure 4-9: Numeric representation of regular and special permissions Slide 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 Linux+ Guide to Linux Certification, 3e46 Slide 47 Linux+ Guide to Linux Certification, 3e47 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) Slide 48 Linux+ Guide to Linux Certification, 3e48 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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