CLI Representation of Files

File Names

• File names serve as identifiers within a directory for end-users.
• File names must be unique within their directory.
• Files in different directories can share the same name.

Paths

• The path must be unique, not the file name.
• File names are components of a longer string called the path.

File Name Length

• POSIX file systems typically limit file names to 255 characters.
• This limit applies only to the file name itself, not the entire path.

Allowed Characters in POSIX

• Most characters are allowed except the null character and forward slash (/).
• Forward slash is a directory separator in paths.
• Avoid spaces and special characters like &, '', "", *, ?, [], and $.
• These characters have other meanings in the shell and can cause issues.

Case Sensitivity

• POSIX file names are case-sensitive (e.g., "Hello" and "hello" are different).
• Windows file names are not case-sensitive.
• Copying files between Windows and Linux can cause problems.

Hidden Files

• In POSIX, file names starting with a dot (.) are hidden.
• Hidden files are not shown by default in standard ls command output.
• The ls -a option lists all files, including hidden ones.
• Hidden files are often used for configuration files.

File Extensions

• File extensions are not required in POSIX systems.
• Using extensions is recommended for file identification.

Essential File and Directory Commands

• ls: Lists files and directories.
  • ls -l: Provides a long listing with details like permissions, owner, size, and modification date.
  • ls -a: Lists all files, including hidden ones.
• cd: Changes the current directory.
  • cd (without arguments): Returns to the home directory.
• rm: Removes (deletes) files.
  • rm -r: Recursively removes directories and their contents (dangerous!).
• rmdir: Removes empty directories.
• cp: Copies files.
• mv: Moves files.
  • Moving a file within the same directory renames it.
• find: Finds files based on various criteria.
• ln: Creates links (hard or symbolic).
  • Symbolic links are similar to shortcuts in Windows.

Demonstration of Commands

• ls -l shows file type, permissions, owner, group, size, and date.
• Colors in the terminal indicate file types and executables.
• Executable files are often green and have an "x" in the permissions list.
• pwd shows the current working directory.
• mv bad.html good.html renames bad.html to good.html.
• cp good.html bad.html creates a copy of good.html named bad.html.

Wildcards and Globing

• Wildcards are special characters that match arbitrary characters or sequences of characters.
• Used to manipulate groups of files.
• *: Matches any sequence of zero or more characters.
• ?: Matches a single character.
• []: Matches any of the characters listed inside the brackets.
• Wildcards do not match hidden files unless the pattern starts with a dot.

Examples of Wildcards

• ls /usr/bin/*zip: Shows all files in /usr/bin/ that end with "zip".
• ls /usr/bin/??zip: Shows files in /usr/bin/ that have two characters followed by "zip".

Path Names

• Every file has a unique absolute path starting from the root.
• Path names consist of file names separated by forward slashes.
• The final component of a path is the base name (file name).
• The preceding portion is the path prefix.
• Absolute paths begin with a forward slash (root directory).

Relative Paths

• The simplest relative path is just the file name itself.
• . denotes the current directory.
• .. denotes the parent directory.
• ~ denotes the user's home directory.

Path Length

• The total maximum length for an entire path is much more than 255 characters.

Absolute vs. Relative Paths

• Given a file /home/u123456/file1.txt and the current directory /home/u123456/csc1030:
  • Absolute path: /home/u123456/file1.txt
  • Relative path: ../file1.txt
  • Relative path using tilde: ~/file1.txt (if the current user is u123456)

Viewing File Content

• more and less: View file content page by page.
  • Spacebar to advance, b to go back, q to quit.
• cat: Displays the entire file content at once.
• head: Shows the first few lines of a file.
• tail: Shows the last few lines of a file.
• bat: An enhanced cat clone with syntax highlighting and line numbers.

File Creation

• touch: Creates a new empty file.
• Output redirection: Saving command outputs to a file.
  • >: Overwrites the file.
  • >>: Appends to the file.
• Automation relies heavily on CLI options.

Examples of File Creation and Redirection

• touch my_new_file.txt && ls my_new_*: Creates a file and lists files starting with my_new_.
• whoami > my_new_file.txt && cat my_new_file.txt: Saves the username to a file and displays its contents.
• date >> my_new_file.txt && cat my_new_file.txt: Appends the current date to the file.
• echo "some text here" >> my_new_file.txt: Appends text to the file.

Searching File Content: grep

• Searches for patterns in a file.
• grep pattern filename: Returns lines containing the pattern.
• Patterns can be literal strings or regular expressions.
• Searching is powerful and is supported by most command line options. Automating searches speeds up file review and anlalysis.

Example of grep

• grep free filename | wc -l: Counts the number of lines containing "free".

Stream Editor: sed

• A non-interactive text editor for modifying files on the fly.
• Uses commands to perform find and replace operations.
• Can use regular expressions for sophisticated matching.

Cutting and Extracting Text: cut

• Extracts specific parts of a text file based on character position or fields.

Options for cut

• -c: Specifies the range of characters to cut.
• -d: Specifies the delimiter between fields.
• -f: Lists the fields or columns to extract.
• -s: Suppresses lines without delimiters.

Examples of Cut

• cut -d : -f 5 file.txt: Extracts the fifth field from file.txt, using : as the delimiter.
• ls -l /usr | grep ^t | cut -c 2-10: Lists files in /usr, filters for those starting with t, and extracts characters 2-10 (permissions).

Joining Files: join

• Joins two files based on a common field.
• Example: join file1 file2, where file1 contains "1 deKaiser" and file2 contains "1 stem", results in "1 deKaiser stem".

Pip pelining

• Output of one command becomes the input of the next.
• command1 | command2 | command3

Pipelining Example

• find /dev -name 't*' 2> /dev/null | grep tty | sort | head -5
• Finds files in /dev starting with t, ignores errors, filters for tty, sorts, and shows the first 5 results.

Fun with Commands

• Commands like fortune, cowsay, and lolcat can be combined for humorous output.
  • fortune | cowsay | lolcat: Displays a fortune cookie in a cow speech bubble with colors.
  • ls -l | lolcat adds color to the output of the long listing.
• htop | lolcat adds color to the output of the htop (process monitoring) command.