Glossary

Quick definitions, A to Z. Each term is explained in depth in its guide; this is the reminder card.

• Argument — input to a command or script ($1, $2, ...); all args as "$@".
• Command substitution — $(...): runs a command, yields its output.
• Exit code — a command's numeric result; 0 is success ($? holds the last one).
• Expansion — Bash replacing $var, $(cmd), globs, etc. before running a command.
• Glob — filename pattern (*.txt, ?.log); expanded by the shell, not the command.
• Heredoc — multi-line input fed to a command (<< 'EOF').
• Parameter expansion — ${var...} string surgery: defaults, stripping, lengths.
• Pipe — |: connects one command's stdout to the next command's stdin.
• PIPESTATUS — array of every stage's exit code in the last pipeline.
• Redirection — >, >>, <, 2>: sending streams to files or merging them.
• Shebang — the #!/usr/bin/env bash first line naming the interpreter.
• ShellCheck — the linter that catches Bash mistakes; run it on everything.
• Source — source file / . file: run a file in the current shell (shares functions/variables).
• stderr — stream 2: errors and diagnostics.
• stdin — stream 0: a command's input.
• stdout — stream 1: a command's normal output.
• Subshell — a child shell copy; variable changes inside don't escape (pipeline stages run in subshells).
• Word splitting — unquoted expansions split on spaces; the reason you quote "$var".
• set -euo pipefail — the three flags every script starts with: exit on error, error on unset variables, pipelines fail if any stage fails.
• trap — run a command on exit/signal; used for cleanup.
• xargs — builds command arguments from stdin lines.