Knowing when to switch into and out of this mode, and what impact it has, is a more advanced subject that should not burden those learning the Borne family.

It is better to start with Almquist, or another pure POSIX implementation, with documentation specific to standard adherence.

More advanced shell features should wait.

When bash is not in POSIX mode, it violates the standard.

  $ ll /bin/sh
  lrwxrwxrwx. 1 root root 4 Nov 24 08:40 /bin/sh -> bash

  $ cat s1
  #!/bin/sh
  alias p=printf
  p hello\\n

  $ cat s2
  #!/bin/bash
  alias p=printf
  p world\\n

  $ ./s1
  hello

  $ ./s2
  ./s2: line 3: p: command not found