File: coreutils.info,  Node: Special built-in utilities,  Next: Exit status,  Prev: Treating / specially,  Up: Common options

2.10 Special built-in utilities
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Some programs like ‘nice’ can invoke other programs; for example, the
command ‘nice cat file’ invokes the program ‘cat’ by executing the
command ‘cat file’.  However, “special built-in utilities” like ‘exit’
cannot be invoked this way.  For example, the command ‘nice exit’ does
not have a well-defined behavior: it may generate an error message
instead of exiting.

   Here is a list of the special built-in utilities that are
standardized by POSIX 1003.1-2004.

     . : break continue eval exec exit export readonly return set shift
     times trap unset

   For example, because ‘.’, ‘:’, and ‘exec’ are special, the commands
‘nice . foo.sh’, ‘nice :’, and ‘nice exec pwd’ do not work as you might
expect.

   Many shells extend this list.  For example, Bash has several extra
special built-in utilities like ‘history’, and ‘suspend’, and with Bash
the command ‘nice suspend’ generates an error message instead of
suspending.