Week 10 Briefing: Essential Linux Commands for File Management
This week, we focus on powerful Linux commands for filtering text, redirecting command output, and archiving files. Mastering these tools is crucial for efficient system administration and scripting.
I. Filtering Text with grep
grep is a fundamental search tool in Linux that filters lines containing specific text from a set of information, returning only what matches your query.
• Purpose:
    ◦ Find specific files or types of files.
    ◦ Search for particular users on a system (e.g., in /etc/passwd).
    ◦ Filter text based on patterns.
• Basic Usage:
    ◦ Directly: grep [what_to_look_for] [file].
        ▪ Example: grep boston datebook.
    ◦ Piping: cat [file] | grep [what_to_look_for]. This sends the output of cat as input to grep. For beginners, these two methods are functionally similar.
• Key Options & Features:
    ◦ -v (invert match): Shows results that do not contain the specified pattern.
        ▪ Example: grep -v boston datebook.
    ◦ Regular Expressions (Regex): grep understands regex for complex pattern matching.
        ▪ ^ (caret): Matches the start of a line.
            • Example: grep '^Z' datebook.
        ▪ $ (dollar sign): Matches the end of a line.
            • Example: grep '57000$' datebook.
    ◦ Piping (|): Allows you to chain commands, sending the output of one command as the input to the next.
        ▪ Example: grep '000$' datebook | grep california (searches for lines ending in "000", then filters those results for "california").
        ▪ Important Note: When piping the output of a grep command to another grep command, do not re-add the original file name to the second grep, as it will search the entire file again.
II. Redirection (>) and Append (>>)
These symbols control where a command's output goes, allowing you to send it to a file instead of displaying it on the screen.
• Redirect (> - single greater than sign):
    ◦ Sends all command output to a text file.
    ◦ Crucially, it will erase any existing contents of the file before writing the new information.
    ◦ Example: ls > testfile (overwrites testfile with the directory listing).
• Append (>> - two greater than signs):
    ◦ Sends all command output to the end of a text file.
    ◦ It will not erase existing file contents; it adds new information to the bottom.
    ◦ Example: ls >> testfile (adds the directory listing to the end of testfile).
III. Displaying and Comparing Files
Various commands help you view and analyze file content:
• cat: Displays the entirety of a file.
• head: Displays the top (default 10) lines of a file.
• tail: Displays the end (default 10) lines of a file.
• od: Displays any file by showing its hex code.
• diff (File Comparison):
    ◦ Used to compare two files.
    ◦ By default, its output is designed to be read by a computer, not a human.
    ◦ Often integrated into tools like Git or used to identify changes in configuration files.
IV. Archiving with tar (Tarballs)
tar (Tape Archive) is used for bundling multiple files and folders into a single file, known as a tarball.
• Purpose:
    ◦ Archiving: To put a "bunch of information" (files and folders) together into one package for easier handling.
    ◦ Backups: Useful for creating backups.
    ◦ Portability: Makes it easy to move, copy, and work with sets of files without forgetting anything.
    ◦ Can be used with compression.
• Creating a Tarball (tar -cvf):
    ◦ Options: c (create), v (verbose - shows what's going in), f (file - specifies the archive name).
        ▪ c and f are mandatory when creating; v is optional but recommended.
    ◦ Crucial Rule for f: The f option must always be the last option given to tar when creating an archive; incorrect placement will cause errors.
    ◦ Arguments Order: Destination (the name and path of the tarball you are creating) then Source (the files/directories to include).
        ▪ Example: tar -cvf ./test_tarball.tar . (creates test_tarball.tar in the current folder, containing everything in the current folder).
        ▪ It's good practice to use the .tar extension for clarity.
• Compression with gzip: To compress a tarball, add the z option (-cvzf) and typically use a .tar.gz or .tgz extension.
V. Recommended Practice
• Experiment with grep on /etc/passwd to search for specific users, use the -v option, and try regex for ^ and $.
• Practice redirection (>) and append (>>) with commands like ls to observe how they affect a target file.
• Use head, tail, and od on existing files to see different display methods.
• Create a simple tarball, paying close attention to the cvf options and the order of destination/source arguments.

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