Week 4 Linux File System Management. Objectives Find files and directories on the filesystem ...

Week 4Linux File System Management

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, 3e 2

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


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


The Filesystem Hierarchy Standard (continued)


Table 4-1: Linux directories defined by the Filesystem Hierarchy Standard

The Filesystem Hierarchy Standard (continued)


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

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


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


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


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


Managing Files and Directories (continued)


Table 4-2: Common Linux file management commands

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


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


Finding Files (continued)


Table 4-3: Common criteria used with the find command

Finding Files (continued)


Table 4-3 (continued): Common criteria used with the find command

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


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.


Linking Files

Symbolic link: one file is a pointer or shortcut to another

Hard link: two files share the same data


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


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



21Linux+ Guide to Linux Certification, 3e

Figure 4-1: The structure of hard linked files

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




Figure 4-2: The structure of symbolically linked files


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


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


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


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


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


Interpreting the Mode


Figure 4-3: The structure of a mode

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


Interpreting Permissions


Table 4-4: Linux permissions

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


Changing Permissions (continued)


Table 4-5: Criteria used within the chmod command



Figure 4-4: Numeric representation of the mode



Table 4-6: Numeric representations of the permissions in a mode

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


Default Permissions (continued)


Figure 4-5: Performing a umask 022 calculation

Default Permissions (continued)


Figure 4-6: Performing a umask 007 calculation

Special Permissions

Three more optional special permissions for files and directoriesSUID (Set User ID)SGID (Set Group ID)Sticky bit


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


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


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


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


Setting Special Permissions (continued)


Figure 4-7: Representing special permissions in the mode



Figure 4-8: Representing special permissions in the absence of the execute permissions



Figure 4-9: Numeric representation of regular and special permissions

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


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)


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


Date post:	13-Dec-2015
Category:	Documents
Upload:	samantha-rich
View:	216 times
Download:	0 times

Week 4 Linux File System Management. Objectives Find files and directories on the filesystem ...

Documents