tr

One line: tr translates, deletes and squeezes characters: one byte stream in, one out, no lines and no fields. Its character classes are where GNU tr on Linux and BSD tr on a Mac disagree, and an unquoted [:lower:] is where bash, zsh and fish do.

Lesson	Level	What it settles
A tr class is a set, and only two of them translate	101	the twelve classes measured byte by byte, -d, -c and -s, and why tr '[:digit:]' '[:xdigit:]' fails on Linux and runs on a Mac
An unquoted [:lower:] is a glob	101	tr [:lower:] [:upper:] working, silently doing nothing after touch e, and refusing to run in zsh, while fish does not care

Planned

Rough order, not a promise:

• A short string2 is padded, and POSIX does not promise how: tr 'abc' 'x', [x*], and GNU's -t
• tr -d '\r' is the CRLF fix, and the one case where it deletes a byte you wanted
• tr on text that is not ASCII is the Encodings library's page, tr and sort work a byte at a time ↗. It is linked, not repeated.