sh

The Wikipedia entry reads
https://en.wikipedia.org/wiki/Shell_(computing)
In computing, a shell is a user interface for access to an operating
system's services. In general, operating system shells use either
a command-line interface (CLI) or graphical user interface (GUI),
depending on a computer's role and particular operation. It
is named a shell because it is the outermost layer around the
operating system.[1] [2]

Rich’s sh (POSIX shell) tricks https://www.etalabs.net/sh_tricks.html
Discussion on Lobsters https://lobste.rs/s/uk5esr/rich_s_sh_posix_shell_tricks