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One line: A Linux shell book is right about the machine it was written on. This library reruns its everyday examples on Linux, on a Mac, and in zsh and fish, and shows where the answer changes.

This library is for someone reading a Linux book or course while typing on a Mac, or in a shell other than the book's. A page from The Ultimate Linux Shell Scripting Guide sets PS1="[\u@\h \W]\$ " and tells you to add it to .bashrc. In fish that line does nothing useful, and on a Mac the .bashrc it names is not the file Terminal reads. The book is right about Fedora; a reader on a Mac needs to know which of its sentences are still true on theirs.

What it assumes

That you have used a terminal: cd, ls, running a command, pressing Tab. Nothing about scripting.

How a lesson works

• The page has one idea, a **One line:** that states the claim, then the evidence.
• examples/ holds the scripts behind the page, one per shell: _sh.sh for bash, _zsh.zsh, _fish.fish. A block marked Verified output was pasted in by a tool from a real run, and CI runs it again on Ubuntu and on macOS on every push. If the block says identical on Linux and macOS, both machines printed those exact bytes. If there are two blocks, labelled, they did not, and the page explains the difference.
• On a Mac and In zsh and fish are two sections on every page. Each answers the question you would otherwise have to test: is this valid here?
• If you are coming from another library links the page in the C, Python, Encodings or Perl library that covers the same thing from the other side of the pipe.

Three differences to know before anything else

1. A Mac's bash is from 2006. /bin/bash on macOS is bash 3.2, the last release under GPLv2. Apple has patched it but never moved it to a newer version. zsh has been the Mac's default login shell since macOS Catalina (2019). Anything a book shows from bash 4 or 5 is either missing on a stock Mac or needs brew install bash.
2. A Mac's command-line tools are BSD's. grep, sed, tr, ls and wc come from FreeBSD, not GNU. They share most options, and the differences are exactly where a copied command breaks: grep -P, sed -i, wc -l padding.
3. fish is not a POSIX shell, on purpose. It has no $?, no PS1, no here-documents, and different quoting rules. Most pages here have a fish section, because "is it valid with fish?" usually gets a no, and the working fish version is usually shorter.

Running an example yourself

git clone https://github.com/masiarek/linux-learning-library
cd linux-learning-library/01_Pipelines/a_pipeline_reports_its_last_command/examples
bash exit_status_sh.sh
fish exit_status_fish.fish
python3 ../../../tools/run_examples.py --only 01_Pipelines

The last command checks your machine's output against the recorded answer key and tells you if yours disagrees.

The chapters

Chapter	What it covers
01 — Pipelines	What | connects, and what it does not carry
02 — Redirection	File descriptors 0, 1 and 2, and every operator that moves them
03 — tee	A T-junction in a pipe
04 — grep	Searching text, GNU against BSD
05 — tr	Character-by-character translation, and its classes
06 — History	What the shell remembers, where, and when it writes it down
07 — fzf	Fuzzy filtering, and Ctrl-R
08 — The prompt	PS1, PROMPT and fish_prompt
10 — Files	The type letter in ls -l, what opening each type does, and the three things a dot can be
09 — Resources	Manuals, books, sibling libraries