(coreutils) Special built-in utilities

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 2.12 Special built-in utilities
 ===============================
 
 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.
 
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