Linux+ Guide to Linux Certification, Second Edition Chapter 5 Linux Filesystem Management

Linux+ Guide to Linux Certification, 2e 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 Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

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-distribution–specific software Linux+ Guide to Linux Certification, 2e

The Filesystem Hierarchy Standard (continued) Table 5-1: Linux directories defined by FHS Linux+ Guide to Linux Certification, 2e

The Filesystem Hierarchy Standard (continued) Table 5-1 (continued): Linux directories defined by FHS Linux+ Guide to Linux Certification, 2e

Managing Files and Directories mkdir command: Creates new directories Arguments specify directory’s 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 Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

Managing Files and Directories (continued) Table 5-2: Common Linux file management commands Linux+ Guide to Linux Certification, 2e

Linux+ Guide to Linux Certification, 2e 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 Linux+ Guide to Linux Certification, 2e

Finding Files (continued) Table 5-3: Common criteria used with find command Linux+ Guide to Linux Certification, 2e

Finding Files (continued) Table 5-3 (continued): Common criteria used with find command Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

Linux+ Guide to Linux Certification, 2e Linking Files Symbolic link: One file is a pointer or shortcut to another Hard link: Two files share the same data Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

Linking Files (continued) Figure 5-1: The structure of hard linked files Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

Linking Files (continued) Figure 5-2: The structure of symbolically linked files Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

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 group Same for directory creation touch command: Create an empty file Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

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, 2e

Linux+ Guide to Linux Certification, 2e Interpreting the Mode Figure 5-3: The structure of a mode Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

Interpreting Permissions Table 5-4: Linux permissions Linux+ Guide to Linux Certification, 2e

Linux+ Guide to Linux Certification, 2e Changing Permissions chmod (change mode) command: Change mode (permissions) of files or directories Permissions stored in a file’s or a directory’s inode as binary powers of two Linux+ Guide to Linux Certification, 2e

Changing Permissions (continued) Table 5-5: Criteria used within the chmod command Linux+ Guide to Linux Certification, 2e

Changing Permissions (continued) Figure 5-4: Numeric representation of the mode Linux+ Guide to Linux Certification, 2e

Changing Permissions (continued) Table 5-6: Numeric representations of the permissions in a mode Linux+ Guide to Linux Certification, 2e

Linux+ Guide to Linux Certification, 2e 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 Linux+ Guide to Linux Certification, 2e

Default Permissions (continued) Figure 5-5: Performing a umask 022 calculation Linux+ Guide to Linux Certification, 2e

Default Permissions (continued) Figure 5-6: Performing a umask 007 calculation Linux+ Guide to Linux Certification, 2e

Linux+ Guide to Linux Certification, 2e Special Permissions SUID (Set User ID) SGID (Set Group ID) Sticky bit Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

Defining Special Permissions (continued) SGID Applicable to files and directories If 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 directory’s group owner is changed to match the file’s group owner Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

Setting Special Permissions (continued) Figure 5-7: Representing special permissions in the mode Linux+ Guide to Linux Certification, 2e

Setting Special Permissions (continued) Figure 5-8: Representing special permissions in the absence of the execute permissions Linux+ Guide to Linux Certification, 2e

Setting Special Permissions (continued) Figure 5-9: Numeric representation of regular and special permissions Linux+ Guide to Linux Certification, 2e