Bash scripting: shebang, variables, parameter expansion, quoting, exit codes and $?

A Bash script is a text file of shell commands run in sequence, letting you automate anything you can type. Linux+ performance-based questions often show a short script and ask what it prints or why it fails, so the core syntax needs to be solid. Most script bugs come from a handful of details: how the script is started, how variables are assigned and expanded, how quoting works, and how success and failure are reported.