tee

One line: tee copies its stdin to stdout and to every file it is named, which puts a T-junction in a pipe: one copy goes on to the next command, another is saved to a file.

Lesson	Level	What it settles
tee saves and passes on	101	cmd \| tee f \| next, -a, several files at once, and 2>&1 (zsh \|&, fish &\|) before the pipe
sudo tee writes where redirection cannot	201	Who opens the file: the shell for >, before the command runs, and tee for \| tee, which is what sudo needs
tee into several commands	201	>(cmd) in bash and zsh, zsh's MULTIOS copying output by itself, and fish's psub, which is input only
tee and the exit status	201	\| tee log hides a failure; PIPESTATUS, pipefail and $pipestatus, plus tee's own 1 and 141

Planned

Rough order, not a promise:

• tee -p is GNU only: what --output-error changes when the reader stops early, measured with a bounded input, and what a Mac's tee says instead
• tee /dev/tty: watching the middle of a pipeline on the terminal while the output goes on to a file