Shell scripting

A shell is the program that reads the commands typed into a terminal. A shell script saves those commands in a file, with variables, loops and conditions, so a job of twenty manual steps becomes one command. Bash is the standard on Linux servers, Zsh is the default on macOS, and plain POSIX sh is the lowest common denominator that runs almost anywhere.

Scripts glue other programs together: a pipe sends one command's output into the next (find the files, filter them with grep, count them with wc), which is why so many CI pipelines, Dockerfiles, deploy steps and cron jobs are shell. On Windows, PowerShell plays the same role but passes structured objects between commands instead of plain text; it is open source and runs on macOS and Linux too.

Shell is easy to start and easy to get wrong: unquoted variables break on file names with spaces, and failed commands are ignored unless the script opts in to stopping, for example with set -euo pipefail. The ShellCheck linter catches most of these mistakes. Once a script grows past a page or two, Python is usually the better tool.