sh is not a single shell. Rather, it is a specification with the POSIX operating system standard for how a shell should work. A script that targets this specification can be executed by any POSIX-compliant shell, such as

• bash
• ksh
• ash and its derivatives, such as dash
• zsh

In a POSIX-compliant operating system, the path /bin/sh refers to a POSIX-compliant shell. This is usually a shell that has features not found in the POSIX standard, but when run as sh, will restrict itself to the POSIX-compliant subset of its features.

Numbers in arithmetic expansions must match the following ERE:

[-+]?(0[0-7]+|[1-9][0-9]*|0[Xx][0-9A-Fa-f]+)

Arithmetic expressions support signed integer operators, comparisons, Boolean expressions, assignments, and ternary expressions from C.

If test(1) is run without any arguments it returns false.