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(bashref.info)Special Builtins

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Special Builtins
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   For historical reasons, the POSIX 1003.2 standard has classified
several builtin commands as _special_.  When Bash is executing in POSIX
mode, the special builtins differ from other builtin commands in three
respects:

  1. Special builtins are found before shell functions during command
     lookup.

  2. If a special builtin returns an error status, a non-interactive
     shell exits.

  3. Assignment statements preceding the command stay in effect in the
     shell environment after the command completes.

   When Bash is not executing in POSIX mode, these builtins behave no
differently than the rest of the Bash builtin commands.  The Bash POSIX
mode is described in Note: Bash POSIX Mode.

   These are the POSIX special builtins:
     break : . continue eval exec exit export readonly return set
     shift trap unset