Bash is a POSIX shell (`sh(1)` compatible)  command interpreter.
Many useful features have been added with  inspiration drawn from the Korn & C shells.
New and unique  extensions are occasionally added to the shell.
There is also support  for custom run-time extensions via the [[enable]] [[builtin]] command.
Bash may be compiled to be POSIX compliant by default as specified by the [[Shell and Utilities]] portion of the [[POSIX]] specification `(IEEE Standard 1003.1)`.