3GNU and Unix Commands

• 3.1Working on the Command Line

  Learn interactive bash basics: shell variables vs environment variables (set, unset, export, env, echo), quoting (single, double, backquote), command history (history, .bash_history), absolute/relative paths and running commands outside PATH, and reading manuals with man.

• 3.2Processing Text Streams with Filters

  Learn filters—commands that transform stdin to stdout: viewers (cat, less, head, tail), extract/format (cut, sort, uniq, wc, nl, tr), join/split (paste, join, split), and others (sed, od, expand/unexpand, fmt, pr).

• 3.3Streams, Pipes, and Redirects

  Learn to rewire command I/O: the three streams (stdin, stdout, stderr), redirection (<, >, >>, 2>&1), chaining with pipes (|), turning output into arguments with xargs, and forking output to screen and file with tee.

• 3.4Searching Text Files with Regular Expressions

  Learn pattern matching with regular expressions: the core metacharacters (^, $, ., *, [ ], \\), grep and its variants (egrep = grep -E, fgrep = grep -F), search-and-replace with sed, and where regex(7) fits.

• 3.5Basic File Editing with an Editor

  Learn the standard editor vi (vim): switching between normal mode and insert mode (i, o, a, ESC), moving with h/j/k/l, editing (dd, yy, p, c, d, y), searching (/ and ?), saving and quitting (ZZ, :w!, :q!, :e!), and changing the default editor (EDITOR, nano, emacs).