Linux File System

Exploring Linux Filesystems

• Understand and navigate the Linux directory structure using relative and absolute pathnames.
• Describe the various types of Linux files.
• View filenames and file types.
• Employ shell wildcards to specify multiple filenames.
• Display the contents of various types of files, including text and binary.
• Search text files for regular expressions using grep.
• Use the vi editor to manipulate text files.
• Identify common alternatives to the vi text editor used today.

The Linux Directory Structure

• A directory organizes files into a logical tree structure and is stored in a filesystem on a specific partition of a hard disk or SSD.
• An absolute pathname provides the path from the root directory to a specific file or directory.
• Root refers to the top-level directory, designated as /root.

Changing Directories

• Each user account has a home directory, the ~ metacharacter refers to this directory.
• The pwd command displays the current directory within the directory tree.
• The cd command changes the current directory.
• A relative pathname specifies the path of a target file or directory relative to the current directory.
• A parent directory is one step closer to the root of the tree and is referenced by two dots (..).
• A subdirectory resides within another directory.
• Tab-completion is a BASH shell feature where pressing the Tab key fills in the remaining characters and alerts the user if multiple matches are possible.

Viewing Files and Directories

• A directory structure organizes files into an easy-to-use format.
• Viewing files and directories involves recognizing various file types, filenames, and commands to select filenames for viewing.

File Types

• Text files contain configuration information.
• Binary data files store information like common functions and graphics
• Linked files have an association with one another.
• Special device files represent different devices on the system.
• Named pipe files identify channels that pass information from one process in memory to another.
• Socket files allow a process on another computer to write to a file on the local computer while another process reads from that file.

Filenames

• A filename is an identifier given to a file, with a length of up to 255 characters (though rarely longer than 20).
• Filenames can contain alphanumeric characters, underscores (_), dashes (-), and periods (.).
• Filename extensions are identifiers following a period (.) at the end of the filename to indicate the file type.
• Most files on Linux do not have filename extensions.

Listing Files

• The ls command displays files in a directory.
• The ls -F command appends a special character at the end of each filename to indicate the file type.
• The ls -l command provides a long listing for each file in a certain directory.
• File components listed with ls -l include the file type character, file permissions, hard link count, owner, group owner, size, modification time, and filename.
• The file command displays the file type of any file, using an argument to specify which file(s) to analyze.
• The file command identifies different types of executable files and empty files.
• Hidden files are not normally displayed to the user and are often configuration files.
• Filenames that start with a dot (.) are hidden.
• The ls -a command displays hidden files.

Wildcard Metacharacters

• Simplify commands by enabling specification of multiple filenames on the command line.
• They can match the entire filename or portions of filenames.
• They can be used with most Linux filesystem commands.
• *: Matches 0 or more characters in a filename.
• ?: Matches 1 character in a filename.
• [aegh]: Matches 1 character, if it is a, e, g, or h.
• [a-e]: Matches 1 character, if it is a, b, c, d, or e.
• [!a-e]: Matches 1 character, if it is NOT a, b, c, d, or e.

Displaying the Contents of Text Files

• Concatenation views an entire text file on the terminal screen,.
• The cat command displays the contents of a text file to the screen.
• The cat -n option displays the line number of each line in the file.
• Log files contain records of events, with the most recent events appended to the end of the file.
• The tac command displays the contents of a text file in reverse order.
• The head command displays the first ten lines of a file.
• The tail command displays the last ten lines of a file.
• Large text files cannot be viewed using the cat command because the screen can display only a portion.
• The more command displays text files page-by-page.
• The less command has the same functionality as more but has the added feature of allowing navigation using arrow keys.
• The more and less commands can be used with output of other commands.

Displaying the Contents of Binary Files

• To view the contents of binary files, you can use the program used to create the file.
• The strings command searches for and displays text characters in a binary file and might indicate the binary file's purpose.
• The od command displays contents of a file in octal format (numeric base 8).
• The od -x option displays the contents in hexadecimal format (numeric base 16).

Searching for Text Within Files

• Text tools include commands that search for and manipulate text, such as database information.
• Regular expressions (regexp) are text wildcards used to ease the search for specific text.
• Regular expressions are recognized by several text tools and programming languages.

Regular Expressions

• Regular expressions are different from wildcard metacharacters.
• Wildcard metacharacters are interpreted by the shell, while regular expressions are interpreted by a text tool program.
• Wildcard metacharacters match characters in filenames (or directory names) while regular expressions match characters within text files.
• Wildcard metacharacters and regular expressions have different definitions.
• There are many more regular expression metacharacters available than wildcard metacharacters.
• Regular expressions fall into two categories: common and extended.
• Common regular expressions are available to most text tools.
• Extended regular expressions are less common and available in certain text tools only.

The grep Command

• Is a global regular expression print command that displays lines in a text file that match common regular expressions.
• The egrep command displays lines that match extended regular expressions.
• The fgrep command does not interpret regular expressions and therefore returns results much faster.
• It requires two arguments: text to search and files to search.
• It is case sensitive.
• The -i option is used for case-insensitive searches.
• It matches patterns of text, but cannot discern words or phrases unless they are specified.

Editing Text Files

• Most system configurations are stored in text files because they contain often-accessed information such as e-mail and program source code.
• Most Linux distributions include multiple text editors that can be used on the command line or in a GUI environment.

The vi Editor

• Is a popular text editor for UNIX OSs.
• The Linux equivalent of vi is vim (vi improved).
• Because it is standard on most Linux distributions and used on Unix and Linux, its advantage is portability, not usability.
• The vi editor supports regular expressions and can perform over 1000 different functions.
• The vi editor is a bi-modal editor.
• Command mode allows text editing tasks not related to inserting text, such as deleting text, copying text, saving changes, and exiting.
• Insert mode inserts text.
• The Esc key returns to command mode.
• Its user environment can be customized through settings at the : prompt.

Other Common Text Editors

• GNU Emacs (Editor MACroS) offers comparable functionality to vi.
• Nano uses Ctrl key combinations for performing functions.
• Gedit is a functional graphical text editor in a GUI environment.

Summary of Linux Filesystems

• The Linux filesystem is arranged hierarchically, with a series of directories that store files.
• The ls command provides an option to modify the view to display filenames,
• Wildcard metacharacters are special keyboard characters that simplify selecting files when using common Linux file commands.
• Text files are the most common file type, and several utilities can display their contents: head, tail, cat, tac, more, and less.
• Regular expression metacharacters are used to specify certain patterns of text.
• Commonly used with certain programming languages and text tool utilities like grep.
• Although command-line and graphical text editors exist, vi (vim) is a powerful bimodal text editor standard on most UNIX and Linux systems.